Mode of action of a proanthocyanidin (PA)-enriched extract from Cranberry (Vaccinium macrocarpon Ait.) against uropathogenic Escherichia coli (UPEC): antiinvasion versus antiadhesion

One of the most common infectious diseases are urinary tract infections (UTI) mainly caused by UPEC with rising antibiotic resistance. Therefore, the need for alternative therapies and prophylaxis is evident. Cranberry fruit extracts are traditionally used for prevention of recurrent UTI with moderate clinical evidence. The common opinion on the mode of action implies that A-type proanthocyanidins inhibit the adherence of UPEC to bladder epithelium by interacting with P-fimbriae and type 1 fimbriae. Our study aimed to investigate potential mechanisms of anti-infective properties of Cranberry extract.

Antiadhesive activity of a PA-enriched, standardized Cranberry extract was investigated using fluorescence-labeled UPEC (strain 2980) and T24 human bladder epithelial cells with flow cytometric evaluation. Surprisingly samples containing Cranberry extract lead to much higher fluorescence intensity compared to the untreated control. This indicates increased bacterial adhesion to the host cells. This result is in total contrast to the common hypothesis that Cranberry inhibits bacterial adhesion to bladder cells. Confocal laser scanning microscopy was done to obtain multidimensional image data sets, which also demonstrated significantly increased adhesion of UPEC to host cells with typical agglutination of UPEC on the outer side of cell membranes at extract concentrations of 10 µg/mL. Clusters were exclusively located on the surface of the bladder cells, while the inside was almost completely free of UPEC.

Also Scanning Electron Microscopy confirmed this finding and clearly indicated typical cluster formation on the epithelial outside.

Fig. 1: Cluster of UPEC after treatment with Cranberry extract (25 µg/ml) and adhesion to T24 bladder cells. SEM 5000 ×

An in vitro invasion assay revealed that UPEC-clusters, adhering to the cell surface, are not internalized anymore: the inhibition of bacterial invasion by the extract (50 µg/mL) was > 88%. Obviously it can be deduced that Cranberry extract exhibits an increase in bacterial adhesion to bladder cells, but inhibits the internalization into them.