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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, E. faecalis helps E. coli to grow.1 It works through a mechanism that helps E. coli form biofilm 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, but does not have the same transport systems as E. coli for acquiring iron.
E. coli colony-forming units (CFUs) increase significantly when E. faecalis is around as part of the co-infection, compared with when only E. coli is present. 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.
Frequently asked questions
Why are E. coli and E. faecalis found together?
When iron is scarce, E. faecalis exports the amino acid ornithine, which cues E. coli to form biofilm and grow. The two support each other, so they are often found together in aerobic vaginitis and urinary tract infections.
Does this show up in a normal urine culture?
No. The effect only appears in biofilms, not in free-floating (planktonic) bacteria grown in a standard culture, which is one reason polymicrobial infections can be missed.
This article is general information and not a substitute for personalised medical advice. If you have a persistent or recurrent infection, please see an experienced practitioner.
- Keogh D, Tay WH, Ho YY, et al. Enterococcal Metabolite Cues Facilitate Interspecies Niche Modulation and Polymicrobial Infection. Cell Host & Microbe. 2016;20(4):493–503.
