A Study On The Active Efflux System In Methicillin-resistant Staphylococcus Aureus

Staphylococcus aureus (SA) is a common clinical pathogenic bacteria, since eighty years twentieth century, when penicillin was advented, infection of SA had been more controled, but with extensive use of penicillin, some strains produced penicillinase, then they expressed as penicillin-resistance. So mankind developed dimethoxy-phenyl penicillin, that is meticillin sodium, to resist penicillinase. Since 1959, after the clinical application of meticillin sodium, infection of SA produced penicillinase had been effectively controled. However, in 1961 in the United Kingdom, the first strain of methicillin-resistant Staphylococcus aureus was found, after that, infection of MRSA was almost all over the world till eighties, up to 58% of the total number of SA. MRSA is an important pathogen in ICU, it has strong pathogenicity, can lead to serious complications, such as sepsis and shock. It was reported that the mortality rate of infection was as high as 63.1% [2]. MRSA is multi-drug resistant and spread fast, can cause outbreak of epidemic. in recent years, vancomycin-resistant Staphylococcus aureus (VRSA) [3] emerged, so it is more and more difficult to treat infection of MRSA. Therefore, to study the drug resistant mechanism of MRSA is great significance. The resistance mechanism of MRSA to meticillin sodium has been clear, mainly because MRSA can produce a new type of penicillin-binding protein (PBP2a), encoded and synthesized by specific mecA gene,and its affinity withβ-lactam antibiotics is very low, when other penicillin-binding proteins are inhibited to affiliate withβ-lactam antibiotics, PBP2a can instead of cell wall synthesis, resulting in resistance. But it can not explain why the different resistance strains of MRSA have different levels of resistance? And the amount of PBP2a produced by MRSA has nothing to do with the level of resistance, let alone explain the complicate multi-drug resistant (resistance to many types of antimicrobial drugs besides) mechanism of MRSAIn recent years, drug resistance mechanism caused by active efflux attracted the attention of scholars, with the gradual deepening of the study, it is found that an increasing number of clinical common pathogenic bacteria with antimicrobial resistance related to active efflux system. And active efflux system of SA can be inhibited by reserpine and CCCP [4]. A study showed that the active efflux system in multi-drug resistance strains can significantly reduce intracellular concentration of antibiotics, and for active efflux system transport substrate extensively, bacteria expresses as higher resistance to completely different antibiotics. Studies suggest that resistance of MRSA may be related to gaining active efflux genes, and exist many kinds of active efflux systems. Markham and Neyfakh [5] as early as 1996, speculated that efflux system in the mechanism of multi-drug resistance MRSA played an important role. In 2004, Xu Fei et al [6] documented active efflux mechanism is one of the important mechanisms for MRSA resistant to quinolones. There are nora, qac, smr and other efflux systems. In qac family, there is a high rate of resistance to a lot of antibiotics, in addition to disinfectants. Quinolones was ever effective in treating MRSA infection, but the quinolone-resistance strains increased rapidly, others, ethidium bromide and quinolones are substrates for many active efflux pumps, but CCCP antagonists and proton pump may hamper this resistance. In this paper, a study will be carried on the status of MRSA infection in an ICU, resistance distribution and active efflux mechanism for quinolone-resistanceObjectivesTo explore the status of Methicillin-resistant Staphylococcus aureus (MRSA) infectious in an intensive care unit (ICU) , and investigate the active efflux mechanism of MRSA.MethodsFrom 2005 to 2008, 102 strains of Staphylococcus aureus (SA) were collected from ICU, by two detection methods: oxacillin-agar dilution and mecA gene detection, eighty isolates were identified as MRSA among 102 strains of SA ,and then the minimal inhibitory concentration (MIC) were determined by agar dilution,waiting for observing the result. The 4 primers of active efflux gene were designed by computer Primer Premier 5.0 software , to amplify the 80 clinical isolates respectively by polymerase chain reaction (PCR), the PCR products were analyzed by agarose gel electrophoresis in order to grasp the situation of the existence of the four genes (norA,qacA,qacB,qacJ) by gel image device, then to conserved and analysed the result. Omeprazole according to a certain concentration were mixed to levofloxacin, to determined the MIC of MRSA with active efflux gene positive, to observe the changes of the susceptibility of MRSA to antibiltics.Results1 In the 102 strains of SA collceted from ICU, 96 strains were from inpatients, and 16 strains were carried by ICU staff's hands. 78 strains were considered as MRSA by oxacillin- agar dilution method, the detection rate was 76.5%, while 80 strains were deemed as MRSA by mecA gene detection method, the detection rate was 78.4%. The age constituent ratio showed that more than 60 years old inpatients with several disease has higher risk of MRSA infection.2 Sensitivity rate of MRSA to both ancomycin and teicoplanin was 100.0%, while to other antibiotics (oxacillin, benzypenicillin, cefuroxime, cefotaxime, ceftazidime, ceftiaxone, cefepime, cefoxitin, imipenem, piperacillin/tazobactam, azithromycin, erythromycin, chloramphenicol, clindamycin, amikacin, gentamicin, levofloxacin, gatifloxacin, ciprofloxacin) MRSA was all multi-drug resistant.3 The detection rates of norA, qacA, qacB and qacJ were 67.5%(54/80), 15.0%(12/80), 21.2%(17/80) and 11.2%(9/80), respectively.4 When the concentration of omeprazole was 20mg/L, MIC of MRSA with active efflux gene positive begain to change. In 31 empirical strains, 4 strians MIC changed clearly, the others MIC decreased one concentration gradient. 11 strains without active efflux gene, MIC was almost no change.Conclusion1 The infection of MRSA in ICU is serious, it is very important to strengthen the hospital staff's hands health for precauting hospital infection of MRSA.2 MRSA is multi-drug resistant, only glycopeptides is effective, it is resistant to all other common antibacterials.3 MRSA possesses active efflux genes norA, qacA, qacB and qacJ, the amount of norA gene is most, and qacB is insecondary, the detection rate of qacJ is lower.4 Omeprazole can inhibit the activity of active efflux genes of MRSA to quinolones, this provide a new idea for clinical treatment to MRSA infection.