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Evaluation of Phoenix Automated Microbiology System for Detecting Extended-Spectrum ß-Lactamases among Chromosomal AmpC-producing Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, and Serratia marcescens

Address correspondence to Yeon-Joon Park, M.D., Department of Laboratory Medicine, College of Medicine, The Catholic University of Korea, Kangnam St. Mary’s Hospital, 505 Banpo-dong, Seocho-ku, Seoul 137-701, Korea; tel 82 2 590 1604; fax 82 2 592 4190; e-mail yjpk{at}catholic.ac.kr.

We evaluated the BD Phoenix Extended-Spectrum ß-Lactamase (ESBL) detection test among chromosomal AmpC-producing Enterobacter cloacae, Enterobacter aerogenes, Citrobacter freundii, and Serratia marcescens. The study was conducted on 72 non-repetitive ESBL producers (33 E. cloacae, 13 E. aerogenes, 14 C. freundii, and 12 S. marcescens) and 77 ESBL non-producers (33 E. cloacae, 9 E. aerogenes, 6 C. freundii, and 29 S. marcescens). The organisms were selected as suspected ESBL-producers based on the double disk synergy test and confirmed by PCR amplification of blaTEM-1, blaSHV-1, blaCTX-M-1, blaCTX-M-2, and blaCTX-M-9. The Phoenix ESBL test, using a 5-well confirmatory test and the BDXpert system, was evaluated. Of the 72 isolates identified as ESBL-producers based on the DDST, 46 isolates harbored CTX-M-type enzymes, 21 harbored TEM type enzymes, and 31 harbored SHV enzymes. The Phoenix system identified ESBL only in 15 isolates. Of the 77 ESBL non-producers, ths Phoenix system identified ESBL in 4 isolates, 3 of which were confirmed to be ESBL-producers. In this study, the Phoenix system was highly specific (76/77, 98.7%), and it identified 3 additional ESBL-producers that were not detected by DDST. However, the Phoenix system’s sensitivity was very low (15/72, 20.8%). Considering the increasing prevalence of ESBL production among AmpC-producers, the BD Phoenix system could not be considered a reliable stand-alone ESBL detection method for the strains tested in our study.

Keywords: Phoenix System, AmpC, extended-spectrum ß-lactamases, Gram-negative bacilli