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The postantibiotic effect (PAE) is a pharmacodynamic parameter that contributes to the choice of antibiotic dosing regimens. It is defined as length of time that bacterial growth is suppressed following brief exposure to an antibiotic (2-4, 10). Odenholt-Tornqvist and coworkers have suggested that during intermittent dosage regimens, suprainhibitory levels of antibiotic are followed by subinhibitory levels that persist between doses, and they have hypothesized that persistent sub-MICs extend the PAE (14, 15). The effect of sub-MICs on growth during the PAE period has been defined as the postantibiotic sub-MIC effect (PA-SME), which represents the time interval that includes the PAE plus the additional time during which growth is suppressed by sub-MICs. In contrast to the PA-SME, SME measures the direct effect of sub-MICs on cultures which have not been previously exposed to antibiotics (14, 15).

We examined the PAE, PA-SME, and SME of gatifloxacin, a fluoroquinolone with a wide spectrum of activity (1, 5-9, 16, 17), against two strains each of penicillin-susceptible, -intermediate, and -resistant Streptococcus pneumoniae; one Enterococcus faecalis strain; one methicillin-susceptible Staphylococcus aureus strain; two methicillin-resistant Staphylococcus aureus strains (for both of which gatifloxacin MICs were 1.0 µg/ml, but of which one strain was susceptible and one was resistant to ciprofloxacin); one Escherichia coli strain; and one Pseudomonas aeruginosa strain. Organisms were identified by standard methods (11).

Standard broth microdilution MIC methodology (13) was used. The PAE was determined by the viable plate count method (4) with Mueller-Hinton broth supplemented with 5% lysed horse blood when pneumococci were tested. The PAE was induced by exposure for 1 h to concentrations that were 10 times the MICs for all strains except the ciprofloxacin-resistant S. aureus isolate and the P. aeruginosa isolate, for which, to approximate achievable levels in serum (12), concentrations 4 times the MICs were used. Tubes containing 5 ml of broth with antibiotic were inoculated with approximately 5 × 10⁶ CFU/ml. After 1 h of exposure, gatifloxacin at 10 times the MIC reduced the starting inoculum by approximately 1 to 2 log₁₀ units. Growth controls with inoculum but no antibiotic were included with each experiment. Tubes were placed in a shaking water bath at 35°C for 1 h. At the end of the exposure period, cultures were diluted 1:1,000 to remove antibiotic. A control containing bacteria preexposed to antibiotic at a concentration 0.01 times the MIC was also prepared.

Viability counts were determined before exposure and immediately after dilution (zero time), and then every two hours until tube turbidity reached a no. 1 McFarland standard. Inocula were prepared by suspending the overnight growth from a blood agar plate in broth. The broth was incubated at 35°C for 2 to 4 h in a shaking water bath until turbidity matched a no. 1 McFarland standard (diluted as required for inocula), and bacteria were checked for viability by plate counting (4).

The PAE was defined according to the formula PAE = T  C, where T is the time required for viability counts of an antibiotic-exposed culture to increase by 1 log₁₀ unit above counts taken immediately after dilution and C is the corresponding time for the growth control (4).

In cultures designated for the determination of PA-SME, the PAE was induced as described above, after exposure to concentrations 4 or 10 times the MICs (see above). Following dilution to 1:1,000, cultures were divided into four tubes. To three tubes, gatifloxacin was added to make final subinhibitory concentrations of 0.2, 0.3, and 0.4 times the MIC. The fourth tube did not receive antibiotic. Viability counts were determined before exposure, immediately after dilution, and then every two hours until their turbidity reached a no. 1 McFarland standard. The PAE was not induced in cultures designated for determination of SME.

The PA-SME was defined according to the formula PA-SME = T_pa  C, where T_pa is the time for cultures previously exposed to antibiotic and then reexposed to different sub-MICs to increase by 1 log₁₀ unit above counts taken immediately after dilution and C is the corresponding time for the unexposed control (14, 15). The SME was defined according to the formula SME = T_s  C, where T_s is the time for the cultures exposed only to sub-MICs to increase 1 log₁₀ unit above counts taken immediately after dilution and C is the corresponding time for the unexposed control. The PA-SME and SME (14, 15) were measured in two separate experiments. For each experiment, viability counts (log₁₀ CFU/ml) were plotted against time and results are expressed as the means of results from two separate assays.

All gatifloxacin pneumococcal MICs were 0.25 µg/ml. Staphylococcal gatifloxacin MICs were 0.06 µg/ml for the methicillin-susceptible strain. For the ciprofloxacin-susceptible (0.5 µg/ml) and ciprofloxacin-resistant (16.0 µg/ml) methicillin-resistant strains, gatifloxacin MICs were 0.06 and 1.0 µg/ml, respectively. The Enterococcus faecalis gatifloxacin MIC was 0.25 µg/ml. The gatifloxacin MIC for E. coli was 0.016 µg/ml, and that for P. aeruginosa was 1.0 µg/ml.

Results are presented in Table 1. Antibiotic at 0.01 times the MIC had no activity. The mean PAE for the six pneumococci was 1.8 h, ranging between 1.2 and 4.0 h. Pneumococcal PA-SMEs were slightly longer than PAEs. At 0.4 times the MIC, PA-SMEs were 3.7 to 8.6 h, with a mean of 6.9 h. Pneumococcal PA-SMEs approximated the sum of PAE and SME, indicating that sub-MICs alone accounted for the slightly longer PA-SMEs.

Staphylococcal PAEs were 1.0 to 2.0 h, with a mean of 1.4 h. Staphylococcal PAEs did not differ significantly in ciprofloxacin-susceptible and -resistant methicillin-resistant S. aureus strains. PA-SMEs were longer than PAEs plus SMEs. PA-SMEs at 0.4 times the MIC ranged from 2.3 to 3.8 h, with a mean of 3.3 h. The Enterococcus faecalis strain had a PAE of 0.5 h and a PA-SME at 4 times the MIC of 1.6 h.

For E. coli and P. aeruginosa, PAEs were 4.8 h and 2.2 h, respectively. For E. coli, PA-SMEs were longer than the PAE plus the PA-SME at 0.2 times the MIC; at 0.3 and 0.4 times the MIC, all SMEs and PA-SMEs were 9.4 h. For the strain of P. aeruginosa, the PA-SME at 0.4 times the MIC was 4.4 h.

Gatifloxacin MICs were similar to those described previously (1, 5-9, 16, 17). Gatifloxacin, like other quinolones, exhibits rapid concentration-dependent bactericidal activity. Longer intervals between doses may be possible when an antibiotic has a long half-life as well as a prolonged PAE and PA-SME, because regrowth continues to be prevented when levels in serum and tissue fall below MICs (2, 4, 10).

PA-SMEs exceeded the PAE plus the SME at 0.2 times the MIC for the E. coli strain, indicating that, for this organism, gatifloxacin sub-MICs had a greater effect on preexposed than on unexposed cultures. The results with unexposed cultures require confirmation by examination of more strains. Therefore, a longer PAE can be achieved by sub-MICs of gatifloxacin when they follow a suprainhibitory level (14, 15). In this study, preexposure concentrations of 4 and 10 times the MIC were within clinically achievable gatifloxacin levels for all strains (12), indicating that concentrations in serum would exceed the MICs for the entire recommended 24-h dosing interval. Our results suggest that a longer dosing interval may be possible for strains with a PAE and PA-SME, because bacterial regrowth would be prevented when levels in serum fall below the MICs.