In vitro efficacy of antibiotics commonly used to treat human plague against intracellular Yersinia pestis.

Yersinia pestis initiates infection as a facultative intracellular parasite in host macrophages; however, little is known about the efficacy of antibiotics commonly used to treat human plague against intracellular Y. pestis. Intracellular minimal bactericidal concentrations (MBCs) were determined using a high-throughput broth microdilution assay in which human THP-1 macrophage-like cells were infected with Y. pestis strain KIM6-2053.1+ and exposed to 2-fold serial dilutions of antibiotics for 24 h in 96-well plates. The numbers of CFU, upon which minimal bactericidal concentrations were based, were determined by counting "microcolonies" in wells of 96-well plates following lysis of tissue culture cells to release surviving Y. pestis, replica dilution, and plating in soft tryptic soy broth agar. For THP-1 cells, streptomycin and ciprofloxacin had comparable efficacies for intra- and extracellular Y. pestis, but the MBCs for chloramphenicol, gentamicin, doxycycline, and amoxicillin were two-, three-, four-, and five 2-fold serial dilutions greater, respectively, for intracellular than for extracellular Y. pestis. During the initial stage of plague, intracellular Y. pestis may be less susceptible to antibiotic killing by particular antibiotics recommended for treatment of plague, such as gentamicin or doxycycline, whereas others, such as streptomycin and ciprofloxacin, may have similar efficacies against extracellular or intracellular Y. pestis. This may be of particular importance in the selection of antibiotics for prophylactic treatment in the case of a bioterrorism event.

Evaluation of a model for Escherichia coli O157:H7 colonization in streptomycin-treated adult cattle.

OBJECTIVE: To develop a repeatable model for studying colonization with streptomycin-resistant Escherichia coli O157:H7 in adult cattle. ANIMALS: 5 adult mixed-breed beef cattle. PROCEDURES: Cattle were surgically cannulated in the duodenum, treated daily with streptomycin (33 mg/kg) via the duodenal cannula prior to and during experimental colonizations, and colonized with 10(10) CFUs of streptomycin-resistant E coli O157:H7 via the duodenal cannula. Colonization of rectal mucus and shedding in feces were monitored. Antimicrobials were administered to eliminate the colonizing strain so that 5 repeated colonization experiments could be performed. A comprehensive analysis of colonization was performed at necropsy. RESULTS: Streptomycin treatment resulted in improved experimental colonization variables, compared with untreated controls, during initiation (days 2 to 6) and early maintenance (days 7 to 12) of colonization. Elimination of the colonizing strain followed by 5 repeated colonizations in the same animals indicated the repeatability of the protocol. Positive results of bacteriologic culture of feces 7 and 12 days after colonization were obtained in 100% and 84% of samples, respectively, across all animals and trials. At necropsy, highest magnitude recovery was in terminal rectal mucus. CONCLUSIONS AND CLINICAL RELEVANCE: The model was highly repeatable and novel with respect to streptomycin treatment, use of duodenal cannulas, and repeated colonizations of the same animals. Its use in adult cattle, from which most bovine-derived food originates, is critical to the study of preharvest food safety. The findings have implications for understanding intermittency of shedding in the field and for proposed vaccine-based interventions.