Characterization of a clinical isolate of staphylococcus aureus, and the action of linezolid on growth properties and toxin production

Staphylococcus aureus emergence as a significant pathogen has been enhanced by its increased resistance to many antibiotics due to its ability to express several virulence factors, and extracellular toxins, as seen in Methicillinresistant S. aureus (MRSA) strains. This study, investigated an S. aureus clinical isolate obtained from Glasgow Royal infirmary MRSA reference laboratory, using phenotypic and molecular methods in assessing its susceptibility to linezolid, ability to produce biofilms, known toxins, and the impact of treatment with sublethal linezolid concentration on toxin(s) expression. Result revealed Minimum Inhibitory Concentration (MIC) of linezolid on S. aureus planktonic cells as 4mg/L, Minimum Bactericidal Concentration (MBC) as 32mg/L, and MBC/MIC ratio of 8. Antibiotic time kill assay revealed linezolid effect on the planktonic cells of the S. aureus isolate as bacteriostatic; as viable count reduction was approximately 2log10. Biomass measurement of the S. aureus isolate by comparison with RP62a; a known biofilm-producing strain, indicated that it formed strong biofilm. In biofilm, linezolid concentration at 10×MIC had no significant effect (p>0.01) on its viability. Whereas at 40×MIC, linezolid effect was significantly greater (p<0.01), but unable to eliminate its viability. The S. aureus clinical isolate was shown to produce Staphylococcal Enterotoxin A and toxic shock toxins, and when challenged with sublethal linezolid concentration (0.25×MIC), its expression of both toxins proteins was downregulated by 2 folds. This study suggests linezolid ability to limit expression of vital S. aureus virulence factors and reinforces linezolid for consideration, in the treatment of severe S. aureus infections.