Multicenter Evaluation of an Automated System Using Selected Bacteria That Harbor Challenging and Clinically Relevant Mechanisms of Resistance to Antibiotics

Abstract

A multicenter study was carried out to evaluate the performance of a new commercial automated system in comparison with that of the reference agar dilution method. Ten clinical microbiology laboratories tested a collection of 61 strains of gram-negative bacilli (49 Enterobacteriaceae and 12 Pseudomonas aeruginosa), and 6 other laboratories tested a collection of 55 strains of gram-positive cocci (10 enterococci and 45 staphylococci) against 10-20 antimicrobial agents. The strains were selected on the basis that they harbored challenging and characterized mechanisms of resistance. In comparison with the agar reference method, the automated system gave an overall essential agreement (±1 dilution) of 94.5%, 93.5%, and 97% for the gramnegative bacilli, enterococci, and staphylococci, respectively. According to the interpretive standards of the National Committee for Clinical Laboratory Standards, the category agreement ranged from 96 to 96.4% for the three sets of organisms. The accuracy of the automated system, as determined by the kappa test, ranged from 0.80 to 0.88, reflecting an almost perfect agreement with the reference technique. Very major, major, and minor errors obtained with the automated system were 0.3%, 2.9%, and 6.6% for gram-negative bacilli, 3.4%, 0%, and 5% for enterococci, and 1%, 1.6%, and 2.7% for staphylococci, respectively. The high rate of very major errors in enterococci was mostly due to a single strain of multidrug-resistant Enterococcus faecium, which was found susceptible to several antibiotics in a majority of participant laboratories. The use of a heavy inoculum and of a broth test medium by the automated system might account for a better expression of certain resistance mechanisms, including β-lactamases, as compared to the agar dilution reference method. The interlaboratory reproducibility was acceptable, as shown by the narrow dispersion of MICs and by the results of quality control.

Characterization of Klebsiella isolates by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and determination of antimicrobial resistance with VITEK 2 advanced expert system (AES)

“…Barry et al (24) found VITEK 2 results to be consistent with routine resistance tests at a rate of 96.5%, 96.7%, and 95.9%, respectively, for staphylococci, enterococci, and gram-negative bacilli. Canton et al (25) found the VITEK 2 method less reliable than the microdiffusion method, whereas Leclercq et al (26) stated that VITEK 2 was more reliable than the agar dilution method. According to Nyberg et al (27), sensitivity and specificity vary in different studies, but high sensitivity and specificity have been achieved with VITEK 2 when testing K. pneumoniae isolates (15,27,28).…”

Multicentre evaluation of the VITEK 2 Advanced Expert System for interpretive reading of antimicrobial resistance tests

“…Limited comparison with MICs determined by Etests was, however, undertaken here, and a fuller comparison of MICs obtained by the VITEK 2 with reference MIC data is published elsewhere. 5,6 By recording and interpreting phenotypes the VITEK 2 AES accurately inferred the presence or absence of mecAmediated methicillin resistance in staphylococci; VanA-, VanB-or VanC-mediated glycopeptide resistance in enterococci; gyrA-and parC-mediated quinolone resistance in staphylococci and Enterobacteriaceae; AAC(6Ј)-APH(2Љ)-mediated gentamicin resistance in enterococci and staphylococci; erm-mediated macrolide resistance in pneumococci; ESBL-mediated cephalosporin resistance in Enterobacteriaceae and P. aeruginosa; and acquired penicillinases in Enterobacteriaceae. Enterococci with the VanA, VanB and VanC determinants were distinguished reliably.…”

“…The VITEK 2 is an automated susceptibility testing system enabling rapid (4-7 h) determination of MICs by the analysis of growth kinetics of bacteria with antibiotics in test cards. [5][6][7] The AES provides standardized interpretive reading of these MICs. 8,9 Briefly, it comprises a database of MIC distribution for different combinations of antibiotics and prevalent resistance mechanisms in different species, together with a series of algorithms.…”