No abstract

The fluoroquinolones ciprofloxacin, norfloxacin, and ofloxacin were examined in vitro against 103 enterococcal isolates and 138 isolates of Pseudomonas aeruginosa, in an attempt to determine which respective proposed interpretive criteria (minimal inhibitory concentration (MIC) versus diameter of agar disk diffusion inhibition zones) correlated best, i.e., resulted in fewer discrepant results. A previously proposed system for grouping discrepant results (very major, major, and minor discrepancies) of Barry and co-workers was modified to comprise 6 categories (very major, major, minor, slight, minimal, and negligible); this expanded system rendered the encountered discrepant in vitro test results more transparent. Overall, the currently employed interpretive criteria for norfloxacin (MICs versus 10-μg disks) resulted in fewer discrepancies with the above two groups of bacterial isolates than those proposed for ciprofloxacin and ofloxacin (MICs versus 5-μg disks, respectively).

Polymyxin B (PB) and polymyxin B nonapeptide (PBNP), when combined with rifampin or novobiocin, but not vancomycin, yielded additive inhibitory effects against test strains of Serratia marcescens of three varieties: those that produced cocarde growths around PB disks (coc⁺); those that grew adjacent to PB disks (coc^-, 6); and those that yielded clear inhibition zones around PB disks (coc^-, clear). However, time kill curve experiments disclosed that only the combination of rifampin + PB exerted a potent bactericidal effect against coc⁺ strains of S. marcescens; rifampin + PBNP and novobiocin + PB or PBNP merely effected transient decreases of colony counts. Assays involving 50% (v/v) of fresh defibrinated human blood + PB or PBNP revealed that only PB clearly augmented the antibacterial activity of blood against coc⁺, and less so against coc^- test strains of S. marcescens.

Twelve multiple-drug-resistant isolates of coagulase-negative staphylococci from diverse clinical sources, which had yielded cocarde growths (double zones of inhibition) around 10 μg imipenem disks, were identified as Staphylococcus haemolyticus. This was the only staphylococcal species which exhibited this unusual phenotypic trait at our institution during a 7-month observation period.