Influence of medium and method on the in vitro susceptibility of Pseudomonas aeruginosa and other bacteria to ciprofloxacin and enoxacin

Abstract

Ciprofloxacin and enoxacin were two-to fourfold less active against Pseudomonas aeruginosa in calcium-and magnesium-supplemented broth compared with unsupplemented broth regardless of inoculum size, presence of serum, or use of inhibitory or bactericidal endpoints (P < 0.01). The effect of cation supplementation was less pronounced and less consistent for Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus.Several new fluorinated piperazinyl quinolone compounds have been shown to have potent antibacterial effects, most notably against gram-negative bacilli (7). However, available pharmacokinetic and microbiological data suggest that the differences between achievable serum or tissue concentrations in vivo and inhibitory or bactericidal concentrations for certain pathogens (e.g., Pseudomonas aeruginosa) may be small. Therefore, relatively small changes in activity found in tests performed by standardized methods in media that more accurately reflect in vivo conditions could be important. The objective of this study was to evaluate the in vitro activities of two new quinolone agents, ciprofloxacin (Miles Pharmaceuticals, West Haven, Conn.) and enoxacin (Warner-Lambert Pharmaceuticals, Ann Arbor, Mich.) in four different media using standardized susceptibility-testing methods.Broth dilution studies were conducted by the microtechnique with two inoculum sizes (8, 11). Three liquid media were used: Mueller-Hinton broth (MHB; Difco Laboratories, Detroit, Mich.), MHB supplemented with calcium (2.5 mM/liter) and magnesium (1.1 mM/liter) (MHB-S), and MHB-S combined 1:1 with fresh heat-inactivated human serum (MHB-S/HS) (18 transformed values) in each medium, the magnitude of increase in MIC or MBC, and the ratios of MBC to MIC for each drug in MHB-S and MHB-S/HS were compared with data obtained in MHB by using the Wilcoxon signed-rank test for matched pairs. Differences were considered significant at the P < 0.05 level for a two-tailed test. The activities of ciprofloxacin and enoxacin against nine strains of P. aeruginosa in the four media tested are summarized in Table 1. Cation supplementation of MHB with or without human serum significantly reduced the activities of both drugs (P < 0.01). MICs and MBCs showed a median two-to fourfold increase at both inoculum sizes tested. The magnitudes of reduction of activity in supplemented broth were similar for ciprofloxacin and enoxacin (P > 0.05). The median MICs obtained by the agar dilution technique were two times higher than in MHB for ciprofloxacin (P < 0.05), whereas no significant difference was found with enoxacin (Table 1).Both quinolones were highly active against non-Pseudomonas strains, with all strains inhibited by 1 ,ug of ciprofloxacin and 2 ,ug of enoxacin per ml. The results obtained with the laboratory reference strains when tested with the larger inocula are presented in Table 2. Compared with P. aeruginosa, the effect of cation supplementation on the activities of the quinolones was quantitatively less pronounced and less consistent among th...

Mechanisms of quinolone resistance in Escherichia coli: characterization of nfxB and cfxB, two mutant resistance loci decreasing norfloxacin accumulation

“…Decreases in pH and increases in magnesium concentration are known to increase the MICs of norfloxacin and other quinolones (4,7,40). These conditions resulted in a fourfold increase in the MIC and also produced 2-to 2.5-fold decreases in the plateau level of norfloxacin accumulation ( Table 4), suggesting that the effects of these conditions on the MIC result from diminished drug accumulation.…”

Resistance to pefloxacin in Pseudomonas aeruginosa

“…Ca2' and Mg2' have an antagonistic effect on the in vitro activity of quinolones in P. aeruginosa (1,4,5,44). It has been hypothesized that quinolones can disorganize the OM structure (16) by chelating membranebound Mg2+ (9), further facilitating the entry of quinolones in a self-promoted pathway like the one described previously for the aminoglycosides (15).…”

Comparative study with enoxacin and netilmicin in a pharmacodynamic model to determine importance of ratio of antibiotic peak concentration to MIC for bactericidal activity and emergence of resistance

“…Enoxacin was administered ql2h (peak concentration, 3 ,ug/ml) or q24h (peak concentration, 6 ,ug/ml).…”