The Phoenix automated microbiology system (BD Diagnostics, Sparks, MD) is designed for the rapid identification (ID) and antimicrobial susceptibility testing (AST) of clinically significant human bacterial pathogens. We evaluated the performance of the Phoenix instrument in comparison with that of the MicroScan WalkAway system (Dade Behring, West Sacramento, CA) in the ID and AST of gram-negative clinical strains and challenge isolates of Enterobacteriaceae (n ‫؍‬ 150) and nonfermentative gram-negative bacilli (NFGNB; 45 clinical isolates and 8 challenge isolates). ID discrepancies were resolved with the API 20E and API 20NE conventional biochemical ID systems (bioMerieux, Durham, NC). The standard disk diffusion method was used to resolve discordant AST results. The overall percentages of agreement between the Phoenix ID results and the MicroScan results at the genus and species levels for clinical isolates of Enterobacteriaceae were 98.7 and 97.7%, respectively; following resolution with conventional biochemical testing, the accuracy of the Phoenix system was determined to be 100%. For NFGNB, the levels of agreement were 100 and 97.7%, respectively. Both systems incorrectly identified the majority of the uncommon nonfermentative nonpseudomonal challenge isolates recovered from cystic fibrosis patients; these isolates are not included in the databases of the respective systems. For AST of Enterobacteriaceae, the rate of complete agreement between the Phoenix results and the MicroScan results was 97%; the rates of very major, major, and minor errors were 0.3, 0.2, and 2.7%, respectively. For NFGNB, the rate of complete agreement between the Phoenix results and the MicroScan results was 89.1%; the rates of very major, major, and minor errors were 0, 0.5, and 7.7%, respectively. Following the confirmatory testing of nine clinical isolates initially screened by the MicroScan system as possible extended-spectrum-␤-lactamase (ESBL)-producing organisms (seven Klebsiella pneumoniae isolates and two Escherichia coli isolates), complete agreement was achieved for eight isolates (one ESBL positive and seven negative); one false positive was obtained with the Phoenix instrument. The MicroScan system correctly detected the 10 ESBL challenge isolates, versus the 6 detected by the Phoenix system. Overall, there was good correlation between the Phoenix instrument and the MicroScan system for the ID and AST of Enterobacteriaceae and common NFGNB. The Phoenix system is a reliable method for the ID and AST of the majority of clinical strains encountered in the clinical microbiology laboratory. Until additional performance data are available, results for all Klebsiella pneumoniae or Klebsiella oxytoca and E. coli isolates screened and confirmed as ESBL producers by any automated system should be confirmed by alternate methods prior to the release of final results.Clinical microbiology laboratories are under increasing pressure and scrutiny from clinicians and administrators to provide rapid, accurate, and timely bacterial identifi...

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ᰔAmpC ␤-lactamases can interfere with extended-spectrum-␤-lactamase (ESBL) confirmatory tests. Resulting failures to detect ESBLs can endanger patients because false susceptibility to cephalosporins may be reported (5). This problem occurs with CLSI and some other ESBL confirmatory tests, but there are tests that can be used to provide more accurate detection of ESBLs in AmpC-producing isolates (4, 5). While inaccurate susceptibility reports are never acceptable, it is of interest to know whether it is common for a lab to encounter ESBL-screen-positive, AmpC-producing isolates that yield negative CLSI confirmatory tests. That is, can it be common to encounter negative ESBL confirmatory test results of unknown accuracy? Apart from a small Escherichia coli study in which 20 of 26 cefpodoxime ESBLscreen-positive, cefoxitin-resistant E. coli isolates produced a plasmid-mediated AmpC ␤-lactamase (4), little is known about how often AmpC production creates uncertainty about the accuracy of CLSI ESBL confirmatory tests. Therefore, a study was conducted at a Louisville, KY, teaching hospital to determine how many ESBL-screen-positive isolates of E. coli, Klebsiella pneumoniae, Klebsiella oxytoca, and Proteus mirabilis were AmpC positive and ESBL confirmatory test negative by CLSI methodology.During the period of 1 May 2006 to 28 February 2007, 952 isolates from patients at the University of Louisville Hospital, Louisville, KY, were analyzed. They comprised 682 E. coli, 152 K. pneumoniae, 44 K. oxytoca, and 70 P. mirabilis isolates. ESBL screening was performed by using CLSI microdilution methodology using cefotaxime, ceftriaxone, ceftazidime, cefpodoxime, and aztreonam with positive isolates then tested by the CLSI ESBL disk confirmatory method (2). Screen-positive isolates with negative ESBL confirmatory tests were investigated for AmpC production using a Tris-EDTA-based disk test (1) to determine if they were AmpC producers. If this test was positive, it meant that the negative ESBL confirmatory result was of unknown accuracy. Representative AmpC-positive isolates of K. pneumoniae were investigated for gene identification by PCR using primers specific for bla DHA , bla FOX , bla CMY , bla ENT , bla ACT , and bla ACC (3).Only 13 of the 57 screen-positive isolates (22.8%) were ESBL producers by CLSI methodology, while 40 were AmpC producers (70.2%) ( Table 1). Other resistance mechanisms were assumed to account for the other four positive screens. The screening-positive isolates comprised 37 E. coli, 16 K. pneumoniae, 3 Klebsiella oxytoca, and 1 P. mirabilis. Of these, 8 E. coli isolates (22% of screen-positive E. coli), 4 K. pneumoniae isolates (25% of screen-positive K. pneumoniae), and 1 P. mirabilis isolate were confirmed as ESBL producers. Of the isolates that gave an ESBL-negative result, 28 E. coli (76% of screen-positive isolates) and 12 K. pneumoniae (75% of screen-positive isolates) were AmpC positive. No K. oxytoca or P. mirabilis isolates were AmpC positive. The AmpC-positive K. pneumoniae isolates were assu...

