Extended-spectrum Beta-lactamase detection with different panels for automated susceptibility testing and with a chromogenic medium.

Infections caused by extended-spectrum beta-lactamase (ESBL)- and ampC beta-lactamase-producing gram-negative bacteria complicate therapy and limit treatment options. Several different panels for ESBL detection with automated systems exist. In addition, a chromogenic agar medium is available for ESBL screening. We compared two automated identification and susceptibility testing systems with regard to their effectiveness in detecting ESBL production in Enterobacteriaceae: the BD Phoenix system (BD Diagnostic Systems, Sparks, MD) and the Vitek 2 system (bioMerieux, Marcy l'Etoile, France). We tested 114 strains using the Etest as the standard, various available panels for both automated systems (for BD Phoenix, the NMIC/ID-50 and NMIC/ID-70 GN Combo panels for combined identification and susceptibility testing of gram-negative bacilli, and for Vitek 2, the ID-GNB panel for identification of gram-negative bacilli and the AST-N020, AST-N041, and AST-N062 panels for susceptibility testing), and a chromogenic agar medium (bioMérieux, Marcy l'Etoile, France). PCR for common ESBL gene families (encoding TEM, SHV, OXA, and CTX-M) and for chromosomal or plasmid-mediated ampC beta-lactamase genes was conducted to complete the study design. For the tested specimens overall, the chromID ESBL agar showed the highest sensitivity (95.8%) but the lowest specificity (10.5%) compared to the sensitivity and specificity of the Etest (chosen as reference by the authors) for the detection of ESBL-producing strains. The BD Phoenix system showed sensitivities of 77.1% and 84.2% and specificities of 61.5% and 75.0%, respectively, for the NMIC/ID-50 andNMIC/ID-70 panels. The sensitivity of the Vitek 2 system ranged from 78.8% (AST-N020) to 80.6% (AST-N062) and up to 84.2% (AST-N041). The specificities of the respective panels were 50.0% (AST-N041 and AST-N062) and 55.6% (AST-N020). In conclusion, the sensitivities and specificities of ESBL detection by the different methods differ depending on the microorganisms under study.

Comparative study using various methods for identification of Staphylococcus species in clinical specimens.

Coagulase-negative staphylococci (CNS) play a predominant role in nosocomial infections. Rapid, reliable identification of these organisms is essential for accurate diagnosis and prompt effective treatment of these infections. Quite recently, the VITEK 2 g-positive (gram-positive [GP]) identification card (bioMérieux) has been redesigned for greater accuracy in the identification of gram-positive cocci. We compared the BD Phoenix (Becton Dickinson) and VITEK 2 (bioMérieux) automated microbiology systems, using their respective update version cards, and the API ID32 STAPH test. The glyceraldehyde-3-phosphate dehydrogenase (gap) gene-based T-RFLP (terminal restriction fragment length polymorphism) method was used for verifying the results. In total, 86 clinical isolates of CNS and 27 reference strains were analyzed. The results show that for identification of CNS, the automated identification methods using the newest VITEK 2 and BD Phoenix identification cards are comparable. However, API ID32 STAPH revealed more correct results compared to both automated microbiology systems. Despite the increased performance of the phenotypic automated identification systems compared to the former versions, molecular methods, e.g., the gap-based T-RFLP method, still show superior accuracy in identifying Staphylococcus species other than Staphylococcus aureus.