Serpil Ercis

A comparison of PCR detection of Meca with oxacillin disk susceptibility testing in different media and sceptor automated system for both Staphylococcus aureus and coagulase-negative staphylococci isolates

PURPOSE To evaluate three methods for 406 isolates of Staphylococcus aureus and coagulase-negative staphylococci (CNS) for the detection of methicillin resistance (MR) using National Committee for Clinical Laboratory Standards (NCCLS) new interpretive criteria. METHODS We used polymerase chain reaction (PCR) as a gold standard method to evaluate three methods [disk diffusion with Mueller-Hinton agar (MHA) and mannitol salt agar (MSA) and Sceptor system (Becton Dickinson, USA)] for the detection of mecA gene. The isolates that were methicillin-resistant with any of the three tests were evaluated further for MR by E-test. RESULTS MHA, MSA and Sceptor showed sensitivities of 100, 100 and 99% for S. aureus and 100, 82.6 and 72.1% for CNS, respectively. The specificities of the same methods were found as 100, 90.1 and 99.3% for S. aureus and 79.2, 95.8 and 97.2% for CNS, respectively. E-test showed 100% sensitivity for both S. aureus and CNS. Forty-eight CNS and 16 S. aureus isolates, which presented discrepancies with the three phenotypic methods (MHA disk diffusion method, MSA disk diffusion method and Sceptor), were correctly classified as resistant/susceptible with the E-test when compared with PCR. Only five CNS isolates, which were mecA-negative with PCR were resistant with E-test. Analysis of 248 S. aureus revealed that MHA is superior to other phenotype-based susceptibility testing methods in detecting MR. When we examined the results of 158 CNS, none of the three methods proved efficient in detecting MR. CONCLUSIONS We conclude that although the accuracy of the MHA disk diffusion test for the detection of MR approaches the accuracy of PCR for S. aureus isolates, the need for easy and reliable methods of detecting MR in CNS still remains.

In vitro susceptibility, tolerance and MLS resistance phenotypes of Group C and Group G streptococci isolated in Turkey between 1995 and 2002.

A total of 105 clinical strains of Group C and Group G streptococci were examined for their susceptibility to penicillin, cefotaxime, erythromycin, meropenem and vancomycin using a broth microdilution method. Minimum bactericidal concentrations of the antimicrobial agents and phenotypes of strains resistant to erythromycin were also evaluated. No resistance to penicillin, cefotaxime, meropenem and vancomycin was found in years 1995-2002, but there was 6.7% resistance to erythromycin. No tolerance was seen for penicillin and vancomycin, but there were strains tolerant to cefotaxime, erythromycin and meropenem. The resistance phenotypes of erythromycin-resistant isolates were determined by the double disc test with erythromycin and clindamycin which showed inducible MLS (57.1%) and M phenotype (42.8%) resistance. This in vitro finding shows that classical antimicrobial agents used for the treatment of GCS and GGS have good activity against clinically significant isolates, but the presence of macrolide resistance and tolerant isolates suggests that careful surveillance of the streptococcal isolates should be carried out.

Rapid 4 to 6 hour detection of extended-spectrum beta-lactamases in a routine laboratory

With the growing frequency of extended-spectrum beta-lactamases (ESBL) among Enterobacteriaceae, treatment of Gram-negative nosocomial infections requires rapid and reliable detection of this enzyme. Quicolor agar (QC agar) (Salubris Inc., Massachusetts, USA) is a novel chromogenic agar medium changing colour within 4 to 6 h due to the metabolic activity of growing bacteria. This study investigated the use of QC agar compared to Mueller Hinton agar (MH) for the detection of ESBL using disk diffusion and E-test. 100 Enterobacteriaceae isolated at Hacettepe University Hospital, of which 50 were predetermined to be ESBL positive and 50 as negative using the CLSI disk diffusion ESBL (phenotypic confirmatory test) criteria. For disk diffusion and E-test, cefotaxime±clavulanate (CT/CTL) and ceftazidime±clavulanate (TZ/TZL) were used, and for E-test, cefepime±clavulanate (PM/PML) was also used. QC agar rapid ESBL results for all strains were in agreement with the standard overnight procedure. All 50 ESBL positives were detected by both methods. For the 50 ESBL negatives, QC agar rapid results from E-test and disk diffusion were in complete accordance with the overnight MH results. Moreover, E-test detected 8 additional ESBL positive strains that disk diffusion missed. For disk diffusion, CT/CTL alone detected all 50 ESBL positives while TZ/TZL alone missed 5 ESBL positives. E-test CT/CTL alone confirmed all 50 ESBL positives and identified 4 additional ESBL-positive strains. When used together, E-test CT/CTL, TZ/TZL and PM/PML identified a total of 58 ESBL positives among the 100 strains tested. QC agar can be used for rapid and reliable ESBL detection within 4 to 6 h, using disk diffusion and E-test ESBL reagents. This rapid method should be further validated using genotype characterized ESBL and other beta-lactamase positive strains.

Quicolor : A novel system for rapid antibacterial susceptibility testing

Early determination of antibacterial susceptibility increases the success of therapy, decreases unnecessary use of antibacterials and side effects and lowers the overall healthcare costs. We have evaluated a rapid antibacterial susceptibility test, Quicolor (Salubris Inc., Massachusetts, USA), which is based on a rapid culture medium that indicates growth early by changing its colour. Quicolor proved to be a reliable rapid test for determining antibacterial susceptibility, having an overall agreement of 97.6% with the conventional CLSI sisk diffusion susceptibility test results. Between two methods overall agreement was 96.7% for Enterobacteriaceae, 96.8% for staphylococci and 94.2% for non-fermentative bacteria. There was only 0.6% major discrepancy in Enterobacteriaceae, 1.7% in staphylococci and 0.9% in non-fermentative bacteria. Since the test provides results in 3.5–6 h, it can provide the means to choose the right treatment regimen the same day the infectious agent is isolated.