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.