Why E. coli and E. faecalis are often found together in infections

Enterococcus faecalis and Escherichia coli are often found together in infections, particularly aerobic vaginitis and urinary tract infections. Researchers discovered that when iron levels are low, that E. faecalis helps E. coli to grow. This is executed via a mechanism that helps E. coli biofilm growth in low iron conditions.

E. faecalis benefits from E. coli surviving and thriving, because the successful co-infection is good for both bacteria. There are often other microbes involved too, and everyone benefits.

Microbes need iron to survive, with some able to tolerate low iron conditions better than others. E. faecalis is nutritionally fastidious (a fussy eater), requiring nutrients from its environment that it cannot synthesise itself. E. faecalis is quite tolerant in low-iron environments, however does not have the same transport systems as E. coli for acquiring iron.

E. coli colony-forming units (CFUs) are increased significantly when E. faecalis is around as part of the co-infection, when compared with just E. coli is around. This applies specifically to biofilm formation, and not free-floating (planktonic) bacterial colonies.

Ornithine is exported by E. faecalis, which is a cue for the E. coli biofilm. E. faecalis alters the environment by changing the types of amino acids that are made available to E. coli. Cell-to-cell interactions are key to this support network with specific strains of E. coli. This doesn’t happen with planktonic bacteria in a culture; only with E. coli biofilms.

References

Keogh D, Tay WH, Ho YY, Dale JL, Chen S, Umashankar S, Williams RB, Chen SL, Dunny GM, Kline KA.2016. Enterococcal metabolite cues facilitate interspecies niche modulation and polymicrobial infection. Cell Host Microbe 20:493–503. doi:10.1016/j.chom.2016.09.004.