Fifty-nine Staphylococcus aureus isolates from one hundred blood cultures containing gram-positive cocci in clusters were identified by conventional methods and the BD GeneOhm StaphSR assay (SR). The SR misidentified three methicillin (meticillin)-resistant S. aureus (MRSA) isolates as methicillin-susceptible S. aureus (MSSA), while one MSSA isolate tested negative for S. aureus. The three MRSA isolates were strains with MREJ types that cannot be detected by the currently available SR.Both the methicillin (meticillin)-susceptible (MSSA) and methicillin-resistant (MRSA) forms of Staphylococcus aureus are significant causes of bacteremia. Several studies have reported that patients with MRSA bacteremia have higher rates of mortality than patients infected with MSSA (2, 7).The observation of gram-positive cocci in clusters from positive blood cultures provides only a presumptive identification of staphylococci. Confirmatory identification and susceptibility testing of staphylococci recovered from blood cultures require a minimum of two days (one day for culture and one day for identification and results of susceptibility testing).This time dependency delays the administration of pathogen-directed therapy, promotes the unnecessary use of vancomycin, and increases the pressure to select resistant organisms.To provide same-day definitive identification, nucleic acid amplification methods for the differentiation of MSSA and MRSA isolates are now commercially available (5,8). The objective of this study was to evaluate the performance of the BD GeneOhm StaphSR assay (SR; BD GeneOhm, San Diego, CA) for the direct detection and differentiation of MSSA and MRSA isolates in positive blood cultures.Blood cultures (aerobic/anaerobic bottles) submitted to the Clinical Microbiology Laboratory of the University of Louisville Hospital for routine culture were eligible for inclusion in this study. Positive blood cultures (in the Bactec 9240 system; BD Systems) containing gram-positive cocci in clusters visible by microscopy were subcultured (blood and chocolate agar) and identified by standard methods (catalase test, tube coagulase test, Staphaurex test [Remel, Lenexa, KS], mecA analysis [Oxoid, Ltd.], and testing for susceptibility to cefoxitin). Confirmatory identification to the species level and susceptibility testing were performed on the MicroScan Walkaway PC29 Combo panel (Siemens Healthcare Diagnostics, Sacramento, CA). Also, the SR was performed on these cultures in accordance with the manufacturer's instructions.Of the 100 blood cultures which contained gram-positive cocci in clusters, conventional methods identified 41 as non-S. aureus, 23 as MSSA, and 36 as MRSA. The SR produced four discrepant results for the 59 S. aureus isolates. Three MRSA isolates, 8.3% of all MRSA isolates, were misidentified as MSSA by the SR, while one MSSA isolate, 4.3% of all MSSA isolates, tested negative for S. aureus.Isolates that yielded discordant PCR results were tested in duplicate (standard concentration and at a 1:100 dilution) and w...

The current BacT/ALERT SA (BTA SA) aerobic blood culture bottle is made from glass, does not require venting, and contains a liquid emulsion sensor (LES). Its performance has been shown to be equivalent to that of the vented standard aerobic culture bottle. A further-improved version of the BTA SA bottle, designated the BacT/ALERT plastic SA (BTA PSA) culture bottle, is made from clear plastic to prevent breakage, does not require venting, and contains a modified LES (LES 2) to reduce the possibility of false positives. The BTA PSA provides a practical alternative to the current glass version of this bottle. The plastic bottle is also comparable to the current glass bottle in transparency and growth performance and additionally minimizes the exposure to infectious agents due to glass bottle breakage.