Barbara S. Reisner

Evaluation of mycobacteria growth indicator tubes for susceptibility testing of Mycobacterium tuberculosis to isoniazid and rifampin

The reliability of mycobacteria growth indicator tubes (MGIT) for determining the susceptibility of Mycobacterium tuberculosis to isoniazid (0.1 microgram/ml) and rifampin (2.0 micrograms/ml) was evaluated by comparing MGIT results to those obtained by the radiometric BACTEC TB method. We tested 29 isolates, many selected for resistance. The turnaround times were 3-8 days (median 6) for MGIT and 4-10 days (median 6) for BACTEC. Isoniazid results by both methods agreed for all isolates tested: 20 resistant and nine susceptible. Rifampin results agreed for 28 isolates: 10 resistant and 18 susceptible. One isolate yielded discrepant results: resistant to rifampin by BACTEC, but susceptible by MGIT. This isolate was also rifampin-resistant by the traditional Method of Proportion and when tested by the MGIT method using 1.0 micrograms/ml of rifampin. The MGIT system is a nonradiometric alternative to the BACTEC for rapid susceptibility testing of M. tuberculosis to isoniazid and rifampin; however, additional testing is needed to determine the optimal concentration of rifampin.

Ampicillin-Resistant Escherichia coli in Gestational Pyelonephritis: Increased Occurrence and Association with the Colonization Factor Dr Adhesin

The pattern of ampicillin resistance and possible association with virulence factors of 78 Escherichia coli isolates taken from 78 pregnant women with pyelonephritis were evaluated. The current incidence of ampicillin resistance among pyelonephritis isolates (46%) was significantly higher than that reported in 1985 (22%). Resistance was found more frequently during the first (60%) and third (53%) trimesters than during the second trimester (33%). Of all dra(+) E. coli isolates, 75% were ampicillin resistant, whereas dra(+) isolates of O75 serotype E. coli accounted for 87% of ampicillin-resistant strains. The significant increase of ampicillin resistance among gestational pyelonephritis E. coli and the association with the dra gene cluster encoding colonization and invasive capacity may warrant further study involving obstetric and neonate wards, with the latter being at the higher risk for potential problems.

Comparison of four methods for identifying Streptococcus pneumoniae

Four methods (bile solubility, optochin, latex agglutination, and DNA probe) were compared for identification of clinical isolates of Streptococcus pneumoniae. Of 209 isolates tested, 151 (Group I) were selected based on typical colony morphology of S. pneumoniae, while 58 (Group II) were selected on the basis of alpha-hemolysis alone. Using the DNA probe as a reference method, 141 isolates from Group I and 10 from Group II were identified as S. pneumoniae. The optochin disk test and the latex agglutination test both exhibited a 100% sensitivity and specificity for Group I isolates; bile solubility identified all but 1 isolate in this group. For Group II isolates, the sensitivity and specificity of optochin testing was 100%, 80 and 94% for latex and 80 and 100% for bile. The results of this study indicate that all methods tested give reliable results for isolates with typical colony morphology of S. pneumoniae. Bile solubility and latex may not be as reliable when testing alpha-hemolytic colonies without colony morphology typical S. pneumoniae.

Comparison of Gen-Probe AccuProbe Group B streptococcus culture identification test with conventional culture for the detection of Group B streptococci in broth cultures of vaginal-anorectal specimens from pregnant women<

The performance of the AccuProbe Group B Streptococcus Culture Identification Test (Gen-Probe Incorporated, San Diego, CA, USA) for the detection of group B streptococci (GBS) directly from LIM broth cultures of vaginal-anorectal swab specimens from pregnant women (two swabs per patient in most cases) was evaluated by comparing results to those of conventional GBS culture. Of 411 specimens analyzed, 82 were positive and 312 were negative for GBS by both methods. After initial testing, the percent agreement was 95.9%. The initial sensitivity, specificity, and positive and negative predictive values for the AccuProbe test were 90.1%, 97.5%, 91.1%, and 97.2%, respectively. Results were discrepant for 17 specimens: eight were GBS positive by probe and negative by culture; nine were negative by probe and positive by culture. To resolve discrepancies, culture plates were re-examined for GBS colonies, AccuProbe testing was repeated on the initial LIM broth cultures, and the second swab (if received) was inoculated to LIM broth for AccuProbe testing after overnight incubation. After discrepant resolution testing, the percent agreement between the two test methods was 97.8%. The final sensitivity, specificity, and positive and negative predictive values for the AccuProbe test were 95.6%, 98.4%, 94.6%, and 98.7%, respectively. These data suggest that the AccuProbe test is a reliable method for detecting GBS in vaginal-anorectal specimens, providing results more rapidly than conventional culture. However, strict adherence to the manufacturers test protocol is necessary to limit technical errors.

