Researchon The Resistance Profile And The Correlation Between Resistance Phenotype And Expression Level Of FemA Of Methicillin-resistant Staphylococcus Aureus

Objective: To analyse the resistance profile of 135 strains of Staphylococcus isolated from our hospital to 8 antimicrobial agents and to investigate the isolation rate of β-lactamase of various resistance level MRSs(methicillin resistant Staphylococcus);To investigate the correlation between the phenotype and expression level of femA of Staphylococcus aureus,to discuss the mechanism of different phenotype in methicillin-resistant Staphylococcus aureus,and to confirm the role of femABX family in the resistance in Staphylococcus aureus to oxacillin.Methods: The minimal inhibitory concentrations(MICs)of 8 antimicrobial agents comprising oxacillin, vancomycin, norvancomycin, teicoplanin, clindamycin, amikacin, levofloxacin and rifampicin against 135 clinical isolates were determined by agar dilution method according to NCCLS.The production of β-lactamase was identified by nitrocephin paperstrip method.The isolation rate of /3-lactamase- producing strains were counted and the correlation between the resistance phenotype and isolation rate of /3-lactamase was analysed by statistics. Real time fluorescent quantitative PCR was used to quantify the mRNA expression of femA of strains that had been choosed as objects(non-j3-lactamase-producing strains) by agar dilution method and nitrocephin paper strip method. gyrB-a. constitutive expression gene is taken as the internal control gene.The correlation between the resistance phenotype and expression level of femA was analysed by statistics.Results: The resistance rate of 83 Staphylococcus aureus and 52 coagulase negative Staphylococcus to oxacillin were 68.67%(57/83) and 57.69%(30/62).The resistance rates to glycopeptide antibiotk^ amikacnu levo- floxacin> rifampicin and clindamycin were 0(0/135)v 1.48%(2/135)> 22.99%(31/135) ^ 49.63%(67/135) and 65.93%(89/135) respectively.The isolation rates of /3-lactamase-producing MRSAn MSSA> MRCNS and MSCNS strains were 43.86%, 76.92%> 43.33% and 63.64% respectively,and that of strains of high- and low-level resistance to oxacillin were 39.62% and 75%(3/4). The standard curve was performed by series dilutions of the heterogeneous resistant strain BB270, the amount of/emvl-specific mRNA in strain BB270 was set to be 100%, the mutant strain BB308 contained approxiamately 37.82%and that of stain BB586 was 240.50%,homogeneous resistant strain COL was 862.61%.The amounts in MSSA strains were from 0.00353% to 29.92%,that in low-level MRSA strains were from 0.00554% to 310%,ortherwise that in high-level MRSA strains were from 13.88% to 55000%.Conclusions: Staphylococcus showed multi-resistant to clinical antimicrobial agents and MSS strains were more sensitive to these drugs than MRS ones.There were marked differences in producting /3-lactamase between MSS and MRS strains but there were no differences between high-level MRS and low-level MRS strains.Expression level of femA in high-level MRSA (non-/3-lactamase- producing) was higher than that in low-level MRSA and MSSA. femA was essential for the expression of high-level methicillin resistance in MRSA,and there might be some mechanisms responsible for upregulated/ downregulated of the expression of femA in Staphylococcus.