Correlation of oxacillin MIC with mecA gene carriage in coagulase-negative staphylococci.
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The National Committee for Clinical Laboratory Standards has recently changed the oxacillin breakpoint from >/=4 mg/liter to >/=0. 5 mg/liter to detect methicillin-resistant coagulase-negative staphylococci (CoNS) because the previous breakpoint lacked sensitivity. To determine the correlation between the new oxacillin breakpoint and the presence of the mecA gene, 493 CoNS of 11 species were tested. The presence of the mecA gene was determined by PCR, and oxacillin susceptibility was determined by the agar dilution method with Mueller-Hinton agar containing 2% NaCl and oxacillin (0. 125 to 4.0 mg/liter). The new breakpoint correctly classified all CoNS strains with mecA as methicillin resistant and strains of Staphylococcus epidermidis, S. haemolyticus, and S. hominis without mecA as methicillin susceptible. The breakpoint of >/=0.5 mg/liter was not specific for S. cohnii, S. lugdunensis, S. saprophyticus, S. warneri, and S. xylosus, in that it categorized 70 of 74 strains of these species without mecA (94.6%) as methicillin resistant. The results of this study indicate that the new oxacillin breakpoint accurately identifies strains of CoNS with mecA but is not specific for strains of certain species of CoNS without mecA. (+info)
Vancomycin and oxacillin synergy for methicillin-resistant staphylococci.
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An increase in oxacillin activity was observed against methicillin-resistant coagulase-negative staphylococci (MRCNS) and methicillin-resistant Staphylococcus aureus (MRSA) in the presence of a sub-MIC of vancomycin. Vancomycin and oxacillin were synergistic against 14 of 21 strains of MRCNS and MRSA. A pattern of enhanced killing was also supported by time-kill studies. These results suggest that combinations of sub-MICs of vancomycin and oxacillin may have therapeutic benefits against methicillin-resistant staphylococci. (+info)
Phenotypic expression of oxacillin resistance in Staphylococcus epidermidis: roles of mecA transcriptional regulation and resistant-subpopulation selection.
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The MICs for many oxacillin-resistant (OR) Staphylococcus epidermidis (ORSE) strains are below the Staphylococcus aureus methicillin or oxacillin resistance breakpoint. The difficulty detecting the OR phenotype in S. epidermidis may be due to extreme heterotypy in resistance expression and/or transcriptional repression of mecA, the OR gene, by MecI. To determine the role of these factors in the phenotypic expression of ORSE, 17 geographically diverse mecI(+) ORSE isolates representing 14 distinct pulsed-field gel electrophoresis pulse types (>3 band differences) were investigated. Thirteen of the 14 types contained mecI and mecA promoter-operator sequences known to be associated with maximal mecA repression, and in all isolates, mecA transcription was repressed. All 17 were heterotypic in their resistance expression. Oxacillin MICs ranged from 1 to 128 microg/ml and increased for 16 of 17 isolates after beta-lactam induction. Allelic replacement inactivation of mecI in three isolates similarly resulted in a four- to sevenfold increase in MIC. In the two of these three isolates producing beta-lactamase, mecA transcription was regulated by both mecI and beta-lactamase regulatory sequences. Heterotypic expression of resistance in these three isolates was unaffected by either beta-lactam induction or mecI inactivation. However, prolonged incubation in concentrations of oxacillin just sufficient to produce a lag in growth (0.5 to 1.0 microg/ml) converted the population resistance expression from heterotypic to homotypic. Homotypic conversion could also be demonstrated in microtiter wells during MIC determinations in one isolate for which the MIC was high. We conclude that the phenotypic expression of S. epidermidis OR in broth can be affected both by mecA transcriptional regulation and by subpopulation resistance expression. (+info)
Activities of taurolidine in vitro and in experimental enterococcal endocarditis.
(20/449)
In vitro, the antimicrobial agent taurolidine inhibited virtually all of the bacteria tested, including vancomycin-resistant enterococci, oxacillin-resistant staphylococci, and Stenotrophomonas maltophilia, at concentrations between 250 and 2,000 microg/ml. Taurolidine was not effective in experimental endocarditis. While it appears unlikely that this antimicrobial would be useful for systemic therapy, its bactericidal activity and the resistance rates found (<10(-9)) are favorable indicators for its possible development for topical use. (+info)
Rapid detection of mecA-positive and mecA-negative coagulase-negative staphylococci by an anti-penicillin binding protein 2a slide latex agglutination test.
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A rapid slide latex agglutination (LA) test, MRSA-Screen (Denka Seiken Co., Niigata, Japan), which detects PBP 2a, was tested for its ability to differentiate between mecA-positive and -negative coagulase-negative staphylococci. A total of 463 isolates from 13 species were included in the study. The mecA gene was detected by PCR, and the oxacillin MIC was determined by the agar dilution method according to the guidelines of the National Committee for Clinical Laboratory Standards (NCCLS). The LA test was performed with oxacillin-induced isolates. The true-positive and true-negative results were defined on the basis of the presence or the absence of the mecA gene. By PCR, 251 isolates were mecA positive and 212 were mecA negative. The sensitivities, specificities, and positive and negative predictive values for the LA test compared to the NCCLS breakpoint for oxacillin resistance (>/=0.5 mg/liter) were as follows: for the LA test, 100, 99.5, 99.6, and 100%, respectively; for the NCCLS breakpoint, 100, 60.8, 75.1, and 100%, respectively. One hundred twenty-five mecA-positive isolates were also tested by the LA test without induction of PBP 2a; only 72 (57.6%) gave a positive result and required 3 to 15 min for reaction. With induction, all 251 isolates were positive within 3 min. The LA test was reliable in classifying mecA-negative isolates, but it classified isolates for which the oxacillin MIC was >/=0.5 mg/liter as oxacillin susceptible. For the reliable detection of oxacillin resistance by the MRSA-Screen in coagulase-negative staphylococci, induction of the mecA gene appears to be necessary. (+info)
Evaluation of three rapid methods for detection of methicillin resistance in Staphylococcus aureus.
