Ynt is the Primary Nickel Import System Used by Proteus Mirabilis and Specifically Contributes to Fitness by Supplying Nickel for Urease Activity

Proteus mirabilis is a Gram-negative uropathogen and frequent cause of catheter-associated urinary tract infection (CAUTI). One important virulence factor is its urease enzyme, which requires nickel to be catalytically active. It is, therefore, hypothesized that nickel import is critical for P. mirabilis urease activity and pathogenesis during infection. P. mirabilis strain HI4320 encodes two putative nickel import systems, designated Nik and Ynt. By disrupting the substrate-binding proteins from each import system (nikA and yntA), we show that Ynt is the primary nickel importer, while Nik only compensates for loss of Ynt at high nickel concentrations. We further demonstrate that these are the only binding proteins capable of importing nickel for incorporation into the urease enzyme. Loss of either nickel-binding protein results in a significant fitness defect in a murine model of CAUTI, but YntA is more crucial as the yntA mutant was significantly outcompeted by the nikA mutant. Furthermore, despite the importance of nickel transport for hydrogenase activity, the sole contribution of yntA and nikA to virulence is due to their role in urease activity, as neither mutant exhibited a fitness defect when disrupted in a urease-negative background.