Protective role of intestinal flora against infection with Pseudomonas aeruginosa in mice: influence of antibiotics on colonization resistance.

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Swiss white mice were given ampicillin, clindamycin, kanamycin, metronidazole, or streptomycin in drinking water for a period of 3 weeks. One week after the initiation of antibiotic administration, the treated mice and untreated control mice were challenged orally with approximately 10(8) viable, streptomycin-resistant (SR) Pseudomonas aeruginosa isolates. All five of the antibiotics decreased the resistance of the mice to intestinal colonization with SR P. aeruginosa, as reflected by an increased fecal carriage of the organism and an increase in population levels of SR P. aeruginosa in feces as compared with untreated controls. Metronidazole was least effective in this regard. The antibiotics lowered the dose of SR P. aeruginosa that resulted in implantation in 50% of the mice ID50 to various degrees. Administration of streptomycin, the most effective antibiotic, caused a 10,000-fold decrease in ID50 as compared with untreated controls. Oral inoculation of approximately 10(8) organisms of SR P. aeruginosa resulted in translocation of the organism to the mesenteric lymph nodes, spleens, or livers of 13 or 17 streptomycin-treated mice, 1 of 20 clindamycin-treated mice, and 1 of 14 metronidazole-treated mice. Translocation was not observed, however, in ampicillin- or kanamycin-treated animals. Antibiotic activity was detected in the cecal contents of streptomycin-, kanamycin, and clindamycin-treated mice but not in the cecal contents of ampicillin- or metronidazole-treated animals.

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