It is thought that β-lactamase enzymes have evolved in bacteria over many millions
of years as a protective mechanism against naturally occurring compounds produced
by other microorganisms.[1]
[2]
[3] Environmental bacteria found in underground caverns, isolated from the outside world
for more than 4 million years, show extensive resistance to commercial antibiotics,
including penicillins and cephalosporins mediated by hydrolyzing β-lactamases.[4] As such, bacterial resistance to β-lactam antibiotics may be nothing “new.” Even
before penicillin had been used to treat clinical infections, Abraham and Chain in
1940 observed a substance produced by Escherichia coli (then named Bacillus coli) that would reduce the inhibitory effect of penicillin on Staphylococcus aureus.[5] Although not known at that time, this was the first scientific description of β-lactamase
activity, in this case the low-level AmpC activity seen in E. coli.[6] However, it is clear that the diversity, distribution, host range, and prevalence
of β-lactamases have expanded dramatically since the introduction of widespread commercial
use of antibiotics.[7]
Extended-Spectrum β-Lactamases
Production of β-lactamase is the primary mechanism by which gram-negative bacteria
express resistance to β-lactams—our most useful and effective antibiotics (see [Fig. 1]). When first recognized, most β-lactamase enzymes showed narrow spectrum activity.
For instance, TEM-1 in E. coli or SHV-1 in Klebsiella pneumoniae are both able to effectively hydrolyze ampicillin, yet most other β-lactam classes
remain unaffected to any clinically significant degree (unless these enzymes become
expressed at very high levels). In response to the increasing prevalence of these
β-lactamases in gram-negative bacteria and their spread to other new host species
(e.g., Haemophilus influenzae or Neisseria gonorrhoeae), third-generation cephalosporins (such as ceftriaxone or cefotaxime) were developed
and showed stability to the effects of these narrow spectrum β-lactamases. As such,
these agents became “workhorse” antibiotics in many hospitals, with a spectrum of
activity that covered common pathogens implicated in many infectious syndromes. However,
within a few years of their introduction into clinical use a bacterial isolate showing
transmissible resistance to third-generation cephalosporins, a key feature of “extended-spectrum”
β-lactamase (ESBL) activity, was described in a nosocomial K. pneumoniae isolate following a point mutation in its “parent” β-lactamase.[8] There are now more than 1,300 unique β-lactamase types described[7] (see www.lahey.org/Studies for a comprehensive list), many of which possess activity against “expanded-spectrum”
cephalosporins—a term used to include third-generation (e.g., ceftriaxone, cefotaxime,
ceftazidime) and fourth-generation (e.g., cefepime) cephalosporins, as well as novel
antistaphylococcal agents such as ceftaroline.[9] ESBLs also typically render bacteria resistant to monobactams such as aztreonam.
Fig. 1 Hydrolysis of β-lactam antibiotics by β-lactamase enzymes.
Classification of β-Lactamases
Several classification schemes for β-lactamases have been proposed over the years,
but two main systems have predominated. The Bush–Jacoby–Medeiros functional classification
scheme defines three main groups of β-lactamase enzymes according to their substrate
and inhibitor profiles: group 1 cephalosporinases not inhibited well by clavulanate;
group 2 enzymes with penicillinase, cephalosporinase, and broad-spectrum β-lactamase
activity generally inhibited by β-lactamase inhibitors; and group 3 metallo-β-lactamase
that hydrolyze penicillins, cephalosporins, and carbapenems that are poorly inhibited
by most β-lactamase inhibitors.[10] This scheme also incorporates several subcategories that have evolved over the years
with the discovery of new β-lactamase types.[7] The Ambler classification scheme relies upon amino acid sequences of β-lactamase
types and includes four categories: types A, C, and D with a serine residues at the
active site and class B metalloenzymes with a Zn2+ cofactor[11] (see [Table 1]).
Table 1
Key β-lactamase enzymes that mediate resistance in Enterobacteriaceae
|
Ambler classification
|
Bush–Jacoby classification
|
Structure and function
|
Genetics
|
Common species
|
Common examples
|
|
Class A
|
Group 2
(2be includes “classical” ESBLs)
|
Contain serine residues at active site. Key feature of ESBL producers is resistance
to third-generation cephalosporins (e.g., ceftriaxone) and monobactams, but not cephamycins.
Inhibited by clavulanate or tazobactam in vitro (except KPC)
|
ESBLs arise from mutations in “parent” narrow-spectrum β-lactamase. Highly transmissible
on mobile genetic elements (e.g., plasmids) often carrying multiple resistance determinants
|
ESBLs most common in E. coli, Klebsiella spp., and Proteus spp. but have been described in most Enterobacteriaceae and Pseudomonas spp.
KPC seen in Klebsiella pneumoniae
|
ESBLs: TEM and SHV variants, CTX-M
Carbapenemase: KPC
|
|
Class B
|
Group 3
|
Contain metal ions (e.g., Zn2+). Carbapenemase activity, not inhibited by clavulanate/tazobactam. Aztreonam not
hydrolyzed
|
Highly transmissible on plasmids carrying multiple other resistance determinants
|
E. coli, Klebsiella spp. But described in many Enterobacteriaceae
|
Carbapenemase: IMP, NDM
(Often called “metallo-β-lactamases”)
|
|
Class C
|
Group 1
|
Contain serine residues at active site. Also known as “AmpC” enzymes. Broad cephalosporinase
activity including hydrolysis of third-generation cephalosporins and cephamycins,
but cefepime usually stable. Not inhibited well by clavulanate, and only limited tazobactam
effect
|
Chromosomally encoded in several species, and may be inducible by exposure to β-lactams.
