Rapid Identification of Gram-Negative Bacteria with and without CTX-M Extended-Spectrum β-Lactamase from Positive Blood Culture Bottles by PCR Followed by Microchip Gel Electrophoresis

We evaluated the usefulness of PCR analysis of the 16S-23S rRNA gene internal transcribed spacer (ITS) and the CTX-M extended-spectrum beta-lactamase (ESBL) followed by microchip gel electrophoresis (MGE) for direct identification and CTX-M detection of Gram-negative bacteria (GNB) from positive blood culture bottles. Of 251 GNB isolated from blood cultures containing a single bacterium, 225 (90%) were correctly identified at the species level directly from positive blood culture bottles by comparing the ITS-PCR patterns of the sample strain with those of the control strains. There were no cases of incorrect identification. Limitations encountered included the inability to detect mixed cultures (four bottles) as well as some species (Enterobacter species and Klebsiella oxytoca) demonstrating identical ITS-PCR patterns. A total of 109 ESBL-producing isolates from various clinical materials obtained between January 2005 and December 2008 were examined for bla(CTX-M), bla(SHV), and bla(TEM) genes by PCR and sequences of PCR products. CTX-M ESBL was detected in 105 isolates, and SHV ESBL was detected in two isolates. The remaining two isolates (K. oxytoca) were shown to harbor bla(OXY). Twenty (19%) of 104 Escherichia coli isolates from blood cultures were suspected to produce ESBL by the combination disk method, and these isolates were shown to harbor CTX-M ESBL by PCR-MGE. The results were obtained within 1.5 h at a calculated cost of $6.50 per specimen. In conclusion, simultaneous detection of ITS length polymorphisms and bla(CTX-M) by single PCR followed by MGE is useful for rapid, cost-effective, and reliable species-level identification of CTX-M ESBL-producing GNB responsible for bloodstream infections.