Evaluation of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of Enterobacteriaceae.

We evaluated the accuracy of the BD Phoenix system for the identification (ID) and antimicrobial susceptibility testing (AST) of 251 isolates of the family Enterobacteriaceae representing 31 species. Organisms were inoculated onto the Phoenix panel according to the manufacturer's instructions. The results from conventional biochemical tests were used for the reference method for ID. Agar dilution, performed according to the CLSI guidelines, was the reference AST method. Essential and categorical agreements were determined. The overall levels of agreement for the genus- and species-level identifications were 95.6% and 94.4%, respectively. Fourteen isolates were incorrectly identified by the Phoenix system; 10 of these were incorrectly identified to the species level. Three of these were Enterobacter (Pantoea) species and four of these were Shigella spp. misidentified as Escherichia coli. For AST results, the essential and categorical agreements were 98.7% and 97.9%, respectively. The very major error, major error, and minor error rates were 0.38%, 0.33%, and 1.8%, respectively. Six isolates (three E. coli isolates and three Klebsiella isolates) were extended-spectrum beta-lactamase producers. All six were flagged by the Phoenix system expert rules. The Phoenix system compares favorably to traditional methods for ID and AST of Enterobacteriaceae.

Evaluation of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of staphylococci and enterococci.

We evaluated the Phoenix automated microbiology system (BD Diagnostic Systems, Sparks, MD) for the identification (ID) and antimicrobial susceptibility testing (AST) of challenge and clinical staphylococci and enterococci recovered from patients in a tertiary-care medical center. In total, 424 isolates were tested: 90 enterococci; 232 Staphylococcus aureus isolates, including 14 vancomycin-intermediate S. aureus isolates; and 102 staphylococci other than S. aureus (non-S. aureus). The Phoenix panels were inoculated according to the manufacturer's instructions. The reference methods for ID comparisons were conventional biochemicals and cell wall fatty acid analysis with the Sherlock microbial identification system (v 3.1; MIDI, Inc. Newark, DE). Agar dilution was the reference AST method. The overall rates of agreement for identification to the genus and the species levels were 99.7% and 99.3%, respectively. All S. aureus isolates and enterococci were correctly identified by the Phoenix panels. For the non-S. aureus staphylococci, there was 98.0% agreement for the ID of 16 different species. The AST results were stratified by organism group. For S. aureus, the categorical agreement (CA) and essential agreement (EA) were 98.2% and 98.8%, respectively. Three of three very major errors (VMEs; 1.7%) were with oxacillin. For non-S. aureus staphylococci, the CA, EA, VME, major errors, and minor error rates were 95.7%, 96.8%, 0.7%, 1.7%, and 2.9%, respectively. The two VMEs were with oxacillin. For the enterococci, there was 100% CA and 99.3% EA. All 36 vancomycin-resistant enterococci were detected by the Phoenix system. The Phoenix system compares favorably to traditional methods for the ID and AST of staphylococci and enterococci.

Antimicrobial-resistant Shigella sonnei: limited antimicrobial treatment options for children and challenges of interpreting in vitro azithromycin susceptibility.

BACKGROUND: Antimicrobial-resistant Shigella sonnei is a growing problem in the United States and poses treatment challenges particularly among children. Azithromycin is recommended as an alternative oral agent for shigellosis. METHODS: All isolates of Shigella submitted to Johns Hopkins clinical laboratory during the outbreak year (2002) were compared with a historical comparison group (1996-2000). Isolates were considered multiresistant if they were resistant to ampicillin and trimethoprim-sulfamethoxazole (TS). Selected outbreak and reference isolates were tested for azithromycin susceptibility by E-test, disk diffusion and broth dilution methods. RESULTS: Between 1996-2000, among the 111 isolates submitted, 63% were from pediatric patients; 63% of isolates were resistant to ampicillin and 12% to TS. In 2002, among the 205 isolates submitted, 82% were from pediatric patients; 91% isolates were resistant to ampicillin and 67% to TS. The proportion of multiresistant isolates increased from 6% in 1996 to 65% in 2002 (P < 0.05). Azithromycin susceptibility by E-test and disk diffusion demonstrated 2 zones of inhibition for S. sonnei. Interpretation using the inner zone resulted in higher MICs (minimal inhibitory concentration) compared with the outer zones by E-test (P < 0.0001) and disk diffusion (P < 0.0001). CONCLUSIONS: With increasing interest in using azithromycin for shigellosis, clinical laboratories should be aware of the interpretation difficulty caused by the dual-zone phenomenon seen with E-test and disk diffusion methods for S. sonnei.