Influence of Inoculum Size and Marbofloxacin Plasma Exposure on the Amplification of Resistant Subpopulations of Klebsiella pneumoniae in a Rat Lung Infection Model

Abstract

We tested the hypothesis that the bacterial load at the infection site could impact considerably on the pharmacokinetic/pharmacodynamic (PK/PD) parameters of fluoroquinolones. Using a rat lung infection model, we measured the influence of different marbofloxacin dosage regimens on selection of resistant bacteria after infection with a low (10 5 CFU) or a high (10 9 CFU) inoculum of Klebsiella pneumoniae. For daily fractionated doses of marbofloxacin, prevention of resistance occurred for an area-under-the-concentrationtime-curve (AUC)/MIC ratio of 189 h for the low inoculum, whereas for the high inoculum, resistantsubpopulation enrichment occurred for AUC/MIC ratios up to 756 h. For the high-inoculum-infected rats, the AUC/MIC ratio, C max /MIC ratio, and time within the mutant selection window (T MSW ) were not found to be effective predictors of resistance prevention upon comparison of fractionated and single administrations. An index corresponding to the ratio of the time that the drug concentrations were above the mutant prevention concentration (MPC) over the time that the drug concentrations were within the MSW (T >MPC /T MSW ) was the best predictor of the emergence of resistance: a T >MPC /T MSW ratio of 0.54 was associated with prevention of resistance for both fractionated and single administrations. These results suggest that the enrichment of resistant bacteria depends heavily on the inoculum size at the start of an antimicrobial treatment and that classical PK/PD parameters cannot adequately describe the impact of different dosage regimens on enrichment of resistant bacteria. We propose an original index, the T >MPC /T MSW ratio, which reflects the ratio of the time that the less susceptible bacterial subpopulation is killed over the time that it is selected, as a potentially powerful indicator of prevention of enrichment of resistant bacteria. This ratio is valid only if plasma concentrations achieve the MPC.

Low or High Doses of Cefquinome Targeting Low or High Bacterial Inocula Cure Klebsiella pneumoniae Lung Infections but Differentially Impact the Levels of Antibiotic Resistance in Fecal Flora

“…A. Ferran, M. Z. Lacroix, P. L. Toutain, and A. Bousquet-Mélou, submitted for publication). This finding is supported by those of previous studies using various classes of antibiotics or bacterial species (13)(14)(15)(16)(17)19) and reinforces the generic relevance of this so-called inoculum effect. A combination of several mechanisms might explain this effect, such as (i) the inoculum size dependency of antimicrobial activity as demonstrated in vitro for several antibiotics (11,12) and (ii) the impact of pathogen burden on the saturable granulocytic clearance of bacteria (21,22).…”

“…A. Ferran, M. Z. Lacroix, P. L. Toutain, and A. BousquetMélou, submitted for publication). In addition, a set of published works on the influence of inoculum size on the in vivo efficacy of fluoroquinolones or beta-lactams against various Gram-positive or Gram-negative pathogens (11,15,17,19,23) supports the hypothesis that efficacious doses can be lower in early than in later interventions. In particular, Mizunaga et al (11) determined that the 50% effective doses (ED 50 s) of 3 carbapenems were 12 to 50 times lower in Pseudomonas aeruginosa-infected mice when the initial bacterial inoculum was 100-fold lower.…”

“…The absence of cefotaxime-resistant (CTX-R) Enterobacteriaceae was checked in the SPF pig fecal samples after plating on MacConkey agar supplemented with cefotaxime (2 g/ml). Pulmonary infection was induced in these rats as previously described (17)(18)(19). Briefly, the trachea of each rat was cannulated under general anesthesia, and the lungs were inoculated with 0.05 ml of a saline suspension of K. pneumoniae containing 2 ϫ 10 6 CFU/ml (low inoculum; n ϭ 8 rats) or 2 ϫ 10 10 CFU/ml (high inoculum; n ϭ 8 rats).…”

“…It was subsequently shown in vivo that lower antibiotic doses given in the early prepatent phase of an infection, when the pathogen burden was still low, were as effective as higher doses administered during the patent phase of infection, as characterized by overt clinical symptoms and a high bacterial burden (13)(14)(15)(16)(17).…”

Emergence of Resistant Klebsiella pneumoniae in the Intestinal Tract during Successful Treatment of Klebsiella pneumoniae Lung Infection in Rats

“…In previous studies on rodent models of Escherichia coli thigh infection and K. pneumoniae lung infection, we showed that the bacterial load at the start of antimicrobial treatment plays an important role in the enrichment of resistant strains at the infection site (4,8). With a low inoculum, an early start of antimicrobial treatment with marbofloxacin, an expanded-spectrum fluoroquinolone extensively used in veterinary medicine, prevented mutant enrichment at the infection site, whereas a late start of the antimicrobial treatment with a high inoculum led to the enrichment of the resistant mutant subpopulation.…”