PBP4 is required for serum-induced cell wall thickening and antibiotic tolerance in .

The bacterial pathogen responds to the host environment by synthesizing a thick peptidoglycan cell wall, which protects the bacterium from membrane-targeting antimicrobials and the immune response. However, the proteins required for this response were previously unknown. Here, we demonstrate by three independent approaches that the penicillin-binding protein PBP4 is crucial for serum-induced cell wall thickening. First, mutants lacking various non-essential cell wall synthesis enzymes were tested, revealing that a mutant lacking was unable to generate a thick cell wall in serum. This resulted in reduced serum-induced tolerance of the mutant toward the last resort antibiotic daptomycin relative to wild-type cells. Second, we found that serum-induced cell wall thickening occurred in each of a panel of 134 clinical bacteremia isolates, except for one strain with a naturally occurring mutation that results in an S140R substitution in the active site of PBP4. Finally, inhibition of PBP4 with cefoxitin prevented serum-induced cell wall thickening and the resulting antibiotic tolerance in the USA300 strain and clinical MRSA isolates. Together, this provides a rationale for combining daptomycin with cefoxitin, a PBP4 inhibitor, to potentially improve treatment outcomes for patients with invasive MRSA infections.