Disruption of waaY results in reduced heptose phosphorylation of the LPS core, thereby diminishing the interaction between AMPs and the bacterial membrane. Although these individual mutations each conferred only minor enhancements in AMP resistance, combining all three mutations in a single mutant further increased AMP resistance. pmrB and waaY mutations appeared together in two of four isolates following serial passage with LL-37 or CNY100HL indicating a possible epistatic effect between these mutations both of which lead to a reduction in AMP interaction with LPS.