Expression regulated by complex systems; mutations in key regulatory genes can lead
to “derepression” and high-level AmpC production. Increasing plasmid transmission
seen
|
Enterobacter cloacae, E. aerogenes, Serratia marcescens, Citrobacter freundii, Providencia spp. and Morganella morganii all contain inducible AmpC enzymes. Plasmid mediated AmpC increasing in E. coli, Klebsiella spp.
|
Cephalosporinase: CMY, DHA, ACT
|
|
Class D
|
Group 2d
|
Contain serine residues at active site. Oxacillinases that may have carbapenemase
activity. Only weakly inhibited by clavulanate
|
May be acquired or naturally occurring chromosomal genes. May be co-located on plasmids
with other β-lactamases (e.g., OXA-48 and CTX-M-15)
|
Increasingly described in Enterobacteriaceae (e.g., K. pneumoniae and OXA-48)
|
Carbapenemase: OXA-types
|
Abbreviation: ESBLs, extended-spectrum β-lactamases.
Although these schemes have been helpful in categorizing β-lactamase types, they have
several drawbacks. The nomenclature can seem impenetrable to the nonspecialist and
has evolved significant complexity to accommodate the expanding variety of β-lactamase
types.[7]
[12] Some β-lactamases do not fit neatly into the category definitions. As a result,
the clinical applicability of these schemes, in terms of determining therapy, defining
infection control responses or policy decisions, may be obscure. The narrow definition
of an ESBL suggests an Ambler class A type, clavulanate-inhibited, Bush–Jacoby group
2be (“e” standing for “extended spectrum”) enzyme that can hydrolyze an oxyimino-cephalosporin
at a rate at least 10% of that for benzylpenicillin. Yet many other enzymes, such
as OXA-type cephalosporinase or carbapenemase, plasmid-mediated AmpC, metallo-β-lactamases,
or KPC-type carbapenemases all share some activity in common with ESBLs, and lead
to key resistance patterns such as resistance to expanded spectrum cephalosporins.
Such β-lactamase types are not considered as “true” or “classical” ESBLs, yet have
equal or greater consequences for infection control and therapeutic decision making.
A simplified nomenclature has been proposed, whereby the term ESBL applies to any
broad-spectrum β-lactamase with a suffix to suggest underlying mechanisms (e.g., using
ESBLCARBA for a KPC-type carbapenemase, ESBLM-C for a “miscellaneous” plasmid AmpC type, or ESBLCARBA-D for OXA-type carbapenemase). This nomenclature is yet to find widespread use or acceptance.[13]
[14]
The Problem with AmpC
In addition to ESBL-type enzymes, Ambler class C (Bush–Jacoby group 1) enzymes may
also effectively hydrolyze third-generation cephalosporins. These enzymes have been
recognized since the 1960s and were termed AmpC-type β-lactamases—a nomenclature that
remains today. Many gram-negative species contain chromosomally located genes encoding
and regulating AmpC. Yet, in several species AmpC is only expressed at clinically
insignificant levels (e.g., E. coli, Shigella spp.), and do not alter the effect of β-lactams, unless their expression is upregulated
by mutations in promoter regions.[15] In some species, AmpC production is controlled by transcription factors that respond
to changes in cell-wall cycling pathways under the influence of β-lactam exposure,
leading to marked increases in AmpC levels—so-called inducible expression.[16] Inducible ampC genes are usually chromosomally located and are intrinsic to certain species: particularly
Enterobacter cloacae, Enterobacter aerogenes, Serratia marcescens, Citrobacter freundii,
Providencia spp., and Morganella morganii. These species have been informally labeled as the “ESCPM” or “SPACE” organisms.[17]
[18] However, there is no clear definition for the term; it can include variable species
(such as Proteus vulgaris or P. penneri, which have a weakly inducible cephalosporinase, but of a class A type[19]) and underestimates the variability in AmpC expression in each species and the clinical
consequences of this. It should also be noted that nonfermenters such as Pseudomonas aeruginosa also possess inducible AmpC enzymes with homology to those seen in Enterobacteriaceae.[20] There are several other additional species that possess AmpC-type enzymes, with
variation in the levels of expression and subsequent clinical significance.[6] Nevertheless, the “ESCPM” term can be useful in encapsulating a complex issue in
shorthand, but one should be mindful of its limitations.
