Increased Antibiotic Resistance of Methicillin-Resistant Staphylococcus aureus USA300 Delta psm Mutants and a Complementation Study of Delta psm Mutants Using Synthetic Phenol-Soluble Modulins

Abstract

Phenol-soluble modulins (PSMs) are responsible for regulating biofilm formation, persister cell formation, pmtR expression, host cell lysis, and anti-bacterial effects. To determine the effect of psm deletion on methicillin-resistant Staphylococcus aureus, we investigated psm deletion mutants including Delta psm alpha, Delta psm beta, and Delta psm alpha beta. These mutants exhibited increased beta-lactam antibiotic resistance to ampicillin and oxacillin that was shown to be caused by increased N-acetylmannosamine kinase (nanK) mRNA expression, which regulates persister cell formation, leading to changes in the pattern of phospholipid fatty acids resulting in increased anteiso-C-15:0, and increased membrane hydrophobicity with the deletion of PSMs. When synthetic PSMs were applied to Delta psm alpha and Delta psm beta mutants, treatment of Delta psm alpha with PSM alpha 1-4 and Delta psm beta with PSM beta 1-2 restored the sensitivity to oxacillin and slightly reduced the biofilm formation. Addition of a single fragment showed that alpha 1, alpha 2, alpha 3, and beta 2 had an inhibiting effect on biofilms in Delta psm alpha; however, beta 1 showed an enhancing effect on biofilms in Delta psm beta. This study demonstrates a possible reason for the increased antibiotic resistance in psm mutants and the effect of PSMs on biofilm formation.