D-Serine degradation by Proteus mirabilis contributes to fitness during single-species and polymicrobial catheter-associated urinary tract infection

Proteus mirabilis is a common cause of catheter-associated urinary tract infection (CAUTI) and secondary bacteremia, which are frequently polymicrobial. We previously utilized transposon insertion-site sequencing (Tn-Seq) to identify novel fitness factors for colonization of the catheterized urinary tract during single-species and polymicrobial infection, revealing numerous metabolic pathways that may contribute to P. mirabilis fitness regardless of the presence of other cocolonizing organisms. One such "core" fitness factor was D-serine utilization. In this study, we generated isogenic mutants in D-serine dehydratase (dsdA), D-serine permease (dsdX), and the divergently transcribed activator of the operon (dsdC) to characterize D-serine utilization in P. mirabilis and explore the contribution of this pathway to fitness during single-species and polymicrobial infection. P. mirabilis was capable of utilizing either D- or L-serine as a sole carbon or nitrogen source, and dsdA, dsdX, and dsdC were each specifically required for D-serine degradation. This capability was highly conserved among P. mirabilis isolates, although not universal among uropathogens: Escherichia coli and Morganella morganii utilized D-serine, while Providencia stuartii and Enterococcus faecalis did not. D-Serine utilization did not contribute to P. mirabilis growth in urine ex vivo during a 6-h time course but significantly contributed to fitness during single-species and polymicrobial CAUTI during a 96-h time course, regardless of D-serine utilization by the coinfecting isolate. D-Serine utilization also contributed to secondary bacteremia during CAUTI as well as survival in a direct bacteremia model. Thus, we propose D-serine utilization as a core fitness factor in P. mirabilis and a possible target for disruption of infection.