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BackgroundBacterial vaginosis is characterized by an imbalance of the vaginal microbiome in which the normally predominant lactobacilli are replaced by other bacterial species. Initiated by strains of the bacterium Gardnerella, a characteristic biofilm forms on the vaginal epithelium, explaining the typical presence of clue cells. This biofilm contributes to the resilience of the bacteria to antibiotic treatment, which may explain the frequent recurrence of BV.ObjectiveIn this study, we investigate whether a therapy based on bacteriophage endolysins which specifically lyse Gardnerella, in particular the drug candidate PM-477, might be a promising alternative to broad-spectrum antibiotics and antiseptics.Study designTo identify Gardnerella specific endolysins, we searched for endolysin-encoding sequences in regions of Gardnerella genomes that are of prophage origin. This search identified fourteen homologous genes predicted to encode 1,4-beta-N-acetylmuramidase-type endolysins. When expressed in Escherichia coli and purified, the recombinant proteins demonstrated strong bactericidal activity against four different Gardnerella species. By shuffling the N-terminal catalytic domains and C-terminal cell wall-binding domains between the homologues, we produced 81 chimeric endolysins. These endolysins were tested for their activity and specificity in vitro and ex-vivo on vaginal samples from fifteen BV positive patients. Fluorescence in situ hybridization was used for visualization.ResultsSeveral engineered endolysins were 10-fold more active than the most active wild-type enzymes. When tested against a panel of 20 Gardnerella strains, the most active endolysin, called PM-477, showed minimum inhibitory concentrations of 0.13–8 µg/ml. PM-477 had no effect on Lactobacillus strains or other species of vaginal bacteria. Furthermore, the efficacy of PM-477 was tested on vaginal samples from fifteen patients with either first time or recurring bacterial vaginosis. In fourteen cases, PM-477 killed the Gardnerella bacteria and physically dissolved the biofilms without affecting the remaining vaginal microbiome.ConclusionThe high selectivity and effectiveness in eliminating Gardnerella, both in cultures of isolated strains as well as in clinically derived samples of natural polymicrobial biofilms, makes PM-477 a promising drug candidate and an alternative to antibiotics for the treatment of bacterial vaginosis, especially in patients with frequent recurrence. |