Chromosomally encoded AmpC enzymes in the species listed earlier render them intrinsically
resistant to some narrow spectrum β-lactams and early generation cephalosporins. Under
exposure to β-lactam antibiotics, the action of regulatory elements (particularly
AmpR, which represses AmpC expression in the absence of an inducer) is altered and
ampC expression can occur at significant levels.[21] For instance, the AmpR protein found in C. freundii downregulates the expression of AmpC by 2.5-fold in the absence of an inducing agent,
but when exposed to a β-lactam inducer, AmpC expression increases to 10- to 200-fold.[22] This process is now understood to be linked to cell-wall recycling involving a complex
interaction of peptidoglycan breakdown products, penicillin-binding proteins, the
ampC gene and its regulators (such as AmpR), enzymes involved in recycling muropeptidases
(such as AmpD), and other modulating elements such as the permease AmpG (see [Fig. 2]).[23]
[24]
[25] This phenomenon is of key importance to antibiotics such as ampicillin, amoxicillin–clavulanate,
and first-generation cephalosporins. The “ESCPM” species are intrinsically resistant
to these agents—to the extent that susceptibility may call the species identification
into question. However, once β-lactam exposure ceases, AmpC levels usually return
to baseline. If mutations occur in genes that contribute to the regulation of ampC transcription, AmpC can become constitutively hyper-expressed.[6]
[26] Such AmpC hyper-producers (sometimes termed “de-repressed mutants”) demonstrate
additional resistance to third-generation cephalosporins, cephamycins (e.g., cefoxitin),
new anti-staphylococcal cephalosporins such as ceftaroline,[9] anti-pseudomonal penicillins (such as piperacillin and ticarcillin), and their β-lactamase
inhibitor combinations.[6]
[27] These variants occur spontaneously at a frequency of ∼10−6 to 10−8 of the bacterial population[23] and may be selected rapidly following β-lactam therapy and predispose to clinical
failure.[27]
Fig. 2 Resistance mediated by inducible AmpC and AmpC overexpression. (1) In the absence of inducing β-lactams, basal AmpC levels are low. Normal peptidoglycan
recycling involves the transport of muropeptides across the inner cell membrane by
AmpG permease, following which, 1,6-anhydro-MurNAc-tripeptides or pentapeptide species
are formed, with levels regulated by AmpD, and recycled to form UDP-MurNAc-pentapeptide
which is incorporated back into the cell wall. UDP-MurNAc-pentapeptides bind to the
AmpR regulatory unit which, under these conditions, predominates and is inhibitory
to the expression of the ampC gene. (2) Under the influence of a strongly inducing β-lactam, cytosolic levels of 1,6-anhydro-MurNAc-tripeptides
or pentapeptides increase; under such conditions, UDP-MurNAc-pentapeptides are displaced
from AmpR causing promotion of ampC expression; phenotypic resistance to strong inducers that remain labile to AmpC is
seen but weak inducers or inducers that are stable to AmpC remain effective. (3) Spontaneous mutations in regulatory elements (such as AmpD) occur at a rate of 1
in 106 to 108 cells and may be selected during antibiotic therapy. Under such circumstances, 1,6-anhydro-MurNAc-peptides
accumulate in the cytoplasm resulting in AmpR-mediated overexpression of AmpC; mutations
in AmpR may also cause a similar phenotype. Such AmpC hyperproduction can occur in
the absence of an inducing agent and mediates resistance to many β-lactams. Please
note, this diagram has been simplified for clarity—more detail can be found elsewhere.[17]
[24]
[221]
[222]
Plasmid-Mediated AmpC
While AmpC is usually chromosomally encoded, we now increasingly see ampC genes mobilized on plasmids, which can easily transfer between species. The first
transmissible cephamycinase (CMY-1) was identified in a K. pneumoniae isolate from a patient in South Korea in 1989.[28] Plasmid-mediated AmpC is now becoming increasingly common as a cause of resistance
in Klebsiella and E. coli.[29]
[30]
[31]
[32] Such isolates can be identified from nosocomial, community onset, and healthcare-associated
infections and may be associated with high mortality.[33] Like ESBL producers, isolates with plasmid AmpC may also frequently be resistant
to other agents such as quinolones or trimethoprim–sulphamethoxazole.[34] In the laboratory, they can give a phenotype similar to an ESBL producer (with resistance
to third-generation cephalosporins) but fail to demonstrate synergy with clavulanate
(the standard test to phenotypically confirm an ESBL) and also show resistance to
cefoxitin.[31] However, other mechanisms (such as outer membrane protein permeability changes)
may confer cefoxitin resistance.[35] Several inhibitors have been proposed to help confirm AmpC production (such as boronic
acid[36] or cloxacillin[31]), but the sensitivity and specificity of such tests have been variable and are not
routinely used. This can cause substantial difficulty for the clinical microbiologist
in knowing which agents to recommend to clinicians for isolates with resistance to
key β-lactams, such as third-generation cephalosporins, without immediately defaulting
to carbapenems (given the implications for antimicrobial stewardship). Furthermore,
plasmid-mediated AmpC can coexist with ESBL enzymes in the same host, making phenotypic
interpretation even less reliable.[29]
Plasmid AmpC genes are usually noninducible, as they lack the genetic apparatus to
regulate expression, but there have been reports of plasmid-mediated inducible AmpC
spreading into new hosts[37]—raising the alarming prospect of making it impossible to predict emergent AmpC-mediated
resistance by species identification alone. Furthermore, there have been increasing
reports of extended-spectrum AmpC β-lactamases, which have developed the ability to
inactivate cefepime.[38]
Treatment
There is a significant evidence gap between our understanding of basic biology of
ESBL- and AmpC-producing bacteria and the clinical application of this information.
Despite many hundreds of studies reporting on resistant gram negatives, the majority
of studies focus on laboratory, epidemiological, or infection-control aspects of these
bacteria—only a handful provide reliable insight into optimal therapy. There have
been several notable but relatively small observational studies reporting treatment
outcomes for ESBL or AmpC producers. While these have been invaluable to our limited
evidence-base, we have been lacking adequately powered, well-designed, international
prospective studies in this area. Particularly, there has never been a randomized
controlled trial reported that specifically addresses these questions, which is unfortunate
given the significance and scale of the problem, but not a surprise given the realities
of clinical research.
It would seem intuitive that selecting appropriate initial empirical therapy is important
in patients with bacteremia, and becomes increasingly difficult when the incidence
of resistance is high. Choosing inappropriate empirical antibiotic therapy for bacteremia
caused by ESBL-producing E. coli or Klebsiella has been associated with increased mortality in some studies,[85] especially in nonurinary infections or with multidrug resistant isolates.[86] However, this has not been a universal finding, with several studies showing no
significant impact of inappropriate empirical therapy on mortality.[67]
[87]
[88]
[89]
[90]
[91] A meta-analysis of 16 studies suggested increased mortality in bacteremia caused
by ESBL producers (RR, 1.85; 95% CI, 1.39–2.47), which increased with delayed therapy
(RR, 5.56; 95% CI, 2.9–10.5), although only 1 study controlled for confounders.[92]
Expanded-Spectrum Cephalosporins for ESBL and AmpC Producers
Soon after the recognition of ESBLs emerging as a concern, clinical failures in patients
treated with third-generation cephalosporins for infections caused by ESBL producers
were reported, even when breakpoints used at that time suggested susceptibility.[93]
[94] This phenomenon was also supported by animal studies.[95] Observational studies suggested that treatment with cephalosporins of bloodstream
infection caused by ESBL producers was associated with poorer outcome when compared
with non-ESBL strains in children[72] and adults.[96] Empirical therapy with ceftriaxone in patients with pyelonephritis, found subsequently
to be caused by ESBL-producing E. coli, was associated with delayed resolution of symptoms, less likelihood of microbiological
resolution at 5 days and longer hospital admissions.[82]
Given the concern that bacteria could harbor ESBLs that would not be detected by the
higher breakpoints for cephalosporins used at the time, the use of third-generation
cephalosporins for ESBL producers (even if susceptible) was discouraged. Regulatory
authorities issued guidance that laboratories should report all ESBL-containing E. coli and Klebsiella spp. as resistant to all penicillins, all cephalosporins, and aztreonam, regardless
of susceptibility results.[97] Although less supported by evidence, most laboratories extended such guidance to
include all other non-carbapenem β-lactams, including inhibitor combination agents.
