Phenotypic and genotypic characterisation of multiple antibiotic-resistant Staphylococcus aureus exposed to subinhibitory levels of oxacillin and levofloxacin

Background: The emergence and spread of multidrug resistant methicillin-resistant Staphylococcus aureus (MDR-MRSA) has serious health consequences in the presence of sub-MIC antibiotics. Therefore, this study was designed to evaluate beta-lactamase activity, efflux activity, biofilm formation, and gene expression pattern in Staphylococcus aureus KACC 10778, S. aureus ATCC 15564, and S. aureus CCARM 3080 exposed to sublethal concentrations of levofloxacin and oxacillin. Results: The decreased MICs were observed in S. aureus KACC and S. aureus ATCC when exposed to levofloxacin and oxacillin, while and S. aureus CCARM remained resistance to streptomycin (512 mu g/mL) in the presence of levofloxacin and imipenem (>512 mu g/mL) in the presence of oxacillin. The considerable increase in extracellular and membrane-bound beta-lactamase activities was observed in S. aureus ATCC exposed to oxacillin (>26 mu mol/min/mL). The antibiotic susceptibility of all strains exposed to EPIs (CCCP and PA beta N) varied depending on the classes of antibiotics. The relative expression levels of adhesion-related genes (clfA, clfB, fnbA, fnnB, and icaD), efflux-related genes (norB, norC, and qacA/B), and enterotoxin gene (sec) were increased more than 5-fold in S. aureus CCARM. The eno and qacA/B genes were highly overexpressed by more than 12- and 9-folds, respectively, in S. aureus CCARM exposed to levofloxacin. The antibiotic susceptibility, lactamase activity, biofilm-forming ability, efflux activity, and gene expression pattern varied with the intrinsic antibiotic resistance of S. aureus KACC, S. aureus ATCC, and S. aureus CCARM exposed to levofloxacin and oxacillin. Conclusions: This study would provide useful information for better understating of combination therapy related to antibiotic resistance mechanisms and open the door for designing effective antibiotic treatment protocols to prevent excessive use of antibiotics in clinical practice.