Gemini Cationic Amphiphiles Control Biofilm Formation by Bacterial Vaginosis Pathogens

Abstract

Antibiotic resistance and recurrence of bacterial vaginosis (BV), a polymicrobial infection, justify the need for novel antimicrobials to counteract microbial resistance to conventional antibiotics. Previously, two series of cationic amphiphiles (CAms) which self-assemble into supramolecular nanostructures with membrane-lytic properties were designed with hydrophilic head groups and nonpolar domains. The combination of CAms and commonly prescribed antibiotics is suggested as a promising strategy for targeting microorganisms that are resistant to conventional antibiotics. Activities of the CAms against Gardnerella vaginalis ATCC 14018, a representative BV pathogen, ranged from 1.1 to 24.4 μM. Interestingly, the tested healthy Lactobacillus species, especially Lactobacillus plantarum ATCC 39268, were significantly more tolerant of CAms than the selected pathogens. In addition, CAms prevented biofilm formation at concentrations which did not influence the normal growth ability of G. vaginalis ATCC 14018. Furthermore, the biofilm minimum bactericidal concentration (MBC-Bs) of CAms against G. vaginalis ATCC 14018 ranged from 58.8 to 425.6 μM, while much higher concentrations (≥850 μM) were required to produce ≥3-log reductions in the number of biofilm-associated lactobacilli. The conventional antibiotic metronidazole synergized strongly with all tested CAms against planktonic cells and biofilms of G. vaginalis ATCC 14018. The synergism between CAms and the tested conventional antibiotic may be considered a new, effective, and beneficial method of controlling biofilm-associated bacterial vaginosis.

Keywords: AMP mimics; Gardnerella vaginalis; antimicrobials; bacterial vaginosis; biofilm.

FIG 1

Structures of the tested CAms showing differences in linker type (ether or ester) and hydrophobic arm length (n = 8 or 10).

FIG 2

MIC-B values of G8 compounds (A) and G10 compounds (B) against G. vaginalis biofilm. The data show biofilm formation (black bars) and bacterial growth (●) after treatment with G8/G10 ether and biofilm formation (gray bars) and bacterial growth (○) after treatment with G8/G10 ester.

FIG 3

MBC-B values of G8 compounds (A) and G10 compounds (B) against a preformed biofilm of G. vaginalis. The data show the log₁₀ CFU per milliliter of biofilm cells (black bars) after treatment with G8/G10 ether and the log₁₀ CFU per milliliter of biofilm cells (gray bars) after treatment with G8/G10 ester.

FIG 4

Isobolograms of metronidazole with CAms against planktonic cells of G. vaginalis. The graphs are for combinations of metronidazole with G8 ether (A), G8 ester (B), G10 ether (C), and G10 ester (D).

FIG 5

Isobolograms of metronidazole with CAms against biofilm cells of G. vaginalis. The graphs are for combinations of metronidazole with G8 ether (A), G8 ester (B), G10 ether (C), and G10 ester (D).

FIG 6

SEM images. (A) CAm-untreated G. vaginalis biofilm; (B) G. vaginalis biofilm treated with 58.8 μM G8 ether; (C) G. vaginalis biofilm treated with a combination of 29.4 μM G8 ether and 40.4 μM metronidazole.