Staphylococcus aureus Virulence Affected by an Alternative Nisin A Resistance Mechanism

Abstract

Nisin A is a bacteriocin produced by <i>Lactococcus lactis</i> and is widely used as a food preservative. <i>Staphylococcus aureus</i> has the BraRS-VraDE system that provides resistance against low concentrations of nisin A. BraRS is a two-component system that induces the expression of the ABC transporter VraDE. Previously, we isolated a highly nisin A-resistant strain with increased VraDE expression due to a mutation in <i>braRS</i> In this study, we isolated <i>S. aureus</i> MW2 mutants with BraRS-VraDE-independent nisin A resistance. These mutants, designated SAN2 ( <i><u>S.</u><u>a</u>ureus</i><u>n</u>isin resistant) and SAN469, had a mutation in <i>pmtR</i>, which encodes a transcriptional regulator responsible for the expression of the <i>pmtABCD</i> operon. As a result, these mutants exhibited increased expression of PmtABCD, a transporter responsible for the export of phenol-soluble modulin (PSM). Characterization of the mutants revealed that they have decreased susceptibility to human β-defensin-3 (hBD3) and LL37, which are innate immune factors. Additionally, these mutants showed higher hemolytic activity than the original MW2 strain. Furthermore, in a mouse bacteremia model, the SAN2 strain exhibited a lower survival rate than the original MW2 strain. These results indicate that the increased expression of <i>pmtABCD</i> due to a <i>pmtR</i> mutation is an alternative nisin A resistance mechanism that also affects virulence in <i>S. aureus</i><b>IMPORTANCE</b> Recently, the emergence of antibiotic-resistant bacteria has resulted in serious problems for chemotherapy. In addition, many antibacterial agents, such as disinfectants and food additives, are widely used. Therefore, there is a possibility that bacteria are becoming resistant to some antibacterial agents. In this study, we investigated whether <i>Staphylococcus aureus</i> can become resistant to nisin A, one of the bacteriocins applied as a food additive. We isolated a highly nisin A-resistant strain designated SAN2 that displayed increased expression of Pmt proteins, which are involved in the secretion of virulence factors called phenol-soluble modulins (PSMs). This strain also showed decreased susceptibility to human antimicrobial peptides and increased hemolytic activity. In addition, SAN2 showed increased lethal activity in a mouse bacteremia model. Our study provides new insights into the possibility that the acquisition of resistance against food preservatives may modulate virulence in <i>S. aureus</i>, suggesting that we need to pay more attention to the use of food preservatives together with antibiotics.