Although this was an understandable response to the challenges faced at the time,
it had the unintended consequence of directing clinicians to use carbapenems increasingly
frequently for ESBL-related infections, even in relatively uncomplicated disease like
cystitis.
In recent years, the Clinical and Laboratory Standards Institute (CLSI) and European
Committee on Antimicrobial Susceptibility Testing (EUCAST) have lowered the susceptibility
breakpoints for third- and fourth-generation cephalosporins against Enterobacteriaceae,
without the need for additional testing. For ESBL production, unless for infection
control or surveillance purposes.[98]
[99] This still begs the question of whether any known ESBL producer which tests susceptible
to an agent, against which the enzyme has potential activity, can be safely used clinically.
For instance, CTX-M producers may retain susceptibility to ceftazidime; yet whether
this would be safe drug to use for a serious infection remains unclear, largely due
to the presence of pronounced inoculum effects and limited clinical data.[100] In theory (at least), given the revised standards, susceptibility should be read
as reported and therapeutic options provided as such.[101] In this way, the current guidance has the implicit message that clinicians should
worry less about the underlying resistance mechanism when selecting therapy. However,
there remains concern that drugs that may act as substrates for ESBLs should still
be avoided for therapy, even if susceptibility is demonstrated.[102]
Emergent resistance during therapy with third-generation cephalosporins for AmpC-producing
Enterobacteriaceae has been a major concern. A key study from 1991 by Chow et al reported
outcomes for patients with bloodstream infections caused by Enterobacter spp. In those treated with a third-generation cephalosporin, 19% experienced relapsed
bacteremia and resistance mediated by high level of AmpC, despite initial susceptibility.[103] This phenomenon has been replicated in larger cohorts, although a lower risk of
clinical failure has been reported with other AmpC-producing species.[104]
[105] When emergent resistance occurs, it has been associated with higher mortality and
healthcare-associated costs.[106] As a result, the use of third-generation cephalosporins for the treatment of significant
infections caused by AmpC producers such as Enterobacter spp. has been strongly discouraged, except perhaps in simple infections (such as
uncomplicated urinary tract infection [UTI]), where a rapid bactericidal effect can
be achieved before selection for hyper-producing mutants can occur.[107] Poor outcomes have also been reported for plasmid AmpC-producing K. pneumoniae treated with third-generation cephalosporins; although such studies often are small,
retrospective and report mortality rates unadjusted for comorbidity.[108]
Cefepime for AmpC and ESBL Producers
Cefepime is the only expanded spectrum cephalosporin with stability to AmpC β-lactamase
and retains in vitro activity to species such as E. cloacae, including constitutively AmpC derepressed strains.[109] Recent retrospective studies would suggest that cefepime is effective for infections
caused by AmpC-producing Enterobacteriaceae. Comparing patients paired by propensity
score matching given either meropenem or cefepime, there were no differences in 30-day
mortality (odds ratio [OR], 0.63; 95% CI, 0.23–2.11) or length of hospital stay (RR,
0.96; 95% CI, 0.79–1.26), although this study included only 64 patients.[110] In a large series of over 300 patients with Enterobacter bacteremia, mortality was similar for patients treated with meropenem or cefepime
after adjustment for comorbidity and propensity score matching.[111]
However, the picture is complicated by the fact that Enterobacter, Citrobacter, and Serratia spp. can frequently acquire additional ESBLs,[112] to which cefepime is not stable, thus elevating minimum inhibitory concentrations
(MICs).[113] Clinical failures from isolates with MICs at or above the previous CLSI breakpoint
of 8μg/mL treated with cefepime have been shown to be associated with an increased
risk of mortality, especially with a dosing regimen of 1 g 12 hourly.[114] ESBLs expressed in AmpC producers may be difficult to reliably detect and discriminate
from chromosomal AmpC with routine laboratory methods. Cefepime may also be subject
to significant inoculum effects with ESBL producers.[115] Eightfold or greater increases in MIC values were observed with several cephalosporins,
including cefepime, when tested against a variety of Enterobacteriaceae at inocula
100-fold higher than standard—a phenomenon not seen with carbapenems.[116] Similarly, cefepime was prone to significant inoculum effects when tested against
K. pneumoniae containing plasmid-mediated AmpC β-lactamase.[117] Resistance to cefepime in Enterobacter has also been described to develop by the overexpression of an altered AmpC enzyme
or porin mutations.[118]
Treatment of ESBL-producing Klebsiella or E. coli with cefepime is controversial. Cefepime, like other cephalosporins, demonstrates
marked inoculum effects in vitro when tested against ESBL producers.[100] Some small case series have reported a role for cefepime, although clinical failures
were observed.[119] In a retrospective study that compared cefepime to a carbapenem for the treatment
of bacteremia caused by susceptible ESBL producers, cefepime was independently associated
with an increased 30-day mortality on multivariate analysis (OR, 9.9; 95% CI, 2.8–31.9).[120] A nonsignificant trend toward increased mortality was also seen for cefepime when
used as empirical therapy for bacteremia caused by ESBL producers (OR, 1.66; 95% CI,
0.71–3.87).[121]
It has been suggested that standard dosing of cefepime should be effective for ESBL
producers that demonstrate an MIC for cefepime of ≤2 mg/L (CLSI) or ≤1 mg/L (EUCAST),
