Bactericidal activity of the fluoroquinolone WIN 57273 against high-level gentamicin-resistant Enterococcus faecalis

Abstract

The fluoroquinolone WIN 57273 showed identical bactericidal activities (MBC for 90%v of the strains = 0.25 ug/ml) for bacteremic strains ofEnterococcusfaecalis with and without high-level gentamicin resistance. WIN 57273 was bactericidal in time-kill measurements with highly gentamicin-resistant, ciprofloxacin-susceptible strains of E. faecalis. However, WIN 57273 was indifferent with penicillin for gentamicin-resistant E. faecalis and was not bactericidal for ciprofloxacin-resistant E. faecalis. antimicrobial agent which inhibits visible growth. The MBC is defined as the lowest concentration of antibiotic that kills .99.9% of the original inoculum. For our study, MBCs were determined by subculturing clear culture broths onto antibiotic-free Mueller-Hinton agar plates. Three E. faecalis strains were used for time-kill measurements: LS-31, which demonstrated high-level gentamicin resistance, high-level streptomycin resistance, and ciprofloxacin susceptibility (Gmr Smr Cips); LS-45, which demonstrated high-level gentamicin resistance, streptomycin susceptibility, and ciprofloxacin susceptibility (Gmr Sms Cips); and LS-49, which demonstrated high-level gentamicin resistance, streptomycin susceptibility, and ciprofloxacin resistance (Gmr Sm, Cipr). Time-kill measurements (10) were performed in triplicate with each strain to determine the extents and rates of killing by WIN 57273 alone or with penicillin and by penicillin with and without an aminoglycoside. Antibiotics were tested at the following concentrations: penicillin, 10 U/ml; WIN 57273, 2 jig/ml; gentamicin, 5 ,ug/ml; and streptomycin, 25 p,g/ml. Antibiotic carryover was eliminated by the use of a minimum dilution factor of at least 100. Organisms were diluted in 20 ml of cation-supplemented Mueller-Hinton broth to give a final inoculum of 5 x 105 CFU/ml and incubated with mechanical shaking at 35°C. At 0, 4, 8, and 24 h after inoculation, 0.5-ml samples were taken from the broth and passed through serial 10-fold dilutions. From each dilution, a 0.1-ml sample was spotted and spread over the entire surface of antibiotic-free agar. A 0.1-ml sample of undiluted broth was also directly inoculated to antibioticfree agar. Colonies were counted after 24 h of incubation. The minimum accurately countable bacterial population was 50 CFU/ml. Bactericidal activity in time-kill measurements is defined as a .3 log10 increase in bacterial killing at 24 h of incubation. Synergy is defined as a .2 logl0 increase in killing by a combination of two antibiotics compared with the killing by the active antibiotic alone, with the other antibiotic present in a concentration which does not affect the growth curve of the organism. If the result of using two drugs in time-kill measurements equals the effect of the more active antibiotic used alone (s1 loglo increase), the antibiotic combination is considered to be indifferent (2).The activity of WIN 57273 was compared with those of other fluoroquinolones, as shown in

Comparative Study of Bacteremias Caused by Enterococcus spp. with and without High-Level Resistance to Gentamicin

“…Enterococcal strains with HLRG were also less susceptible to penicillin and ciprofloxacin. This lower degree of susceptibility of enterococcal strains with HLRG to quinolones has been described previously (4,22).…”

Current perspectives on glycopeptide resistance

“…Other compounds showing promise include PD 138312 and PD 140248 (193). Although strains of E. faecalis and E. faecium resistant to these newer quinolones are encountered (300,400), several investigations have included strains resistant to other nonquinolone antimicrobial agents and obtained similar results (105,208,255,289,300,425); in one case, gentamicin-resistant strains were significantly more resistant to the quinolone CP 99219 than gentamicin-susceptible strains (105). Temafloxacin has been withdrawn from clinical development because of unexpected toxicity, and tosufloxacin is not being developed further (193).…”

“…Clinical experience with the newer quinolones against glycopeptide-resistant strains is not reported. Potential problems are the comparatively poor bactericidal activity of some compounds, though not of all quinolones or against all strains of enterococci (289,300,302); the possibility of resistance emerging during treatment (302); and the effect of the trend that has been observed towards ciprofloxacin resistance among multiresistant enterococci (339), which may result in higher MICs and lack of bactericidal activity by the newer quinolones.…”

Improved efficacy with nonsimultaneous administration of first doses of gentamicin and ceftazidime in vitro

“…Culture sample bacterial counts were determined by plating serial 10-fold dilutions on Columbia sheep's blood agar plates. Most samples were diluted by more than 100-fold before plating; dilution by at least 100-fold has been used extensively in other studies as a means of negating antibiotic carryover effect (9,14,15). To exclude a significant antibiotic carryover effect in samples that were not diluted by 100-fold, an additional experiment was performed using a published method (5).…”