[Genotyping and drug resistance of methicillin-resistant Staphylococcus aureus]

Abstract

Objective: To investigate the genotype of staphylococcal chromosomal cassette mec (SCCmec) in methicillin-resistant Staphylococcus aureus (MRSA) isolated from burn wards and its current status of drug resistance.

Methods: One hundred and seventy-nine strains of Staphylococcus aureus were isolated from wound excretion, blood, and sputum samples of patients that were admitted to ICU or public wards of our Department of Burns and Plastic Surgery from September 2012 to September 2013. Among them, 68 strains were from ICU and 111 strains from public wards. The MRSA phenotype of Staphylococcus aureus was detected with cefoxitin K-B disk diffusion method, and the isolation rates of MRSA in ICU and public wards were compared. Genotyping of SCCmec was performed by PCR in strains of MRSA. In the meantime, the identification result of MRSA by K-B method was verified through detecting methicillin-resistant determinant mecA. The antimicrobial resistance of MRSA and methicillin-sensitive Staphylococcus aureus (MSSA) to 23 kinds of commonly used antibiotics in clinic were detected by K-B disk diffusion method. Except for the antibiotics to which the resistant rates of MRSA were 100.0% or 0, the resistant rates of SCCmecIII MRSA and non-SCCmec III MRSA to the rest of antibiotics were compared. Data were processed with Pearson chi-square test or corrected chi-square test.

Results: One hundred and forty-eight strains out of the 179 Staphylococcus aureus were identified as MRSA (accounting for 82.7%), among which 62 were originated from ICU and 86 from public wards. The rest 31 strains of Staphylococcus aureus were MSSA, accounting for 17.3%. The percentage of MRSA in the isolated Staphylococcus aureus was 91.2% (62/68) in ICU, which was significantly higher than that in the public wards [77.5% (86/111), χ2 = 5.526, P = 0.019]. PCR detection showed that the 148 strains of MRSA harbored the mecA gene, out of which 106 strains were SCCmec III positive, accounting for 71.6%. The percentages of SCCmec III type MRSA in MRSA isolated from ICU and public wards were respectively 72.6% (45/62) and 70.9% (61/86), showing no statistically significant difference (χ2 = 0.048, P = 0.826). The 148 strains of MRSA were 100.0% resistant to a total of 8 kinds of antibiotics including penicillin and cephalosporins, but it was 0 for vancomycin, teicoplanin, linezolid, tigecycline, nitrofurantoin, and quinupristin/dalfopristin. Except for the 6 kinds of antibiotics to which the resistant rates of MRSA and MSSA were 0, resistant rates of MRSA to the remaining 17 kinds of antibiotics were significantly higher than those of MSSA (with χ2 values from 4.091 to 138.546, P < 0.05 or P < 0.01). Resistant rates of the 106 strains of SCCmecIII type MRSA to levofloxacin, ciprofloxacin, rifampicin, tetracycline, erythrocin, lincomycin, gentamicin, clindamycin were respectively 56.6% (60/106), 85.8% (91/106), 89.6% (95/106), 86.8% (92/106), 84.9% (90/106), 78.3% (83/106), 92.5% (98/106), 74.5% (79/106), and they were significantly higher than those of the 42 strains of non-SCCmec III type MRSA [33.3% (14/42), 61.9% (26/42), 71.4% (30/42), 66.7% (28/42), 69.0% (29/42), 57.1% (24/42), 71.4% (30/42), 52.4% (22/42), with χ2 values from 4.801 to 11.377, P < 0.05 or P < 0.01].

Conclusions: Isolation rate of MRSA from burn wards in our hospital is high, and drug resistance status of this strain against antibiotics is very serious. SCCmec III is the major genotype of the isolated MRSA, but no strains resistant to the glycopeptide antibiotics are found.