Novel anti-virulence compounds disrupt exotoxin expression in MRSA

Hemolysins are lytic exotoxins expressed in most strains of S. aureus, but hemolytic activity varies between strains. We have previously reported several novel anti-virulence compounds that disrupt the S. aureus transcriptome, including hemolysin gene expression. This report delves further into our two lead compounds, loratadine and a structurally related brominated carbazole, and their effects on hemolysin production in methicillin-resistant S. aureus (MRSA). To gain understanding into how these compounds affect hemolysis, we analyzed these exotoxins at the DNA, RNA, and protein level after in vitro treatment. While lysis of red blood cells varied between strains, DNA sequence variation did not account for it. We hypothesized that our compounds would modulate gene expression of multiple hemolysins in two hospital-acquired strains of MRSA, both with staphylococcal cassette chromosome mec (SCCmec) type II. RNA-seq analysis of differential gene expression in untreated and compound-treated cultures revealed hundreds of differentially expressed genes, with a significant enrichment in genes involved in hemolysis. The brominated carbazole and loratadine both displayed the ability to reduce hemolysis in strain 43300 but displayed differential activity in strain USA100. These results corroborate gene expression studies as well as western blots of alpha hemolysin. Together, this work suggests that small molecules may alter exotoxin production in MRSA but that the directionality and/or magnitude of the difference are likely strain dependent.IMPORTANCEMethicillin-resistant S. aureus (MRSA) is a deadly human pathogen. In addition to evading antibiotics, these bacteria produce a wide range of toxins that negatively affect the host. Our work aims to identify and characterize novel compounds that can decrease the pathogenic effects of MRSA. Two lead compounds investigated in this study triggered changes in the production of multiple toxins. These changes were specific to the strain of MRSA investigated. Specifically, this work sheds light on novel compounds that decrease MRSA's ability to lyse host red blood cells. Importantly, it also highlights the importance of examining strain-specific differences in response to therapeutic interventions that could ultimately affect clinical outcomes.