Study On Antimicrobial Susceptibility And The Resistance Mechanism Of Staphylococcus Aureus

Objective To study the characteristics of the antimicrobial resistance and to investigate the conditions of antibiotic resistance genes in staphylococcus aureus; to investigate the distribution and antimicrobial resistance of different methicillin-resistant staphylococcus aureus (MRSA) SCCmec genotype in our area ; to study the resistance mechanism of MRSA according to the result of SCCmec typing.Methods The antimicrobial sensitivity tests of staphylococcus aureus were tested by Vitek automatic microbial monitor GPS109; the methicillin resistance gene mecA, the aminoglycoside resistance gene aac(6')/ aph(2'')and aph(3')-III, the macrolides resistance gene ermA and ermC, and the tetracycline resistance gene tetM and tetK of 175 clinical staphylococcus aureus were detected by polymerase chain reaction(PCR). The characteristic genes of SCCmec typing were tested by PCR methods and the clinical isolates of MRSA were typed by the results of characteristic genes.Results Among the 35 isolates of community-associated staphylococcus aureus, MRSA was not be found and among the 175 isolates of hospital-associated staphylococcus aureus , 84 isolates were MRSA. The resistance rates to cefazolin, gentamicin and levofloxacin of hospital-associated methicillin-sensitive staphylococcus aureus(HA-MSSA) were higher than that of community-associated methicillin-sensitive staphylococcus aureus (CA-MSSA), P﹤0.05. The resistance rates to cefazolin, erythromycin, clindamicin, tetracycline, gentamicin, levofloxacin and rifampin of hospital-associated methicillin-resistant staphylococcus aureus (HA-MRSA) were higher than that of CA-MSSA and HA-MSSA (P﹤0.05). The detection rates of ermA, ermC, tetM, tetK, aac(6')/ aph(2''), aph(3')-III in HA-MRSA were 75.0%, 8.3%, 50%, 54.8%, 63.1%, 3.6%; in HA-MSSA were 14.3%, 46.2%, 16.5%, 4.4%, 3.3%, 0%; in CA-MSSA were 11.4%, 40.0%, 11.4%, 2.8%, 0%, 0%. Among the 84 isolates of MRSA, 9(10.7%) isolates were belongde to SCCmecII, 70 (83.3%) isolates were belonged to SCCmecIII, 5(6.0%) isolates were belongde to SCCmecIV in this test and no isolates were belonged to SCCmecⅠgenotype. Drug resistance analysis showed the resistance rates to rifampin of all strain was lower, to vancomycin and linezolid were completely sensitive, there were not significant difference among the SCCmec genotype(P >0.05). The resistance rates to other non-β-lactamase antibiotics of the strains was as follows in order SCCmecⅢ>SCCmecⅡ>SCCmecⅣ. The strains of SCCmecII and SCCmecIII were multi-resistant and their resistance rates were manifest higher than SCCmecIV(P <0.05).Conclusions CA-MSSA and HA-MSSA were more sensitive to all antibiotics than HA-MRSA. All of the antibiotic resistance genes can be detected in staphylococcus aureus and the positive rates were difference in MRSA and MSSA. Resistance to aminoglycoside, macrolide and tetracycline was closely associated with methicillin resistance in Staphylococci aureus . SCCmecIII genotype was the major epidemic isolates in our area and showed multiple resisitance to antibiotics. SCCmec was closely associated with methicillin resistance in Staphylococci aureus .