In VitroEfficacy of Antibiotics Commonly Used To Treat Human Plague against IntracellularYersinia pestis

ABSTRACTYersinia pestisinitiates 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 intracellularY. 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 withY. pestisstrain 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 survivingY. pestis, replica dilution, and plating in soft tryptic soy broth agar. For THP-1 cells, streptomycin and ciprofloxacin had comparable efficacies for intra- and extracellularY. 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 extracellularY. pestis. During the initial stage of plague, intracellularY. pestismay 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 intracellularY. pestis. This may be of particular importance in the selection of antibiotics for prophylactic treatment in the case of a bioterrorism event.