Accurate characterization of ofloxacin susceptibility with Enterobacteriaceae using a modified GNS F6 card and the bioMerieux Vitek System.

Routine antimicrobial susceptibility testing of Enterobacteriaceae using the Vitek System (bioMerieux Vitek, Hazelwood, MO) and the GNS-F6 card revealed discrepancies between the activity of ofloxacin and ciprofloxacin, primarily with isolates of Klebsiella pneumoniae. Specifically, during a one-year period, 12% of 618 ciprofloxacin-susceptible isolates were determined to be ofloxacin resistant with the GNS-F6 card. A similar problem, but one of lower magnitude, was observed with Serratia marsescens. That these represented false ofloxacin resistance results was confirmed by comparison of broth microdilution determinations of ofloxacin MICs with F6 results on a collection of 203 fresh clinical isolates of K. pneumoniae and 39 isolates of S. marsescens. The GNS-F6 card was then modified by the manufacturer to include a new formulation of ofloxacin and assessed using a collection of 224 recent clinical isolates of Enterobacteriaceae and Pseudomonas aeruginosa, and 78 stock cultures of enteric Gram-negative bacilli selected specifically because of disproportionately high rates of fluoroquinolone resistance. No ofloxacin false resistant results were observed with this collection when the modified GNS-F6 card was evaluated in comparison to a standardized broth microdilution MIC test. The current clinical version of the Vitek System appears to accurately assess ofloxacin susceptibility.