CHARACTERIZATION OF PVL (PANTO-VALANTINE LEUCOCIDINE) GENE IN METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS (MRSA) ISOLATES FROM SOME HOSPITALS IN BAGHDAD CITY
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Cefoxitin is a potent inducer of the mecA regulatory system. It is being recommended for detection of methicillin resistance in Staphylococcus aureus (MRSA) when using disk diffusion testing. The aim of our study was to evaluate the efficacy of cefoxitin disc diffusion test to characterize MRSA and compare it with oxacillin agar screening and detection of mecA gene by PCR.
Fifty strains of S. aureus isolated from clinical samples were used in the study. Routine antibiotic susceptibility testing was performed including oxacillin disk. Oxacillin screen agar plates with 4% NaCl and 6 microg/ml of oxacillin were inoculated and interpreted as per standard guidelines. Cefoxitin disc diffusion test was performed using 30 microg disc and zone sizes were measured. PCR for amplification of the mecA gene was performed.
Out of the 50 isolates, 28 were found to be methicillin resistant by oxacillin disc diffusion test, 30 were resistant by oxacillin screen agar method, and 32 were resistant with cefoxitin disc diffusion. For these 32 isolates mecA gene was positive.
Results of cefoxitin disc diffusion test is in concordance with the PCR for mecA gene. Thus, the test can be an alternative to PCR for detection of MRSA in resource constraint settings.
We examined factors associated with penicillinase production by nasal carriage Staphylococcus aureus strains in 648 children aged 3 to 6 years attending 20 randomly sampled playschools. The children were prospectively monitored
for drug use and medical events for 6 months and were then screened for S. aureus carriage. Isolates were tested for their susceptibility to penicillin G and methicillin, and penicillinase production by
methicillin-susceptible, penicillin-resistant strains was quantified. S. aureus was isolated from 166 children (25.6%). Exposure to amoxicillin-clavulanate during the previous 3 months was associated with
higher penicillinase production by penicillin-resistant, methicillin-susceptible strains (odds ratio, 3.6; P = 0.03). These results suggest that use of the amoxicillin-clavulanate combination could induce a herd selection process
of S. aureus strains producing higher levels of penicillinase.
Genomic diversity of mutation in the mecI gene or mecA promoter/operator region was analyzed for clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and Staphylococcus epidermidis (MRSE). In most MRSA strains, a single base substitution was detected in either the mecI (three different positions) or the mecA promoter (two different positions), while a 28-base deletion in mecI was found in one strain. In contrast, no mutation was detected in these gene sequences of MRSE strains.
Multiresistant staphylococci (82 Staphylococcus aureus and 114 coagulase-negative staphylococci) were characterized by testing with rapid multiplex polymerase chain reaction (PCR)
assays for species identification and detection of associated antibiotic resistance genes. These 196 staphylococci were isolated
from 149 adult patients who developed wound infection after elective coronary artery bypass grafts and/or valve surgery. The
multiplex PCR assays allowed identification of the most common staphylococcal species with S. aureus- and Staphylococcus epidermidis-specific primers as well as the detection of the erythromycin resistance genes ermA, ermB, ermC and msrA, the aminoglycoside resistance gene aac(6′)-aph(2″), the oxacillin resistance gene mecA and the penicillin resistance gene blaZ. There was a very good correlation between the genotypic analysis by PCR and the phenotype determined by standard methods
of susceptibility testing and identification of staphylococcal species: 100% for erythromycin resistance, 98.0% for gentamicin
resistance, 99.0% for oxacillin resistance, 100% for penicillin resistance and 100% for S. aureus and S. epidermidis species identification. This study suggests that the incidence and distribution of the tested clinically relevant antibiotic
resistance genes in staphylococci associated with infections after cardiac surgery do not differ from those in strains from
other infections. These multiplex PCR assays may be used as diagnostic tools to replace or complement standard methods of
susceptibility testing and identification of staphylococci.
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