Metronidazole Degradation by the Gram-positive Bacterium Enterococcus Faecalis

Abstract

Enterococcus faecalis is a species of Gram-positive bacterium that natively inhabits the human gastrointestinal tract. This bacterium is capable of causing sepsis and endocarditis and is associated with the decreased effectiveness of antibiotic treatments for other bacterial infections. One such antibiotic is metronidazole, a 5-nitroimidazole drug once used commonly in the treatment of Clostridioides difficile infections. We hypothesize that E. faecalis reduces the efficacy of metronidazole in treating C. difficile infections. The molecular mechanism for how E. faecalis degrades metronidazole is unknown. In this work, we demonstrate how different strains of E. faecalis affect levels of metronidazole in in vitro culture. Through the use of spectrophotometry, we screened for E. faecalis transposon mutants unable to degrade metronidazole and used DNA sequencing to determine the insertion sites of the transposons. Screening has yielded two E. faecalis strains of interest, dubbed E. faecalis A7 and E. faecalis H8. These strains have disruptions in genes that encode the proteins AroA and AroB, respectively, which indicates that the chorismate synthesis pathway is crucial to E. faecalis’s ability to degrade metronidazole. These data lead us to propose that E. faecalis utilizes extracellular electron transfer to degrade metronidazole, a process that relies on a reliable supply of chorismate to synthesize demethylmenaquinone. This compound, alongside other membrane proteins, is utilized in extracellular electron transfer in E. faecalis. Since the reduction of metronidazole to a toxic form is seen in C. difficile, we theorize that E. faecalis uses extracellular electron transfer to reduce metronidazole to a different compound that is non-toxic to bacteria. In summary, we hypothesize that the work of demethylmenaquinone and other extracellular electron transfer proteins is the link behind E. faecalis and metronidazole degradation due to the nature of extracellular metronidazole reduction mechanisms that exist in Gram-positive bacteria.