but higher or more frequent dosing would be required for an MIC between 4 and 8 mg/L.[122] It should be noted that the method of susceptibility testing for cefepime against
ESBL producers may provide variable results; lack of concordance between gold standard
agar dilution and Vitek2 microbroth dilution methods have been reported and could
lead to major interpretative errors, especially when lowered breakpoints are used.[123]
Cephamycins
Although rarely used in many countries, cephamycins (such as cefoxitin, flomoxef,
and cefmetazole) remain stable to hydrolysis by ESBLs, but are susceptible to AmpC
enzymes. Cefoxitin was effective in a murine model of UTI caused by CTX-M-15–producing
E. coli, when compared with a carbapenem.[124] A small study from Japan compared cefmetazole to meropenem for the treatment UTI
caused by ESBL-producing Enterobacteriaceae and showed no differences in clinical
or microbiological cure rates or adverse events.[125] However, for dialysis patients with ESBL K. pneumoniae bacteremia and high acuity of illness, use of flomoxef was independently associated
with mortality (OR, 3.52; 95% CI, 1.19–58.17).[126]
Carbapenems
Carbapenems have long been considered the first-line treatment option for significant
infections caused by ESBL or AmpC producers. Carbapenems are generally stable to hydrolysis
by ESBLs or AmpC. They are less affected by inoculum effects in vitro[117] and in animal models.[95] They demonstrate excellent pharmacodynamic exposure in vitro. Monte Carlo simulation
of carbapenems against 133 ESBL-producing isolates showed that the bactericidal cumulative
fraction of response (defined as ≥40% of the proportion of the dosing interval for
which free drug levels were above the MIC) was achieved for 96.3% of isolates against
ertapenem and >99% for imipenem and meropenem.[127]
Several observational studies have demonstrated that carbapenems are associated with
improved outcome when compared with cephalosporins or other alternatives for bloodstream
infections caused by ESBL producers.[71]
[90]
[128]
[129]
[130]
[131] However, superiority has never been demonstrated in a randomized trial.
Ertapenem, a carbapenem lacking activity against Pseudomonas, has been increasingly used for directed therapy against ESBL and AmpC producers.
Testing both ertapenem and meropenem against ESBL-producing E. coli or Klebsiella with a range of MICs in an animal model showed that both drugs had similar efficacy
when MICs were low, but meropenem had greater efficacy against isolates with ertapenem
MICs ≥ 2 μg/mL.[132] Ertapenem achieved clinical success in 80% of patients with ventilator-associated
pneumonia caused by ESBL producers, although this study only enrolled 20 patients
and lacked any control group.[133] Similarly, a clinical success rate of 78% and microbiological cure rate of 92% were
seen in a series of patients treated with ertapenem for a variety of infections caused
by ESBL producers, although this study also had only 50 evaluable patients and no
comparison group.[134] Favorable clinical response rates of up to 96% have been reported in its use against
ESBL bacteremia.[135] When ertapenem was compared with carbapenems (such as meropenem) for the treatment
of bacteremia caused by ESBL-producing Enterobacteriaceae in a cohort of 261 patients,
no difference in mortality was seen even after controlling for the propensity to receive
ertapenem (OR, 0.50; 95% CI, 0.12–2.1).[129] Other studies of ESBL bacteremia have also shown an equivalent mortality between
ertapenem and other carbapenems.[136]
Although most studies have concentrated on Klebsiella and E. coli, being the most common ESBL producers, some studies have examined treatment options
for other ESBL-producing species. Huang et al[137] assessed the 14-day survival of 54 adult patients with bacteremia caused by ESBL
producers other than E. coli or Klebsiella spp. (including intrinsic AmpC producers such as E. cloacae or C. freundii) and compared carbapenem to noncarbapenem therapy. Although improved survival (90.9%,
20/22) was seen with carbapenems compared with noncarbapenems (71.9%, 23/32), with
ciprofloxacin as the main alternative choice, this difference was not statistically
significant.[127] As with many small retrospective cohorts, such studies may be underpowered to detect
true differences in treatment regimens.
The treatment options for inducible AmpC-producing Enterobacteriaceae that also express
ESBLs are limited. Among 31 patients with ESBL-producing E. cloacae, all (8/8) patients who received a carbapenem survived, whereas 38.9% died when given
a noncarbapenem (p = 0.06).[138] In a study that compared patients treated for bacteremia caused by ceftriaxone nonsusceptible
E. cloacae, with or without ESBL production, carbapenems were associated with lower mortality
in the ESBL group when compared with those treated by noncarbapenem β-lactam (5/53,
9.4% vs. 13/44, 29.5%; p = 0.01), although the difference was not significant in a multivariate analysis;
breakthrough bacteremia was more common in the noncarbapenem β-lactam group (18/31,
58% vs. 3/31, 9.6%; p < 0.001).[139]
However, emergent carbapenem resistance has been described during carbapenem therapy,
leading to clinical failure. In a patient with pneumonia caused by a CTX-M–producing
K. pneumoniae treated with ertapenem, carbapenem resistance developed via the loss of a porin.[140] Carbapenem resistance in Enterobacteriaceae may occur either by the acquisition
of a carbapenemase, hyperproduction of AmpC, or an ESBL combined with porin mutations
or via efflux pumps.[118]
[141]
[142]
[143] Resistance to ertapenem has also been described by chromosomal AmpC mutations that
allow carbapenemase activity,[144] especially when combined with loss of outer membrane proteins.[145]
β-Lactam/β-Lactamase Inhibitor Combinations
By definition, ESBLs (Ambler class A enzymes) are inhibited by clavulanate and tazobactam.
