الفهرس 
Introduction & aims of the work
Methicillin resistant staphylococcus aureus (MRSA)Only 14 pages are availabe for public view
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Abstract
A major problem in MRSA-infected patients is the co-infection with Gram-negative bacteria which are naturally resistant to Vancomycin (VA) & linezolide (LZD). In the past; the use of a combination of VA & ß-lactam antibiotics was one of the solutions for treating of such condition depending on synergism. In the recent years; a class of MRSA that becomes resistant to VA only in the presence of ß-lactam antibiotics (BIVR) has been emerged meaning that there is antagonism. This type of VA resistance is mainly due to stimulation of peptidoglycan metabolism & repair system, by ß-lactams, so rapidly depleting free VA to a level below its MIC. This means that ß-lactams should remain intact in BIVR culture, although most MRSA cells are known to produce ß-lactamase, meaning that the BIVRs either did not carry the ß-lactamase gene (blaZ) or this gene was suppressed. Aim : To isolate MRSA from different sites of infection among patients admitted to Mansoura University Hospitals (MUHs). To detect the frequency of BIVR among patients already receiving β-lactam antibiotics & VA in MUHs.To screen for the tendency of VA sensitive MRSA to become VA resistant MRSA when exposed to β-lactam antibiotics in vitro. As a result, we can expect MRSA possessing the capability to become BIVR in vivo.To investigate the relation between the ״ BIVR phenomenon״ & the ß-lactamase activity. Setting : Microbiology Diagnostics and Infection Control Unit (MDICU), in the Medical Microbiology and Immunology Department, Faculty of Medicine, Mansoura University. Results : A total of 642 Staph. Aureus isolates were isolated during the first 6 months of the year 2015. BIVR testing of the130 MRSA strains revealed that 13.8% of the MRSA strains were BIVR positive while 86.2 % were BIVR negative. All the BIVRs showed an undetectable ß-lactamase activity by nitrocefin test, most of them (83.3%) lacked the blaZ gene & the remaining (16.7%) carried the blaZ gene but showed an undetectable ß-lactamase either by nitrocefin test or spectrophotometrically, indicating that this gene was down regulated or suppressed in them by certain mechanism. Most (94.6%) non-BIVRs carried the blaZ gene &most of them(79.2%)actively produced detectable ß-lactamase by nitrocefin test. Conclusion: BIVRs gain vancomycin resistance only in presence of ß-lactam antibiotics, so preserving ß-lactams in milieu, by preventing ß-lactamase production, either by lacking or suppressing the blaZ gene.Accordingly, we recommend that; Avoid the concomitant use of ß-lactam antibiotics & VA in treating VA sensitive MRSA that have tendency to become BIVR, & if there are Gram negative bacilli which are common co-pathogens treat them by other effective antibiotics. Also, if BIVRs isolated from a patient, ß-lactams should be avoided for 5 successive days till the organisms become sensitive to VA, & during this period other effective antibiotics, other than the ß-lactams, should be used.