(22/449)
The probe-based Velogene Rapid MRSA Identification Assay (ID Biomedical Corp., Vancouver, British Columbia, Canada) and the latex agglutination MRSA-Screen (Denka Seiken Co., Tokyo, Japan) were evaluated for their ability to identify methicillin-resistant Staphylococcus aureus (MRSA) and to distinguish strains of MRSA from borderline oxacillin-resistant S. aureus (BORSA; mecA-negative, oxacillin MICs of 2 to 8 microgram/ml). The Velogene is a 90-min assay using a chimeric probe to detect the mecA gene. MRSA-Screen is a 15-min latex agglutination test with penicillin-binding protein 2a antibody-sensitized latex particles. We compared these assays with the BBL Crystal MRSA ID System (Becton Dickinson, Cockeysville, Md.) and with PCR for mecA gene detection. A total of 397 clinical isolates of S. aureus were tested, consisting of 164 methicillin-susceptible strains, 197 MRSA strains, and 37 BORSA strains. All assays performed well for the identification of MRSA with sensitivities and specificities for Velogene, MRSA-Screen, and BBL Crystal MRSA ID of 98.5 and 100%, 98.5 and 100%, and 98.5 and 98%, respectively. Three MRSA strains were not correctly identified by each of the Velogene and MRSA-Screen assays, but repeat testing with a larger inoculum resolved the discrepancies. The BBL Crystal MRSA ID test misclassified four BORSA strains as MRSA. Both the Velogene and the MRSA-Screen assays are easy to perform, can accurately differentiate BORSA isolates from MRSA isolates, and provide a rapid alternative for the detection of methicillin resistance in S. aureus in clinical laboratories, especially when mecA PCR gene detection is unavailable. (+info)
Tn551-mediated insertional inactivation of the fmtB gene encoding a cell wall-associated protein abolishes methicillin resistance in Staphylococcus aureus.
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A Tn551 insert in a gene termed fmtB was shown to reduce oxacillin as well as Triton X-100 resistance in highly methicillin-resistant Staphylococcus aureus (MRSA) COL. Backcrosses of fmtB::Tn551 into S. aureus COL and into two genetically distinct MRSA strains, KSA8 and NCTC10443, confirmed the linkage of fmtB::Tn551 with loss of oxacillin resistance. The fmtB gene codes for a protein of a deduced molecular mass of 263 kDa that contains 17 tandem repeats of 75 amino acids and a C-terminal LPXTG cell wall-sorting motif. Immunoblots with anti-FmtB antibodies confirmed its localization in the cell wall fraction. The fmtB gene was mapped downstream of the phosphoglucosamine mutase operon glmM which catalyses formation of glucosamine-1-phosphate. Oxacillin resistance was not restored in fmtB mutants by trans-complementation with fmtB. However, although GlmM production was not affected by fmtB inactivation, oxacillin resistance was increased in fmtB mutants by introducing a plasmid-borne glmM gene, presumably by GlmM overexpression. Interestingly, a similar phenotypic complementation was obtained in fmtB mutants by including substrate level concentrations of N-acetylglucosamine or glucosamine in the growth medium. Inactivation of the fmtB gene seems therefore to have an indirect effect on methicillin resistance which can be relieved by increasing the production of the cell wall precursor glucosamine-1-phosphate. (+info)
Adults with meningitis caused by oxacillin-resistant Staphylococcus aureus.
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Since 1995, 11 adult patients with oxacillin-resistant Staphylococcus aureus (ORSA) meningitis have been identified at Chang Gung Memorial Hospital-Kaohsiung, in Kaohsiung, Taiwan. The 11 patients were 8 men and 3 women, aged 17-78 years. A postneurosurgical state was an underlying condition for all, and fever and disturbances in consciousness were the most common clinical manifestations. Infection with S. aureus only was found in 8 patients, and mixed infection was found in the other 3. The 8 patients with meningitis caused by S. aureus only were mainly treated with intravenous vancomycin, 2-4 g/day; 4 of these patients died. Although ORSA meningitis is uncommon among adults with culture-proven bacterial meningitis, its incidence has been increasing in recent years. The diagnosis of adult ORSA meningitis can be confirmed only with a positive culture of cerebrospinal fluid, and the choice of initial empirical antibiotics must be guided by the accumulated data concerning the relative frequency of the implicated pathogens found at each institution. Despite the high rate of mortality associated with ORSA meningitis, intravenous vancomycin therapy seems to be one of the best choices for management of this condition in adults. (+info)