Indeed, phenotypic confirmation of an ESBL in E. coli, Klebsiella spp., and Proteus mirabilis relies upon this phenomenon.[98]
[146] These inhibitors act as suicide substrates by irreversibly binding to β-lactamase
enzymes.[147] Despite this inhibition, the currently available BLBLI agents (such as amoxicillin–clavulanate,
piperacillin–tazobactam, ampicillin–sulbactam, cefoperazone–sulbactam, and ticarcillin–clavulanate)
have generally been avoided for infections caused by ESBL producers in favor of carabapenems.[148]
In general, piperacillin–tazobactam has retained good in vitro activity against ESBL
producers, especially for E. coli, although K. pneumoniae are often less susceptible.[149]
[150] MICs for BLBLIs tested against ESBL producers may tend to cluster around susceptibility
breakpoints, so a single dilution change (within the margin of error) can alter the
categorization. BLBLIs, especially piperacillin–tazobactam, may also be subject to
significant inoculum effects in vitro.[100]
[115] Although piperacillin–tazobactam exhibits significant MIC elevations against ESBL
producers tested using a high inoculum, this phenomenon is less marked than that observed
with expanded-spectrum cephalopsorins.[116] Inoculum effects are not universal to all BLBLIs. In time-kill studies of amoxicillin–clavulanate,
bactericidal killing of ESBL-producing E. coli was maintained over 24 hours in the presence of a high inoculum, in contrast to piperacillin–tazobactam.[151] However, an inoculum effect was also seen for piperacillin–tazobactam against non-ESBL
strains, which suggests that the effect is more likely a property of the drug rather
than related to β-lactamase activity alone. The significance of the inoculum effect
has been debated and has been argued to represent a laboratory phenomenon of limited
clinical significance.[152] However, some animal models appear to reproduce the effect.[153] In a murine model of pneumonia caused by ESBL-producing K. pneumoniae at higher inoculum, 100% of mice died with piperacillin–tazobactam treatment, in
contrast to 100% survival with meropenem.[154] However, in animal models at standard inocula, piperacillin–tazobactam appeared
to be efficacious against ESBL-producing K. pneumoniae, whereas ceftazidime was not; although imipenem was the most effective agent.[155]
Another theoretical concern relating to the use of BLBLIs for ESBL producers is the
co-location of other β-lactamase types on acquired plasmids, some of which may be
poorly inhibited (such as plasmid AmpC or OXA-1). Bacteria may overexpress other non-ESBL
“parent” enzymes that can overcome β-lactamase activity.[156]
[157] Resistance may also occur by the development of inhibitor-resistant enzymes, porin
mutations, or efflux pumps.[158] It should be noted that BLBLIs have been used for many years against isolates with
narrow spectrum β-lactamases, even in critical infections, without clear concerns
over loss of efficacy.
However, there were early reports of clinical failure with piperacillin–tazobactam
against ESBL producers.[159]
[160] There were concerns over the reliability of tazobactam to inhibit some ESBL variants
or if expression occurs at high levels[156]
[161] and limited experience with the use of BLBLIs for this indication. As a result,
a view was formed that these agents could not be relied upon.[148]
In recent years, clinical evidence has accumulated that may support the use of BLBLIs
in the treatment of infections caused by ESBL producers. Piperacillin–tazobactam was
effective in treating a small series of patients with UTI caused by ESBL producers,
as well as 90% of infections from other sites, provided the MIC was ≤16 μg/mL.[162] In a small study from Thailand, a predictor of mortality in patients with bloodstream
infection caused by ESBL-producing E. coli or Klebsiella was failure to receive either a carbapenem or BLBLI for empirical therapy (93 vs.
43%; p = 0.002), although all patients switched to carbapenem therapy once susceptibility
was determined.[163] After adjustment for confounders, no association between empirical use of piperacillin–tazobactam
and increased mortality was found in a study of 114 patients from Korea with bacteremia
caused by ESBL-producing E. coli or K. pneumoniae (OR, 0.55; 95% CI, 0.16–1.88).[164] In a large study of 387 ESBL E. coli bacteremia cases, piperacillin–tazobactam was associated with lower mortality when
compared with carbapenems, provided treatment was adequate.[165]
Much of the current evidence to support the use of BLBLIs has been derived form a
large Spanish cohort of patients with bacteremia caused by ESBL-producing E. coli. A post hoc analysis of six prospective studies compared BLBLI treatment with carbapenems
and found no differences in mortality for empirical (hazard ratio [HR], 1.14; 95%
CI, 0.29–4.40) or definitive therapy (HR, 0.76; 95% CI, 0.28–2.07).[166] However, for nonurinary infection, an MIC ≤2 mg/L to piperacillin–tazobactam appears
to be predictive of better outcome.[167] A larger international observational study, including 656 patients, has recently
been reported and also suggests noninferiority for BLBLIs used for ESBL bloodstream
infection in comparison to carbapenems, with an adjusted HR for 30-day mortality of
0.97 (95% CI, 0.48–2.03).[168]
Optimized dosing of piperacillin–tazobactam to reach therapeutic drug targets may
be necessary in critically ill patients,[169] who frequently demonstrate altered pharmacokinetics through variations in key variables
such as renal clearance, increased capillary permeability, hypoalbuminemia and increased
volumes of distribution.[170] Continuous infusions of β-lactams may improve outcomes in critically ill patients.[171]
BLBLIs such as piperacillin–tazobactam may offer a carbapenem-sparing “step-down”
option once susceptibility is proven—especially if the MIC is low and the burden of
infection has been reduced.[150] This seems most reliable for urinary infections. However, further evidence is required
to allow confidence in efficacy for a wider set of clinical circumstances. An international
randomized-controlled trial registered with www.clinicaltrials.gov that compares piperacillin–tazobactam with meropenem for the definitive treatment
of bloodstream infections caused by ceftriaxone nonsusceptible E. coli or Klebsiella spp. is currently recruiting (Trial registration number NCT02176122).
