Abstract
Many scientists investigate the potential of finding new antibiotics from plants,
leading to more than a thousand publications per year. Many different minimum inhibitory
concentrations of extracts have been proposed to decide if an extract has interesting
activity that could lead to the discovery of a new antibiotic. To date, no rational
explanation has been given for the selection criteria different authors have used.
The cumulative percentage of plant extracts with different activities from a large
experiment determining the activity of 714 acetone tree leaf extracts of 537 different
South African tree species against 4 nosocomial pathogenic bacteria and 2 yeasts was
calculated using a widely accepted serial dilution microplate method with p-iodonitrotetrazolium violet as indicator of growth. All the extracts were active
at a concentration of 2.5 mg/mL. The formula, % of active extracts = 439 × minimum
inhibitory concentration in mg/mL1.5385,
describes the results for minimum inhibitory concentrations below 0.16 mg/mL,
with a correlation coefficient of 0.9998. A rational approach could be to determine
the minimum inhibitory concentrations of the most active 1, 3, 9, 25, 50, and > 50%
of a large number of plant extracts investigated against these six important microbial
pathogens. Starting with an extract concentration of 10 mg/mL, I propose the following
classification based on minimum inhibitory concentrations:
outstanding activity < 0.02 mg/mL,
excellent activity 0.021 – 0.04 mg/mL,
very good activity 041 – 0.08 mg/mL,
good activity 0.081 – 0.16 mg/mL,
average activity 0.161 – 0.32 mg/mL, and
weak activity > 0.32 mg/mL. Higher minimum inhibitory concentrations may still be effective in
ethnopharmacological studies.
Key words
antimicrobial - minimum inhibitory concentration - tree leaves - classification of
activity
