Study On The Antibiotic Resistant Mode And SCCmec Genotype Of Methicillin-resistant Staphylococci Isolates

Objectives:Staphylococci played an important role in nosocomial infections.Recently,the infections of staphylococci isolates have become the focus of clinician around the world,for their increasing isolating rates and antibiotic resistance rates.In this study, the isolate rates and resistant rates of staphylococci isolated from children were investigated,and the spectrum of resistance was made to provide basic rule for proper drug administration in clinical practice.Moreover,the mechanism of methicillin resistance in staphylococci was studied through the detection of mecA gene and genotyping of SCCmec.To predict and prevent the development of MRS and provide information for anti-infective therapy,molecular epidemiology of MRS recovered from patients with nosocomial infection was also studied.Methods:(1)138 staphylococcal strains isolated from patients with nosocomial infection in Tianjin Children's Hospital were identified and the clinical data were collected.(2)The antibiotic susceptibility pattern of the staphylococcal isolates to 16 clinical routing used antibiotics was studied,and then the strains were screened with vancomycin-containing plates and verified by E-test.(3)The Nitrocefin plates were used for detectingβ-lactamase activity in two types of strains,the original isolates and the isolated induced by oxacillin-containing plates at the concentration of 0.05μg/ml.(4)The status of gene mecA carrying was investigated with PCR amplification to identify MRS strains.(5)The genotyping of SCCmec with multiplex PCR was progressed on the mecA gene positive strains,to explain the molecular epidemiological characteristic.Results:(1)The total of 138 staphylococcal isolates included 65 Staphylococcus aureus strains (SA)and 73 coagulase-negative staphylococci strains(CNS),the oxacillin-resistant rates were 10.8%and 80,8%,respectively.SA strains were mainly isolated from sputum of respiratory tract,and CNS strains were mostly from blood cultures.(2)The CNS strains suggested higher resistance to oxacillin,erythromycin, cidomycin,benzyl-sulbactam,cefradine,ciprofloxacin,cefepime and rectocillin than SA strains(P＜0.05);The resistance of MRCNS strains to cefradine,cefatrizine, rectocillin,benzyl-sulbactam,erythromycin were higher than the MSCNS strains(P＜0.05).The MRCNS strains were characterized with multi-drug resistance.All isolates were sensitive to vancomycin.(3)Theβ-lactamase isolating rates of SA and CNS strains changed after induced by oxacillin-containing plates.Theβ-lactamase positive rate of SA increased from 50.8% to 89.2%and CNS from 61.6%to 90.4%.It indicates we could reduce the false negative rate by oxacillin induced in clinical detection of theβ-lactamase.(4)The mecA carrying rate was 38%.5 of 7 MRSA strains were carrying mecA.In 59 MRCNS isolates,47 isolates were mecA positive,while other 13 isolates were negative(false rate is 22%).No mecA positive strains were detected in phenotypic MSSA,and only one was mecA positive in phenotypic MSCNS.(5)In 5 mecA positive SA isolates,three were carrying SCCmec typesⅠ,ⅢandⅣ, respectively.47 mecA positive CNS strains including 6 for typeⅠ,10 for typeⅢ,9 for typeⅣ,3 for typeⅤand novel variants of SCCmec 4 strains for typeⅠ-like,5 strains for typeⅠA-like,the rest 10 CNS strains and 2 SA strains were not-typeable.Conclusions:The antibiotic resistance of CNS is much more severe than that of SA,Which needs more attention from clinician.All the strains are sensitive to vancomycin. When detecting theβ-lactamase,the strains induced by oxacillin could decrease the false negation.More than half of the SCCmec in CNS could be typed by the current multiplex PCR assays,but there are two novel variants of SCCmec in the staphylococci isolates,tentatively designated typeⅠ-like and typeⅠA-like.