Evaluation of a latex agglutination assay for rapid detection of oxacillin resistant Staphylococcus aureus

Oxacillin resistance in Staphylococcus aureus is mediated by the mecA gene, resulting in production of penicillin-binding protein 2a (PBP2a), which is not present in the oxacillin susceptible strains. We evaluated the ability of a 30-min latex agglutination (LA) test (Seiken, Tokyo, Japan) to detect production of PBP2a in 315 clinical isolates of S. aureus. The LA results were compared with results of susceptibility testing using the Vitek GPS-SV test card. The latex test was positive for all 206 isolates determined to be methicillin resistant by Vitek (sensitivity 100%), the latex test was negative for 108 of 109 isolates determined to be oxacillin susceptible by Vitek, and the latex test was positive for 1 isolate determined to be susceptible by Vitek (specificity 99.1%). The discrepant isolate was negative for the mecA gene by polymerase chain reaction (PCR). The LA test is a rapid and reliable method for detecting oxacillin resistant S. aureus.

Evaluation of the Vitek Card GPS105 and VTK-RO7.01 Software for Detection of Oxacillin Resistance in Clinically Relevant Coagulase-Negative Staphylococci

ABSTRACT The performance of Vitek cards GPS105 with software version VTK-R07.01 for detection of oxacillin resistance in coagulase-negative staphylococci (CoNS) was compared to disk diffusion and PCR detection for mecA. The sensitivity and specificity of the Vitek GPS105 method were 97.6 and 85.5%, respectively.

Evaluation of CPS ID 2 chromogenic agar as a single medium for urine culture

The CPS ID 2 (CPS) chromogenic agar was compared to routine media for use in the isolation, enumeration, identification, and susceptibility testing of bacteria recovered from urine specimens. Of 487 urine specimens, 318 were culture negative, 12 were positive on CPS only, 16 positive on routine only, and 141 positive on both. The enumeration of microorganisms agreed for 96 of the 141 cultures. Fewer organisms were recovered on CPS for 25 cultures, more for 20 cultures. The identification of bacteria from CPS and routine media agreed for all isolates. Susceptibility testing was performed for 100 isolates. The categorical susceptibility test results from isolates grown on CPS agreed with results from routine media for 77 isolates. For the remaining 23 isolates, one or more discrepancies were seen involving 16 different antimicrobial agents; 27 minor, 1 major, and 6 very major errors. After additional testing, there were 10 confirmed errors, 7 minor and 3 very major errors. This study demonstrates that CPS agar is a reliable medium for culturing urine. Susceptibility testing directly from this medium resulted in low error rates for all antimicrobial agents tested.

Clinical evaluation of the Vitek automated system with cards GNS 122 and 127 and VTK-R07.01 software for antimicrobial susceptibility testing of Pseudomonas aeruginosa

The performance of the Vitek Automated Susceptibility Testing System software version VTKR07.01 (bioMerieux Vitek, Hazelwood, MO), for testing Pseudomonas aeruginosa was evaluated by comparing results for 200 clinical isolates with those of disk diffusion and manual broth microtiter dilution testing. For cefepime, the restricted major error rate was 0.53% and the minor error rate was 12.5%. For piperacillin, the restricted major error rate was 2.15%. For ticarcillin/clavulanic acid, restricted very major and major error rates of 6.5% and 3.2%, respectively, occurred. The results of our study indicate that the Vitek system performs within acceptable limits when testing piperacillin, but remains problematic for testing cefepime and ticarcillin-clavulanic acid.

Evaluation of a combined acid-fast-trichrome stain for detection of microsporidia and Cryptosporidium parvum.

M and Cryptosporidium parvum are important agents of enteritis, capable of causing severe chronic disease in immunocompromised individuals.1,2 Currently, detection of these organisms in stool specimens requires at least 2 special stains or procedures; a chemofluorescent agent or a modified trichrome stain is commonly used to detect microsporidia,3–5 and an acid-fast stain, direct immunofluorescent assay (DFA), or enzyme immunoassay is used to detect C parvum.6–8 Recently, Ignatius et al9 described a combination acid-fast–trichrome (AFT) stain, which allows detection of these organisms, as well as Cyclospora cayetanensis and Isospora belli, with a single procedure. We modified this AFT procedure, incorporating the use of commercially available reagents, and compared the results of the modified AFT stain with those of our standard procedures for stool specimens received in the laboratory for a 15-month period.

Prospective study of IgM to Toxoplasma gondii onTechlab Product Insert, 1997 Beckman Coulter’s Access™ immunoassay system and comparison with Zeus ELISA and gull IFA assays

Abstract We compared a new assay for Toxoplasma IgM on the Access™ analyzer (Beckman Coulter, Inc., Chaska, MN, USA), a random access instrument based on the principle of paramagnetic particle enzyme immunoassay with an enzyme-linked immunosorbent assay (ELISA) (Zeus Scientific, Inc., Raritan, NJ, USA) and an immunofluorescent assay (IFA) (Gull Laboratories, Inc., Salt Lake City, UT, USA). Four hundred fresh, unfrozen clinical samples from pregnant women ( n = 154), HIV positive patients ( n = 41), and patients in whom infection with Toxoplasma gondii was suspected ( n = 200) were collected and assayed over a three month period. The specificity of the Access assay was compared to the consensus results. Results that were discrepant between the ELISA and IFA were resolved using a third IFA (Zeus). Once resolved, the specificity for the Access assay, the Zeus ELISA and the Gull IFA were 99.22%, 97.91%, and 99.45%, respectively. We conclude that the Access assay specificity is comparable to consensus results, minimizing false positive results; and because it is a random access instrument, it may be preferable over batch methods.