Kirby-Bauer disc approximation to detect inducible third-generation cephalosporin resistance in Enterobacteriaceae

Xuan Qin¹ , Scott J Weissman² , Mary Frances Chesnut¹ , Bei Zhang³ and Lisong Shen⁴

¹Microbiology Laboratory, Department of Laboratories and Pathology, Children's Hospital and Regional Medical Center, Seattle, WA 98105, USA

²Division of Infectious Disease, Department of Pediatrics

³Children's Hospital and Regional Medical Center, Seattle, WA 98105, USA

⁴Laboratory Diagnostic Center, Xin Hua Hospital and Shanghai Children's Medical Center, Shanghai Second Medical University, Shanghai, China

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Annals of Clinical Microbiology and Antimicrobials 2004, 3:13doi:10.1186/1476-0711-3-13


Published:	15 July 2004

Abstract

Resistance to β-lactam antibiotics in enteric Gram-negative bacilli may be difficult to detect using standard methods of either Kirby-Bauer disc diffusion (KBDD) or broth dilution for minimal inhibitory concentration (MIC). This difficulty is due to genetic differences in resistance determinants, differences in levels of gene expression, and variation in spectra of enzymatic activity against the substrate β-lactams used for susceptibility testing. We have examined 95 clinical isolates reportedly susceptible to ceftazidime and ceftriaxone, as originally determined by either KBDD or MIC methods. The organisms studied here were isolated in 2002 from two pediatric hospital centers (Seattle, USA and Shanghai, China). They belong to the inducible β-lactamase producing Gram-negative bacilli, such as Enterobacter spp., Citrobacter spp., Serratia spp., Morganella spp., Providencia spp., and Proteus vulgaris. A Kirby-Bauer disc approximation (KBDA) method identified inducible phenotypes of third-generation cephalosporin resistance in 76% of isolates, which would otherwise be considered susceptible by standard KBDD methods.