Salmonella enterica serovar Typhimurium waaP mutants show increased susceptibility to polymyxin and loss of virulence in vivo

In Escherichia coli, the waaP (rfaP) gene product was recently shown to be responsible for phosphorylation of the first heptose residue of the lipopol ysaccharide (LPS) inner core region. WaaP was also shown to be necessary fo r the formation of a stable outer membrane. These earlier studies were perf ormed with an avirulent rough strain of E. coli (to facilitate the structur al chemistry required to properly define waaP function); therefore, we unde rtook the creation of a waaP mutant of Salmonella enterica serovar Typhimur ium to assess the contribution of WaaP and LPS core phosphorylation to the biology of an intracellular pathogen. The S. enterica waaP mutant described here is the first to be both genetically and structurally characterized, a nd its creation refutes an earlier claim that waaP mutations in S. enterica must be leaky to maintain viability. The mutant was shown to exhibit chara cteristics of the deep-rough phenotype, despite its ability to produce a fu ll-length core capped with O antigen. Further, phosphoryl modifications in the LPS core region were shown to be required for resistance to polycationi c antimicrobials. The waaP mutant was significantly more sensitive to polym yxin in both wild-type and polymyxin-resistant backgrounds, despite the dec reased negative charge of the mutant LPSs. In addition, the waaP mutation; was shown to cause a complete loss of virulence in mouse infection models. Taken together, these data indicate that WaaP Is a potential target for the development of novel therapeutic agents.