Therapies which target quorum sensing (QS) systems that regulate virulence in methicillin-resistant Staphylococcus aureus (MRSA) are a promising alternative to antibiotics. QS systems play a crucial in the regulation of MRSA antibiotic resistance, exotoxin production, antioxidant protection and immune cell evasion, and are therefore attractive therapeutic targets to reduce the virulence of a pathogen. In the present work the the effects of bioactive peptides isolated from two strains of lactic acid bacteria were tested against antibiotic resistance, carotenoid production, resistance to oxidative killing and biofilm structure in two clinical MRSA isolates. The results obtained from fractional-inhibitory concentration assays with bulk and semi-purified bioactive molecules showed a significant synergistic effect increasing cefoxitin mediated killing of MRSA. This was coupled to a six-fold decrease of the major membrane pigment staphyloxanthin, and a 99% increase in susceptibility to oxidative stress mediated killing. Real-time quantitative PCR analysis of the QS-genes agrA and luxS, showed differential expression between MRSA strains, and a significant downregulation of the hemolysin gene hla. Light microscopy and scanning electron microscopy revealed alteration in biofilm formation and clustering behavior. These results demonstrate that bioactive metabolites may be effectively applied in tandem with beta-lactam antibiotics to sensitize MRSA to cefoxitin. Moreover, these results shown that several key QS-controlled virulence mechanisms are diminished by probiotic metabolites.

Antimicrobial resistance is a growing threat to food safety, medical advancement, and overall global health. Methicillin resistant Staphylococcus aureus (MRSA) is typically a commensal species that, given an opportunity to establish an infection, transforms into a formidable pathogen with high rates of mortality and morbidity. Therefore, it is globally recognized that new therapies to combat this pathogen are desperately needed. A potential strategy in combating MRSA resistance and infections is the development of alternative therapeutics that interfere with bacterial quorum sensing (QS) systems involved in cell-to-cell communication. QS systems are crucial in the regulation of many virulence traits in MRSA such as methicillin resistance, exotoxin and surface protein expression, antioxidant production and immune cell evasion. Based on our previous research, in which we have shown that probiotic bioactive metabolites act as novel QS-quenching compounds, we propose in this letter that the same probiotic compounds can be used in tandem with a beta-lactam antibiotic to “re-sensitize” MRSA clinical isolates to cefoxitin. Moreover, we show that these probiotic metabolites decrease production of carotenoids and alpha-hemolysin in active cultures of MRSA, resulting in reduced toxicity and diminished resistance to hydrogen peroxide cytotoxicity in vivo.