Comparison of the Phoenix automated system, the Etest method and broth microdilution in determining temocillin susceptibility of Enterobacteriaceae

Sir, Temocillin, a semi-synthetic 6-a-methoxy derivative of ticarcillin, has been shown to be clinically efficacious in infections caused by extended-spectrum b-lactamase (ESBL)and AmpC-producing Enterobacteriaceae. It is licensed in the UK for bacteraemia, urinary tract infections (UTIs) and lower respiratory tract infections where susceptible Gram-negative bacteria are suspected or confirmed. Currently, only the BSAC has defined temocillin breakpoints for Enterobacteriaceae (susceptible if MIC ≤8 mg/L for systemic infections and ≤32 mg/L for UTIs). Here, we report a comparison of three temocillin susceptibility testing methodologies [BD PhoenixTM Automated Microbiology System (instrument version 5.15A, software version 6.01A/V5.15A) (Becton Dickinson, Oxford, UK), Etest (AB Biodisk, Solna, Sweden) and broth microdilution (BMD)]. Our data indicate that, whereas the level of agreement for ‘susceptible’ results is excellent, the Phoenix system overcalls temocillin non-susceptibility. A total of 281 consecutive clinical Enterobacteriaceae isolates from distinct patients were collected from urine, blood culture, fluid, respiratory and tissue specimens. Isolates comprised Escherichia coli (194), Klebsiella pneumoniae (19), Proteus mirabilis (16), Enterobacter cloacae (9), Serratia marcescens (7), Citrobacter koseri (6), Enterobacter aerogenes (5), Citrobacter freundii (4), Morganella morganii (4), Klebsiella oxytoca (3), Proteus vulgaris (3), Citrobacter farmeri (2), Pantoea agglomerans (2), Serratia liquefaciens (2), Citrobacter braakii (1), Enterobacter gergoviae (1), Kluyvera sp. (1), Providencia rettgeri (1) and Raoultella ornithinolytica (1). Twenty-seven (9.6%) were ESBL producers, 16 (5.7%) were inducible AmpC b-lactamase producers and 8 (2.8%) were derepressed AmpC b-lactamase producers. Identification and susceptibility testing were performed on overnight cultures using the BD PhoenixTM AP instrument (automated nephelometry) and the BD PhoenixTM Automated Microbiology System (instrument version 5.15A, software version 6.01A/ V5.15A) with Gram-negative Phoenix panels (NMIC-84). Temocillin MICs were also determined by BMD in cation-adjusted Mueller– Research letters