Examining the Impact of Enteroccocus faecalis on Pseudomonas aeruginosa Metabolism and Behavior

Pseudomonas aeruginosa is an opportunistic pathogen commonly found in a variety of infection sites, namely the lungs of those with cystic fibrosis. A flexible carbon metabolism and robust biofilm formation are fundamental to P. aeruginosa's ability to colonize and adapt to many environments. In addition to its adaptability, P. aeruginosa also harbors an arsenal of virulence factors that allow it to outcompete other bacterial species in polymicrobial infections. While competitive interactions between P. aeruginosa and staphylococcus aureus have been the most well-characterized in the cystic fibrosis lung, not as much attention has been given to interactions with other bacterial species that have also been identified in infections with P. aeruginosa. One bacterium in particular, Enterococcus faecalis, has been frequently co-isolated with P. aeruginosa in many infection sites. Little research has been done to investigate interactions between these two bacteria. Previous research identified both species as present in prosthetic joint infections. Here, I show that E. faecalis can impact P. aeruginosa's metabolism and behavior in both liquid and surface growth. This work highlights the importance of characterizing interspecies interactions in a variety of growth conditions, as nutrient availability and specificity can shift the outcomes for each bacterium. My research shows that E. faecalis can survive in coculture with P. aeruginosa in part by dampening the P. aeruginosa alkyl quinolone (AQ) response, an important component of P. aeruginosa's competition with other bacteria. Additionally, I show that E. faecalis can alter pyocyanin production, a significant P. aeruginosa virulence factor. I identified a metabolite from E. faecalis, L-ornithine, which is key to mediating this interaction. L-ornithine alone can also diminish P. aeruginosa AQ response and pyocyanin production. I also further studied the effect of L-ornithine on P. aeruginosa growth and behavior. Despite a limited ability to utilize L-ornithine as a carbon or nitrogen source, P. aeruginosa growth is increased when the amino acid is added to minimal medium containing hexose and pentose sugars. L-ornithine also impacted P. aeruginosa surface growth and motility by altering the AQ production and the RhlR/I quorum sensing system. Lastly, I determined that E. faecalis can promote the growth of P. aeruginosa when grown in media containing sugars not metabolized by P. aeruginosa, such as ribose, arabinose, and maltose. This increase in growth is observed in both planktonic and surface growth and is not contact-dependent. Despite L-ornithine from E. faecalis being important for other interactions with P. aeruginosa, it does not seem to play a significant role in these interactions centered around sugar metabolism.