[The action of active efflux system on multi-drug resistance in methicillin resistant Staphylococcus aureus].

OBJECTIVE: To study the active efflux gene qacB, qacJ and smr in methicillin resistant Staphylococcus aureus (MRSA) and to investigate their effect on the multi-drug resistance (MDR) of MRSA. METHODS: The three pairs of ideal primers of active efflux gene qacB, qacJ and smr were designed by computer with Primer Premier 5.0 software. A total of 124 clinical isolates of MRSA were amplified respectively by polymerase chain reaction (PCR) with above mentioned primers. The PCR products were separated by electrophoresis on an 1.5% agarose gel with 0.5 microg/ml ethidium bromide. Reserpine inhibition test was used to observe the changes of the susceptibility to antibiotics of MRSA which had qacB, qacJ and smr genes separately. RESULTS: Of the 124 strains of MRSA, 86 strains had qacB, 45 strains had qacJ and 32 strains had smr gene. Reserpine inhibition test showed that the minimal inhibitory concentration (MIC) decreased 2 to 32 times for MRSA to levofloxacin and rifampin. CONCLUSION: MRSA have qacB, qacJ and smr active efflux systems, which play a very important role in multiple antibiotic resistance.

[Study on the molecular mechanism of transferable multiple-antibiotic resistance in extended-spectrum beta-lactamase-producing clinical isolates].

OBJECTIVE: To investigate the molecular mechanism of transferable multiple-antibiotic resistance in extended-spectrum beta-lactamases (ESBLs) producing isolates. METHODS: Antibiotics susceptibility was tested by E-test method, and multi-resistance plasmids were screened and isolated by extracting transformant plasmids. Inserted gene Cassettes of class 1 integron were amplified and analyzed by polymerase chain reaction (PCR) and DNA sequencing. RESULTS: Eight of the nine ESBL-producing plasmids were found to comprise class 1 integron sequence, of them 7 harbored 1 or 2 antibiotic resistant gene cassettes which encoding resistance to aminoglycosides (aacA4, aadA2 or aadA5), trimethoprim (dhfrA12 or dfrA17), rifampicin (arr-3) and chloramphenicol (cmlA6). The function of these gene cassettes corresponded to the resistance profiles of their electro-transformants. CONCLUSION: Multi-resistance gene cassettes located on plasmids and mediated by class 1 integron may play an important role in causing the development and dissemination of multiple-antibiotic resistance in ESBL-producing clinical isolates.

[Study of susceptibility and genotype characterization of plasmid-mediated class I cephalosporinase in gram-negative bacteria].

OBJECTIVE: To investigate the susceptibility and genotype characteristics of gram-negative bacteria producing plasmid-mediated class I cephalosporinase (AmpC beta lactamase) epidemic in Southern China. METHODS: A total of 1,187 clinical isolates of nonrepetive gram-negative bacteria were collected from different cities in Southern China. AmpC beta lactamase producing isolates were identified by cefoxitin three-dimensional test, and antimicrobial susceptibility test was identified by Kirby-Bauer agar diffusion test; plasmid conjugation, plasmid extraction, universal PCR for gene amplication of corresponding group was done, and the PCR products were sequenced subsequently. RESULTS: The positive rate of cefoxitin three-dimensional test in gram-negative bacteria was 5.9% (70/1,187), and the prevalence of plasmid-mediated AmpC beta lactamase was: E. coli: 4.2% (19/451), Klebsiella: 4.7% (16/339), Enterobacter: 2.1% (4/190), Alcaligenes: 5.3% (1/19), Acinetobacter: 2.2% (1/45) and the total positive rate was: 3.5% (41/1,187). The susceptibility test showed that compared with the clinical isolates, the transconjugations remained resistance to cephamycins and ampicillin, and susceptible to cefepime and imipenem. PCR amplication and sequencing confirmed them to be bla(DHA-1) gene and bla(ACT-1) gene, and they were mainly distributed in Klebsiella and Escherichia. CONCLUSIONS: DHA-1 and ACT-1 were the most common genotypes in plasmid-mediated AmpC beta-lactamase produced by clinical isolates in Southern China. Fourth-generation cephalosporins and carbapenems could be better choices for the treatment of infection caused by AmpC betalactamase producers.