Aims: Determine the antimicrobial effects of 5 μmol ml-1 sodium chlorate, 9 μmol ml-1 nitroethane or 2-nitropropanol as well as lauric acid, myristic acid and the glycerol ester of lauric acid Lauricidin® , each at 5 mg ml-1 , against representative methicillin-resistant staphylococci, important mastitis- and opportunistic dermal-pathogens of humans and livestock.
Methods and results: Three methicillin-resistant Staphylococcus aureus and two methicillin-resistant coagulase-negative staphylococci were cultured at 39°C in 5 μmol ml-1 nitrate-supplemented half-strength Brain Heart Infusion broth treated without or with the potential inhibitors. Results revealed that 2-nitropropanol was the most potent and persistent of all compounds tested, achieving 58-99% decreases in mean specific growth rates and maximum optical densities when compared with untreated controls. Growth inhibition did not persist by cultures treated solely with chlorate or nitroethane, with adaptation occurring by different mechanisms after 7 h. Adaptation did not occur in cultures co-treated with nitroethane and chlorate. The medium chain fatty acid compounds had modest effects on all the staphylococci tested except the coagulase-negative Staphylococcus epidermidis strain NKR1.
Conclusions: The antimicrobial activity of nitrocompounds, chlorate and medium chain fatty acid compounds against different methicillin-resistant staphylococci varied in potency.
Significance and impact of the study: Results suggest that differential antimicrobial activities exhibited by mechanistically dissimilar inhibitors against methicillin-resistant staphylococci may yield potential opportunities to combine the treatments to overcome their individual limitations and broaden their activity against other mastitis and dermal pathogens.
Keywords: Staphylococci; antimicrobials; food safety; resistance; veterinary.
© 2019 The Society for Applied Microbiology.