Evaluation of the OSIRIS video reader system for disk diffusion susceptibility test reading.

OBJECTIVE: The objective of this study was to compare the performance of the OSIRIS video-assisted reading system for disk diffusion susceptibility testing with conventional manual reading. METHODS: Prospectively collected clinical isolates (n = 119) and isolates with well-characterised resistant mechanisms, including extended-spectrum (ESBL) or inhibitor-resistant TEM (IRT) beta-lactamases producing Enterobacteriaceae (80), methicillin-resistant Staphylococcus aureus (MRSA) (16) and vancomycin-resistant enterococci (14) were studied using the National Committee for Clinical Laboratory Standards disk-diffusion technique. The OSIRIS reading (inhibition zone in mm) was compared with manual reading (reference value). RESULTS: Essential agreement (< or =3 mm discrepancy with manual reading) was 91.6% in routine isolates and 94.8% in those with well-characterised resistant mechanisms, respectively. Overall agreement for susceptibility testing interpretation was slightly higher in the former (95.5%) than in the latter (93.2%) group. The presence of ESBL enzymes enhanced variations of measurements due to synergy among amoxicillin-clavulanate and cephalosporins, as a consequence of closer disk placement. The poor growth characteristic of enterococci affected the video reading; on the other hand, there was a high performance with MRSA isolates. Combining all interpretative results, 4.1% minor, 1.0% major and 2.8% very major errors were observed. CONCLUSION: The OSIRIS system is a useful tool for the reading and interpretation of inhibition zone sizes in disk diffusion susceptibility testing.

Performance of the VITEK2 system for identification and susceptibility testing of routine Enterobacteriaceae clinical isolates.

The VITEK2 system was evaluated with 138 fresh consecutive routine clinical Enterobacteriaceae isolates. Susceptibility results to 10 beta-lactams, three aminoglycosides and a quinolone were compared with those obtained following the NCCLS standard microdilution. API20E was used as reference method for identification. All but three isolates were correctly identified in 3 h at species level (97.8%), two isolates (1.4%) at genus level and only one isolate was misidentified. Overall essential agreement for susceptibility testing was 97.1%. Discrepancies were mainly observed with piperacillin (1.1%), cefuroxime (0.6%) and amoxycillin/clavulanate (0.3%). Discrepancies for aminoglycosides and ciprofloxacin were low (<0.1%). Minor, major and very major errors (NCCLS categories) were 4.1%, 0.2% and 6.1%, respectively. Very major errors were due to piperacillin (4.5%), ampicillin (0.8%) and amoxycillin/clavulanate (0.8%). The VITEK2 system gave accurate identification and susceptibility testing results of routine Enterobacteriaceae clinical isolates.

Evaluation of the Wider system, a new computer-assisted image-processing device for bacterial identification and susceptibility testing.

The Wider system is a newly developed computer-assisted image-processing device for both bacterial identification and antimicrobial susceptibility testing. It has been adapted to be able to read and interpret commercial MicroScan panels. Two hundred forty-four fresh consecutive clinical isolates (138 isolates of the family Enterobacteriaceae, 25 nonfermentative gram-negative rods [NFGNRs], and 81 gram-positive cocci) were tested. In addition, 100 enterobacterial strains with known beta-lactam resistance mechanisms (22 strains with chromosomal AmpC beta-lactamase, 8 strains with chromosomal class A beta-lactamase, 21 broad-spectrum and IRT beta-lactamase-producing strains, 41 extended-spectrum beta-lactamase-producing strains, and 8 permeability mutants) were tested. API galleries and National Committee for Clinical Laboratory Standards (NCCLS) microdilution methods were used as reference methods. The Wider system correctly identified 97.5% of the clinical isolates at the species level. Overall essential agreement (+/-1 log(2) dilution for 3,719 organism-antimicrobial drug combinations) was 95.6% (isolates of the family Enterobacteriaceae, 96.6%; NFGNRs, 88.0%; gram-positive cocci, 95.6%). The lowest essential agreement was observed with Enterobacteriaceae versus imipenem (84.0%), NFGNR versus piperacillin (88.0%) and cefepime (88.0%), and gram-positive isolates versus penicillin (80.4%). The category error rate (NCCLS criteria) was 4.2% (2.0% very major errors, 0.6% major errors, and 1. 5% minor errors). Essential agreement and interpretive error rates for eight beta-lactam antibiotics against isolates of the family Enterobacteriaceae with known beta-lactam resistance mechanisms were 94.8 and 5.4%, respectively. Interestingly, the very major error rate was only 0.8%. Minor errors (3.6%) were mainly observed with amoxicillin-clavulanate and cefepime against extended-spectrum beta-lactamase-producing isolates. The Wider system is a new reliable tool which applies the image-processing technology to the reading of commercial trays for both bacterial identification and susceptibility testing.

[Prospective study of Staphylococcus aureus with reduced susceptibility to glycopeptides]. / Estudio prospectivo de Staphylococcus aureus con sensibilidad disminuida a los glucopéptidos.

The emergence of clinical methicillin-resistant Staphylococcus aureus with reduced susceptibility to glycopeptides has recently been documented. Heterogeneous levels of glycopeptides susceptibility have been demonstrated in these isolates. Prospectively, we investigated the presence of S. aureus isolates with reduced susceptibility to vancomycin in our hospital from October 1997 to January 1998. The vancomycin and teicoplanin susceptibility of 248 consecutively methicillin-susceptible and -resistant S. aureus isolates from clinical and surveillance specimens was studied by broth microdilution and agar dilution methods. In addition, 108 CFU/mI of overnight cultures were plated onto BHI-agar plates containing 6 and 10 microg/ml of vancomycin and were incubated for 48 hours at 35 degrees C. Under these conditions selected colonies were subcultured onto vancomycin plates and vancomycin susceptibility (E-test) was determined again. Vancomycin MIC(90) values by the agar dilution technique for methicillin-susceptible and -resistant isolates were 2 and 4 microg/ml, respectively No selected colonies were observed on 10 microg/ml vancomycin plates. In contrast, 12 isolates, including 8 (3.9%) methicillin-susceptible and 4 (9. 1%) methicillin-resistant isolates, showed subpopulations at a frequency rate of 10(-6)-10(-7) on 6 microg/ml vancomycin plates. Nevertheless, a significant increase in MIC values for vancomycin was not observed in these S. aureus subpopulations.