While some bacterial infections are caused by a single species, we now know that most infections are polymicrobial in origin. We know that the activities of these species are often synergistic, as interactions between species enhances virulence, persistence, and tolerance to antibiotics. The end result is patient outcomes are typically worse in co-infection compared to mono-infection. Staphylococcus aureus is the leading cause of skin and soft tissue infections. The bacterium is also the most common organism isolated from chronic wounds and is frequently found with the opportunistic pathogens Enterococcus faecalis and Pseudomonas aeruginosa. Interactions between S. aureus and E. faecalis are relatively understudied. More broadly, our knowledge of the molecular mechanisms governing these interactions in chronic wounds is incomplete. S. aureus relies heavily on the de novo synthesis of branched-chain fatty acids for membrane biogenesis that cannot be obtained from the host during infection. These membrane fatty acids are essential for avoiding phase separation, protein aggregation, and cell death. They are also required to promote the activity of the Sae two-component system, a major regulator of S. aureus virulence. We recently discovered a new pathway specific to human-associated staphylococci for salvaging and synthesizing branched-chain fatty acids from the metabolic byproducts of co-infecting bacteria found in chronic wounds. We are working to reveal mechanistically how E. faecalis helps S. aureus construct its membrane using this pathway during infection, and how this might increase pathogenicity during coinfection.

E. faecalis cross-feeds S. aureus during co-infection, and during laboratory growth. A. WT or ΔlpdA mutant were grown overnight, washed in PBS, and 100 μl of inoculum (5x10⁶ CFUs for mono, 2.5x10⁶ each for co-infection) was injected under the bandage of wounds. 4 days post infection (DPI), CFUs were determined as described in the application. Data are log fold-change CFUs 4 DPI/inoculum to normalize for inoculum differences. 1 dot per mouse; 4 mice per condition. B. E. faecalis conditioned medium or control medium was used to feed the indicated strains. C. The indicated S. aureus strains were grown in wound-like medium in monoculture (black). Growth was rescued with branched carboxylic acids (orange) or co-culture with E. faecalis (blue). For C, n=3; D, n=2 independent experiments.