Bactericidal activity of levofloxacin against Mycoplasma pneumoniae

527 percentiles calculated.2 Applying the BSAC formula4 to pharmacokinetic data (Cmax of ∼3.2 mg/L following an oral dose of 250 mg with a terminal half-life of 7–8 h), an MIC susceptible breakpoint (BP) of 0.4 mg/L was calculated. MIC data were reviewed to ascertain the ABT-492 MIC ranges for the ‘wild sensitive’ populations. For Enterobacteriaceae, M. catarrhalis, haemolytic streptococci, S. milleri, N. meningitidis and E. faecalis, ABT-492 MICs for the population lacking a mechanism of resistance were 0.015–0.12 mg/L, 0.004–0.06 mg/L, 0.008–0.06 mg/L, 0.004–0.06 mg/L, 0.001 mg/L and 0.06–2 mg/L, respectively. In the case of staphylococci, the MIC range for the susceptible population was 0.001–0.015 mg/L. In contrast, for the 25 MRSA isolates that had reduced susceptibility to ciprofloxacin (ciprofloxacin MIC range 64–>128 mg/L), ABT-492 MICs ranged from 0.25 to 1 mg/L. For S. pneumoniae, ABT-492 MICs ranged between 0.002 and 0.015 mg/L except for the three isolates of S. pneumoniae with ciprofloxacin MICs of 128 mg/L. These organisms had corresponding ABT-492 MICs of 0.03 mg/L. In the case of H. influenzae, MICs for the susceptible population were between 0.001 and 0.004 mg/L. For the isolate with no zone of inhibition to a nalidixic acid 30 μg disc (ciprofloxacin MIC 0.06 mg/L), the ABT-492 MIC was 0.004 mg/L and therefore indistinguishable from the susceptible population. Five isolates of N. gonorrhoeae with reduced susceptibility to the quinolones (no zone of inhibition to a nalidixic acid 30 μg disc) had ABT-492 MICs between 0.03 and 0.25 mg/L. For the organisms lacking a mechanism of resistance, MICs ranged between 0.001 and 0.002 mg/L. For all isolates except E. faecalis, zones for disc contents of 2 and 5 μg were unacceptably large, susceptible organisms having zones larger than 40 mm (data not shown). Zone diameter data for a 1 μg disc were therefore analysed for all genera except E. faecalis, looking at the populations lacking a mechanism of resistance. A summary of MIC and zone diameter BPs for interpreting susceptibility and expected MIC and zone diameter ranges for the control strains is shown in Table 1. Although not the drug of first choice, quinolones have been used to treat enterococcal urinary tract infections.5 In this case, an MIC BP of 0.4 mg/L would seem inappropriate as concentrations of ABT-492 in urine are significantly higher than those found in blood. Recommendations are therefore given based on microbiological BPs and a disc content of 5 μg (Table 1). These data indicate that an ABT-492 disc content of 1 μg is the most appropriate concentration for determining susceptibility by BSAC methodology except for E. faecalis where a 5 μg disc is more suitable. For the detection of low-level quinolone resistance in H. influenzae and Neisseria spp., it would be prudent for a 30 μg nalidixic acid disc to be used (recommended by the BSAC).