BLBLIs for AmpC Producers
AmpC enzymes are generally poorly inhibited by clavulanate or tazobactam, although
the concentration of tazobactam needed to inhibit AmpC β-lactamase is much lower than
for clavulanate.[172] Clavulanate is a powerful inducer of AmpC and poorly inhibits its activity, and
so may antagonize the activity of ticarcillin when used in combination against isolates
with inducible β-lactamase.[147] Conversely, tazobactam is much less potent inducer of AmpC.[147] However, once again, the clinical efficacy of drugs such as piperacillin–tazobactam
against AmpC producers is controversial. Isolates with derepressed AmpC frequently
demonstrate high MIC values to piperacillin–tazobactam. It is a curiosity that the
AmpC enzyme produced by M. morganii is well inhibited by tazobactam, even when highly expressed.[173]
[174] However, no clinical studies exist to corroborate efficacy in significant M. morganii infections.
Clinical studies to assess the efficacy of piperacillin–tazobactam in serious infections
caused by AmpC producers are limited. Many laboratories do not report piperacillin–tazobactam
susceptibility results for AmpC producers such as Enterobacter spp., over concerns of clinical failure and emergent resistance. This practice is
somewhat extrapolated from the poor outcomes seen with third-generation cephalosporins.[103] However, piperacillin–tazobactam was not associated with the emergence of cephalosporin
resistance in the treatment of Enterobacter bacteremia (RR, 1.1; 95% CI, 0.4–2.7) in contrast to third-generation cephalosporins
(RR, 3.3; 95% CI, 1.8–6.0).[104] In another study that examined 377 Enterobacter bacteremia events in adults, the only factor independently associated with a reduction
in 30-day mortality was the early use of piperacillin–tazobactam.[175] However, piperacillin–tazobactam use may still cause selection pressure for isolates
with derepressed AmpC. The risk of isolating a resistant Enterobacter following piperacillin–tazobactam or broad-spectrum cephalosporin was equal in one
study (2% in both groups, RR = 1.02; p = 0.95).[176]
Fosfomycin
Fosfomycin has been used for many years in some countries as a single-dose treatment
for uncomplicated UTIs caused by E. coli. It has a bactericidal effect by inhibiting cell wall synthesis.[177] There has been renewed interest in its use against urinary infections caused by
ESBL- or plasmid AmpC-producing E. coli or K. pneumoniae, as it demonstrates excellent in vitro activity against such strains.[177]
[178]
[179]
[180]
[181]
[182]
[183] Although only a handful of clinical studies have examined fosfomycin for the treatment
of UTI caused by ESBL-producing E. coli, clinical response rates of >78% have been reported.[184] In a prospective observational study from Turkey, oral fosfomycin (given alternate
days for 3 doses) was compared with carbapenems (given for 14 days) for ESBL E. coli causing lower UTIs in 47 patients, with complicating factors such as catheterization
or urological surgery, but no signs of pyelonephritis.[185] Clinical and microbiological success rates were similar in both the fosfomycin and
carbapenem groups, with significant cost savings seen and no adverse effects reported
in those given fosfomycin.[185] It achieves high concentrations in prostate tissue and may be a useful prophylactic
antibiotic before transrectal prostate biopsy or for treatment of prostatitis caused
by resistant gram-negative bacteria.[186] Increased use of fosfomycin has been associated with a rising burden of resistance.
In Spain, the incidence of fosfomycin resistance in ESBL-producing E. coli has increased from 0% in 2005 to 14.4% in 2011.[187] Rates of resistance remain low, but high-level resistance can occur via single-step
mutations,[188] or may be acquired on plasmids.[189] It should be noted that studies of fosfomycin resistance require attention to the
methods used. Resistance can be overestimated by disk diffusion or microbroth dilution
susceptibility testing, when compared with a reference agar dilution.[178]
Data regarding the use of fosfomycin outside the urinary tract are sparse. In a murine
model of ESBL-producing E. coli implant infections that compared combinations of fosfomycin, tigecycline, gentamicin,
and colistin, fosfomycin was the only single agent able to eradicate biofilm in a
small number of cases (17%) and, when combined with colistin, had the highest cure
rate (8/12, 67%) and was superior to fosfomycin alone.[190] Intravenous formulations of fosfomycin are available in some countries. It has been
used successfully as salvage therapy in combination with meropenem for refractory
Lemierre syndrome, bacteremia and cerebral abscesses caused by ESBL-producing K. pneumoniae.[191] In a literature review of available evidence that examined 62 studies involving
1,604 patients with various infections treated with fosfomycin alone or in combination,
an overall cure rate of 81.1% was reported.[192]
Tigecycline
Tigecycline, a first in class glycylcyline, has activity against most ESBL- and AmpC-producing
Enterobacteriaceae.[193] It should be noted that tigecycline has limited penetration into the urinary tract
and may not be effective at this location, although successful treatment has been
reported.[194] It may also achieve poor serum levels because of a very large volume of distribution,
which may limit effectiveness in bacteremia. Breakthrough infections have been reported.[195] However, it has been used successfully as salvage therapy for a complex infection
caused by a carbapenem-resistant, ESBL-producing K. pneumoniae.[196] A recent meta-analysis has suggested excess mortality for tigecycline,[197] limiting enthusiasm for its use in serious infections when alternatives exist.
Mecillinam and Pivmecillinam
Mecillinam is an amidinopenicillin with a wide spectrum of gram-negative activity.
