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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 β-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 μg/mL) in the presence of levofloxacin and imipenem (>512 μg/mL) in the presence of oxacillin. The considerable increase in extracellular and membrane-bound β-lactamase activities was observed in  S. aureus  ATCC exposed to oxacillin (>26 μmol/min/mL). The antibiotic susceptibility of all strains exposed to EPIs (CCCP and PAβN) 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.    Electronic supplementary material  The online version of this article (doi:10.1186/s12866-016-0791-7) contains supplementary material, which is available to authorized users.

bmc microbiology

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