Comparison of agar dilution, disk diffusion, MicroScan, and Vitek antimicrobial susceptibility testing methods to broth microdilution for detection of fluoroquinolone-resistant isolates of the family Enterobacteriaceae.

Fluoroquinolone resistance appears to be increasing in many species of bacteria, particularly in those causing nosocomial infections. However, the accuracy of some antimicrobial susceptibility testing methods for detecting fluoroquinolone resistance remains uncertain. Therefore, we compared the accuracy of the results of agar dilution, disk diffusion, MicroScan Walk Away Neg Combo 15 conventional panels, and Vitek GNS-F7 cards to the accuracy of the results of the broth microdilution reference method for detection of ciprofloxacin and ofloxacin resistance in 195 clinical isolates of the family Enterobacteriaceae collected from six U.S. hospitals for a national surveillance project (Project ICARE [Intensive Care Antimicrobial Resistance Epidemiology]). For ciprofloxacin, very major error rates were 0% (disk diffusion and MicroScan), 0.9% (agar dilution), and 2.7% (Vitek), while major error rates ranged from 0% (agar dilution) to 3.7% (MicroScan and Vitek). Minor error rates ranged from 12.3% (agar dilution) to 20.5% (MicroScan). For ofloxacin, no very major errors were observed, and major errors were noted only with MicroScan (3.7% major error rate). Minor error rates ranged from 8.2% (agar dilution) to 18.5% (Vitek). Minor errors for all methods were substantially reduced when results with MICs within +/-1 dilution of the broth microdilution reference MIC were excluded from analysis. However, the high number of minor errors by all test systems remains a concern.

Characterization of staphylococci with reduced susceptibilities to vancomycin and other glycopeptides.

During the last several years a series of staphylococcal isolates that demonstrated reduced susceptibility to vancomycin or other glycopeptides have been reported. We selected 12 isolates of staphylococci for which the vancomycin MICs were > or =4 microg/ml or for which the teicoplanin MICs were > or =8 microg/ml and 24 control strains for which the vancomycin MICs were < or =2 microg/ml or for which the teicoplanin MICs were < or =4 microg/ml to determine the ability of commercial susceptibility testing procedures and vancomycin agar screening methods to detect isolates with reduced glycopeptide susceptibility. By PCR analysis, none of the isolates with decreased glycopeptide susceptibility contained known vancomycin resistance genes. Broth microdilution tests held a full 24 h were best at detecting strains with reduced glycopeptide susceptibility. Disk diffusion did not differentiate the strains inhibited by 8 microg of vancomycin per ml from more susceptible isolates. Most of the isolates with reduced glycopeptide susceptibility were recognized by MicroScan conventional panels and Etest vancomycin strips. Sensititre panels read visually were more variable, although with some of the panels MICs of 8 microg/ml were noted for these isolates. Vitek results were 4 microg/ml for all strains for which the vancomycin MICs were > or =4 microg/ml. Vancomycin MICs on Rapid MicroScan panels were not predictive, giving MICs of either < or =2 or > or =16 microg/ml for these isolates. Commercial brain heart infusion vancomycin agar screening plates containing 6 microg of vancomycin per ml consistently differentiated those strains inhibited by 8 microg/ml from more susceptible strains. Vancomycin-containing media prepared in-house showed occasional growth of susceptible strains, Staphylococcus aureus ATCC 29213, and on occasion, Enterococcus faecalis ATCC 29212. Thus, strains of staphylococci with reduced susceptibility to glycopeptides, such as vancomycin, are best detected in the laboratory by nonautomated quantitative tests incubated for a full 24 h. Furthermore, it appears that commercial vancomycin agar screening plates can be used to detect these isolates.

Ability of commercial and reference antimicrobial susceptibility testing methods to detect vancomycin resistance in enterococci.

