Study on the antibacterial activity of linezolid combined with other antibiotics against MRSA in vitro

Abstract

Objective To investigate the antibacterial activity of linezolid combined with three kinds of commonly used antibiotics ( rifampicin,levofloxacin,gentamicin) against MRSA in vitro,and provide basis for treating MRSA infection in clinic.Methods Bacteria were isolated by conventional methods to obtain pure cultured microorganism with the VITEK automatic analyzer to identify Staphylococcus aureus,and then according to CLSI (2006) standard,the broth method with the determination of oxacillin minimal inhibitory concentr ation (MIC) values,MIC value determined result ≥ 4 μg/ml for the MRSA strain.Broth dilution method was used to test the susceptibility.Determination of four kinds of antimicrobial drugs first tested the MIC values alone.The combined sensitivity test method designed by the board to determine linezolid in conjunction with the other three kinds of antibiotics used for MIC.Inhibition index calculated part of the FIC (fractional inhibitory concentration index).FIC index =MIC Adrug combined with / MIC A drug alone + MIC B drugs combination / MIC B drug alone.Interpretation of standards:FIC index ≤0.5,synergy; ＞0.5 - 1.0,Additive effect; ＞ 1.0 -2.0,no relevant effect; ＞2.0,antagonism.Results Linezolid MIC against 30 strains of MRSA in the range of 0.5 -4.0 μg/ml,MIC90 was 4 μg/ml,levofloxacin against 30 strains of MRSA in the MIC range of 16 -256 μg/ml,MIC90for the 256 μg/ml,lrifampicin level of the 30 MRSA the MIC range of 8 -256 μg/ml,MIC90 for the 256 μg/ml,and gentamicin against 30 strains of MRSA in the MIC range of 128 - 1024 μg/ml,MIC90was 1024 μg/ml.Linezolid combined with levofloxacin MIC90 reduced to 2 μg/ml,linezolid and rifampicin combined with MIC90 reduced to 1 μg/ml,linezolid combined with gentamicin MIC90reduced to 2 μg/ml.Conclusions Linezolid has high sensitivity of the MRSA,linezolid and rifampicin in combination with synergistic based,combination of linezolid and levofloxacin,gentamicin has synergistic and additive effects,both of no antagonism. Key words: Methicillin-resistant staphylococus aureus; Minimal inhibitory concentration; Combined treatment