Undecaprenyl pyrophosphate phosphatase confers low-level resistance to bacitracin in Enterococcus faecalis

Undecaprenyl pyrophosphate phosphatases (UppPs) have been implicated in bacitracin resistance in some bacterial genera and the aim of this study was to determine the role of UppPs in mediating low-level bacitracin resistance in Enterococcus faecalis. The uppP gene was identified in the genomes of laboratory (JH2-2) and clinical (V583) strains of E. faecalis. Gene fusions (uppP-lacZ) and 5-RACE were used to study uppP expression. The uppP gene in both strains was inactivated and mutants were studied for antimicrobial susceptibility and their susceptibilities to various stress agents. The UppP protein from E. faecalis showed high sequence identity to the Escherichia coli BacA-type UppP and was predicted to be a hydrophobic protein with eight transmembrane helices. The expression of uppP-lacZ was constitutive and not affected by bacitracin or cell wall-active antimicrobials. E. faecalis uppP mutants showed no significant changes in growth rate, colony morphology and biofilm formation. The uppP mutants exhibited increased susceptibility to bacitracin (MICs36 mg/L) relative to the isogenic wild-type (MICs3248 mg/L). When uppP was expressed in a wild-type background, the MIC of bacitracin increased to 128256 mg/L. The MICs of cefoxitin, teicoplanin, vancomycin, gentamicin, enrofloxacin and d-cycloserine were unaltered in the uppP mutant relative to the wild-type, as were susceptibilities to other stress agents (glycine, lysozyme, NaCl, SDS, low and high pH, oxidative stress and ethanol). The results demonstrate that low-level bacitracin resistance in E. faecalis is mediated by a BacA-type UppP.