We evaluated the abilities of 10 commercially available antimicrobial susceptibility testing methods and four reference methods (agar dilution, broth microdilution, disk diffusion, and the agar screen plate) to classify enterococci correctly as vancomycin susceptible or resistant using 50 well-characterized strains of enterococci. There was a high level of agreement of category classification data obtained with broth-based systems (Sceptor, MicroMedia, Pasco, and Sensititre), agar dilution, and an antibiotic gradient method (E test) with data obtained by reference broth microdilution; no very major or major errors were seen, and minor errors were < or = 6%. Increased minor error rates were observed with disk diffusion (12%), Alamar (16%), Uniscept (16%), and conventional (overnight) MicroScan panels (16%). The errors were primarily with Enterococcus casseliflavus strains and organisms containing the vanB vancomycin resistance gene. Very major error rates of 10.3 and 20.7% were observed with Vitek and MicroScan Rapid (MS/Rapid) systems, respectively; however, only the MS/Rapid system produced major errors (13.3%). On repeat testing of discrepant isolates, the very major error rate with the Vitek system dropped to 3.4%, while the very major error rate with the MS/Rapid system increased to 27.6%; major errors with the MS/Rapid system were not resolved. Many of the commercial systems had only 4 dilutions of vancomycin, which resulted in up to 84% of values being off scale (e.g., Uniscept). Of the methods tested, most conventional broth- and agar-based methods proved to be highly accurate when incubation was done for a full 24 h, although several of the tests had high minor error rates. Automated systems continued to demonstrate problems in detecting low-level resistance.

Prevalence of Neisseria meningitidis relatively resistant to penicillin in the United States, 1991. Meningococcal Disease Study Group.

To estimate the prevalence of Neisseria meningitidis relatively resistant to penicillin in the United States, antimicrobial susceptibility testing was performed on all US meningococcal isolates submitted to the Centers for Disease Control and Prevention in 1991, including isolates identified through population-based surveillance for invasive meningococcal disease in selected areas of the United States. Three of the 100 isolates tested had MICs of penicillin of 0.125 microgram/mL. All were serogroup B, beta-lactamase-negative, and unique by multilocus enzyme electrophoresis subtyping. None of the 3 patients had been treated solely with penicillin; all recovered completely. About 4% of the isolates obtained from the population-based surveillance system were relatively penicillin-resistant. Given the low prevalence and uncertain clinical significance of infection with these organisms, routine susceptibility testing of meningococcal isolates is not indicated at this time; however, continued surveillance is necessary to monitor trends in antimicrobial susceptibility of meningococci in the United States.

Comparison of the E Test to agar dilution, broth microdilution, and agar diffusion susceptibility testing techniques by using a special challenge set of bacteria.

The E Test (AB Biodisk, Solna, Sweden) is a new method for performing antimicrobial susceptibility tests. It consists of an impervious carrier (5- by 50-mm strip) with a predefined antimicrobic gradient which is placed on an inoculated agar plate and processed like a disk diffusion test. Results are generated directly as MICs from a continuous concentration gradient covering 15 twofold dilutions, and MICs are read where the edge of the inhibition zone intersects the strip. We compared the E Test with disk diffusion, broth microdilution, and agar dilution tests by using a challenge set of 195 gram-positive and gram-negative bacteria for 14 antimicrobial agents. Also, disk diffusion, broth microdilution, and agar dilution tests were compared with each other. All test method comparisons gave greater than 94% agreement for the category of susceptibility. The E Test category agreement with disk diffusion and broth microdilution was 95.1%, and with agar dilution it was 95.2%. The E Test results were as reliable as the results obtained by the standard antimicrobial susceptibility testing methods.

Evaluation of the MicroScan antimicrobial susceptibility system with the autoSCAN-4 automated reader.

The American MicroScan (American MicroScan, Mahwah, N.J.) identification and antimicrobial susceptibility system consists in part of an automated reading system (autoSCAN-4) with data management capabilities. We evaluated the system with 404 gram-negative and 170 gram-positive facultative anaerobic and aerobic bacteria. We compared MicroScan results read automatically and visually with each other and with the results obtained by the reference method (read visually). The overall agreement within +/- 1 log2 dilution was 94.3% when the MicroScan system (read automatically) was compared with the reference method (read visually), 96.4% when MicroScan panels (read visually) were compared with reference panels, and 97.4% when the autoSCAN-4 automated reading was compared with the visual reading of the MicroScan panels. Total discrepancies (susceptibility interpretation category changes) for the MicroScan system compared with the reference method were 7%, with 6.2% considered a minor discrepancy. The autoSCAN-4 and the complete MicroScan system yielded accurate results compared with the reference method.