Seven-week-old C57BL/6 mice (Janvier) were pretreated with 100 mg mL−1 streptomycin. Mice were co-infected intragastrically with 107 colony forming units (CFU) each of two Salmonella Typhimurium strains that contained a different antibiotic resistance cassette. Small intestine, cecum and colon were isolated 2 days post-infection and plated on respective antibiotics resistance plates. The CFU were counted and reported as CFU per gram tissue. Competitive indices (CI) were calculated by normalizing the percentage of each strain to the inoculum and the challenge strain.
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