The study shows that when the bacteria Staphylococcus aureus is repeatedly exposed to the human antimicrobial peptide LL‑37 (and similar peptides), it can become resistant not only to those peptides but also to many antibiotics, while still being able to cause disease. This resistance develops without making the bacteria weaker, meaning the resistant strains can stick around.

The clinical development of antimicrobial peptides (AMPs) is currently under evaluation to combat the rapid increase in MDR bacterial pathogens. However, many AMPs closely resemble components of the human innate immune system and the ramifications of prolonged bacterial exposure to AMPs are not fully understood. We show that in vitro serial passage of a clinical USA300 MRSA strain in a host-mimicking environment containing host-derived AMPs results in the selection of stable AMP resistance. Serial passage experiments were conducted using steadily increasing concentrations of LL-37, PR-39 or wheat germ histones. WGS and proteomic analysis by MS were used to identify the molecular mechanism associated with increased tolerance of AMPs. AMP-resistant mutants were characterized by measuring in vitro fitness, AMP and antibiotic susceptibility, and virulence in a mouse model of sepsis. AMP-resistant Staphylococcus aureus mutants often displayed little to no fitness cost and caused invasive disease in mice. Further, this phenotype coincided with diminished susceptibility to both clinically prescribed antibiotics and human defence peptides. These findings suggest that therapeutic use of AMPs could select for virulent mutants with cross-resistance to human innate immunity as well as antibiotic therapy. Thus, therapeutic use of AMPs and the implications of cross-resistance need to be carefully monitored and evaluated.