Comparison of the in vitro efficacies of moxifloxacin and amoxicillin against Listeria monocytogenes

Abstract

Listeria monocytogenes is a facultative intracellular bacterium that causes severe infections associated with a high mortality rate. Moxifloxacin presents extended activity against gram-positive bacteria and has recently been suggested to be a potential alternative in the treatment of listeriosis. We evaluated the in vitro efficacy of moxifloxacin against L. monocytogenes using a combination of epidemiological and experimental approaches. The median MIC of moxifloxacin for a large collection of L. monocytogenes strains of various origins (human, food, and environment) was 0.5 microg/ml (MIC range, 0.064 to 1 microg/ml). No differences were observed, irrespective of the origin of the strains. Moreover, no cross-resistance with fluoroquinolones was detected in strains that have been reported to be resistant to ciprofloxacin. The in vitro activities of moxifloxacin and amoxicillin were compared by time-kill curve and inhibition of intracellular growth experiments by using a model of bone marrow-derived mouse macrophages infected by L. monocytogenes EGDe. Both moxifloxacin and amoxicillin were bactericidal in broth against extracellular forms of L. monocytogenes. However, moxifloxacin acted much more rapidly, beginning to exert its effects in the first 3 h and achieving complete broth sterilization within 24 h of incubation. Moxifloxacin has a rapid bactericidal effect against intracellular reservoirs of bacteria, whereas amoxicillin is only bacteriostatic and appears to prevent cellular lysis and the subsequent bacterial spreading to adjacent cells. No resistant bacteria were selected during the in vitro experiments. Taken together, our results suggest that moxifloxacin is an interesting alternative to the reference treatment, combining rapid and bactericidal activity, even against intracellular bacteria.

FIG. 1.

Distribution of MICs of moxifloxacin for a collection of L. monocytogenes strains isolated in 2005 from humans (black) and food-processing environments (white). Arrows indicate the critical concentrations used for susceptibility interpretation classification (<1 and ≥2 μg/ml).

FIG. 2.

In vitro efficacies of moxifloxacin and amoxicillin against extracellular forms of L. monocytogenes. Bactericidal activity was evaluated in time-kill curve experiments for the nontreated control (⧫) and in MH broth supplemented with various concentrations of antibiotics: amoxicillin at 1× MIC (0.125 μg/ml; ▪), amoxicillin at 4× MIC (0.5 μg/ml; ▴), amoxicillin at 400 × MIC (C_max or peak concentration, 50 μg/ml; •), moxifloxacin at 1× MIC (0.5 μg/ml; □), moxifloxacin at 4× MIC (2 μg/ml; ▵), or moxifloxacin at 8× MIC (C_max or peak concentration, 4 μg/ml; ○). The results shown correspond to the means ± standard errors from three experiments. Arrows indicate the start of treatment.

FIG. 3.

Effects of amoxicillin and moxifloxacin on morphological aspects of macrophages infected with L. monocytogenes. Infected bone marrow-derived macrophages from BALB/c mice infected with L. monocytogenes were examined microscopically. Macrophages stained with May-Grunwald-Giemsa were observed at 3, 6, and 24 h of incubation with various concentrations of moxifloxacin or amoxicillin (1× MIC and C_max) or with no antibiotic for the nontreated control. C_max corresponds to the maximum serum concentration (or peak concentration) after the administration of clinically relevant doses of moxifloxacin and amoxicillin in humans (8× MIC and 400× MIC, respectively). Filopodium-like projections are shown by arrowheads. Bars, 15 μm.

FIG. 4.

In vitro efficacies of moxifloxacin and amoxicillin against intracellular reservoirs of L. monocytogenes cells. Bactericidal activity was evaluated for the nontreated control (⧫) and in bone marrow-derived macrophages of BALB/c mice infected with L. monocytogenes and treated with various concentrations of antibiotics: amoxicillin at 1× MIC (0.125 μg/ml; ▪), amoxicillin at 4× MIC (0.5 μg/ml; ▴), amoxicillin at 400× MIC (C_max or peak concentration, 50 μg/ml; •), moxifloxacin 1× MIC (0.5 μg/ml; □), moxifloxacin at 4× MIC (2 μg/ml; ▵), or moxifloxacin at 8× MIC (C_max or peak concentration, 4 μg/ml; ○). The results shown correspond to the means ± standard errors from three experiments. Arrows represent the start of treatment. Dotted lines extrapolate the results of the bacterial counts after 24 h of incubation due to alterations in the macrophage monolayer.