There has been interest in its use against resistant Enterobacteriaceae since the
1970s.[198] It appears to act by binding penicillin-binding protein-2 to inhibit cell wall synthesis.
However, given the poor oral bioavailability of this drug, the prodrug pivmecillinam
has been developed and is approved for the use against uncomplicated UTIs. It is now
included in the Infectious Disease Society of America (IDSA) guidelines for the treatment
of cystitis, although there are concerns that it has inferior efficacy for pyelonephritis
when compared with other agents.[199] However, (piv)mecillinam has the advantage of retaining activity against ESBL and
plasmid AmpC-producing E. coli,[200]
[201] even if expressing multiple β-lactamase types.[202] In a series of 100 ESBL-producing E. coli isolated from patients with (predominantly community acquired) UTI, 85% demonstrated
susceptibility to pivmecillinam.[183] Against E. coli strains that expressed various β-lactamase types, it showed excellent activity when
compared with other penicillins against isolates that contained TEM, IRT, and AmpC
producers.[203]
[204] The combination with clavulanate may also enhance its activity and mitigate against
inoculum effects.[201] In a small study of patients with lower UTI caused by ESBL-producing E. coli or K. pneumoniae, patients treated with pivmecillinam achieved good clinical responses (8/8) but low
(2/8) bacteriological cure rates (defined as <103 CFU/mL at 30-day follow-up). The drug is not approved for use in the United States
and has not been widely used outside European countries, but deserves greater attention
in an era of increasing community-acquired gram-negative resistance. It has also been
suggested that, although resistance to pivmecillinam may develop by mutations in the
genes affecting the bacterial elongation process, the risk for clonal spread is low
and may be associated with limited epidemiological fitness.[205]
Temocillin
Temocillin is a carboxypenicillin derivative of ticarcillin, which has been modified
to improve stability to AmpC and ESBL enzymes, although it has less activity against
Pseudomonas spp., gram positives, and anaerobes. Having initially received little market interest,
it was withdrawn from the United Kingdom, but continued to be used in Belgium for
infections caused by resistant Enterobacteriaceae,[206] and it has now been relaunched.[207] It demonstrated in vitro efficacy against 88% of 846 isolates with ESBL or AmpC
phenotypes or K. oxytoca K1 hyperproducers,[207] and >90% of multiresistant ESBL-producing E. coli.[206] However, temocillin efficacy may be affected by porin mutations and is not stable
to OXA-48 and NDM-1 carbapenemases. It achieves excellent levels in the urine and
may be a useful agent for infections at this site, although clinical data are limited.
One study reported outcome for 92 adults treated with temocillin, mainly for urinary
or bloodstream infections caused by ESBL or derepressed AmpC producers.[208] Good clinical (86%) and microbiological (84%) cure rates were observed, especially
when dosed at 2 g twice daily (clinical cure rate 97%, 36/37 patients with ESBLs or
AmpC); a low risk of C. difficile (2%) also seems an advantage.[208] Further prospective studies of temocillin as a carbapenem-sparing option for ESBL
or AmpC producers would be of interest, especially in nonurinary infections.
Nitrofurantoin
Although nitrofurantoin is only effective in the context of uncomplicated UTIs, a
significant proportion of ESBL-producing E. coli retain susceptibility to this agent, especially when community acquired.[55]
[183]
[209] In a retrospective study of 75 patients treated with nitrofurantoin for uncomplicated
UTI caused by ESBL-producing E. coli, clinical and microbiological success rates of 69 and 68%, respectively, were reported,
although there was no control group.[210]
Other Agents
Although ESBL producers are frequently multidrug resistant, they may still demonstrate
susceptibility to other standard antimicrobials such as trimethoprim–sulphamethoxazole,
quinolones, or aminoglycosides (especially amikacin). Some of these agents have limitations
in terms of toxicity (e.g., amikacin) and there are few published studies examining
the clinical efficacy against ESBL producers, especially for critical infections.
These may be reasonable alternatives for less complex infections, especially where
oral options are limited. However, coresistance to agents such as quinolones is very
common in ESBL producers.[56] Some studies have shown inferiority of quinolones in comparison to carbapenems,
even when susceptible in vitro.[85]
[211] Although not widely used, sitafloxacin (a quinolone) showed excellent in vitro efficacy
against ESBL-producing E. coli or K. pneumoniae from Japan, even when strains showed resistance to levofloxacin.[179]
New Agents in Development
Ceftolozane is a novel oxyimino-aminothiazole cephalosporin, which has additional
activity against P. aeruginosa in comparison to ceftazidime or cefepime, but may be inactivated by ESBL or AmpC
enzymes. However, in combination with tazobactam, it has demonstrated greater in vitro
activity against almost 3,000 gram-negative isolates from U.S. and European patients
with pneumonia, including ESBL- and AmpC-producing Enterobacteriaceae, when compared
with current cephalosporins and piperacillin–tazobactam.[212] It even maintained reasonable activity against Enterobacteriaceae with multidrug
or extensive drug-resistant phenotypes.[212] It is currently being evaluated in phase III trials in combination with tazobactam.[213]
There has been a renewed interest in developing novel β-lactamase inhibitor compounds.[214] One of the most promising is Avibactam, a new non-β-lactam inhibitor of β-lactamase
that shows efficacy against class A, C, and some class D enzymes. Coformulations with
ceftazidime, ceftaroline, and aztreonam are currently under investigation. It has
been shown to retain its effect against extend-spectrum AmpC variants.[215] It also has limited ability to induce the expression of AmpC.[216] In vitro, when combined with ceftazidime, it showed broad-spectrum efficacy against
a large series of clinical isolates from the United States.[217] Several trials are currently underway, including phase I, II, and III studies, for
various combinations of avibactam with β-lactams or monobactams.[214]
