RO 23–9424, a new cephalosporin 3'-quinolone: in-vitro antimicrobial activity and tentative disc diffusion interpretive criteria

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Abstract
The susceptibility of 410 clinical bacterial isolates to RO 23-9424, a novel cephalo sporin 3'-quinolone, was determined. Overall, 97% of Enterobacteriaceae and 100% of staphylococci were inhibited by ≤8·0 mg/L of RO 23-9424. Only 60% of Pseudomonas aeruginosa and 80–90% of Pseudomonas spp. and Xanthomonas maltophilia were inhibited by this concentration. Enterococci and Listeria monocytogenes were resistant to RO 23-9424. Clinical isolates of Moraxella catarrhalis, Streptococcus spp., and Corynebacterium jekeium were all susceptible to ≤8·0 mg/L of RO 23-9424. This drug's antimicrobial activity was superior to that of its two components fleroxacin and desacetylcefotaxime against the organisms tested. Using ≤8·0 mg/L and ≥32 mg/L respectively as the susceptible and resistant MIC breakpoints for RO 23-9424, the regression analysis-derived disc diffusion zone diameter breakpoints for the 30 μg disc are: susceptible ≥19 mm, intermediate 16–18 mm, and resistant ≤15 mm.

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