The present study assessed the antibiotic susceptibility patterns of Escherichia coli and Salmonella isolates of raw milk from zero grazed cows. A total of 65 milk samples were collected for analysis. The standard membrane filtration technique and HiCrome E. coli agar were used in isolation of E. coli from milk samples. Isolation of Salmonella species employed pre-enrichment in buffered peptone water followed by enrichment in Rappaport and Vassilidis broth prior to Xylose lysine deoxychocolate agar as a differential media. The isolates were analyzed for antimicrobial susceptibility to eight different types of antibiotics using disc diffusion method. The prevalence of E. coli was 16 (16.7%) and all the samples tested were negative for Salmonella. The average colony forming unit for E. coli was 2cfu/mL. All E. coli isolates tested were resistant to penicillin (100%) and amoxicillin-clavulanic acid (100%) while 15(93.8%) were sensitive to ciprofloxacin. Resistance was also observed in sulfamethoxazole-trimethoprim (43.8%), chloramphenicol (12.5%), oxytetracycline (68.8%), streptomycin (12.5%) and gentamicin (25%). Of the isolates tested, 14 (87.5%) showed multi-drug resistance pattern. These results confirm that milk from zero grazed cows in Arusha was contaminated with E. coli, and that most of the E. coli strains isolated were resistant to at least one of the antimicrobial agent commonly used in treatment of human diseases.
Key words: Salmonella, Escherichia coli, prevalence, antibiotic susceptibility.
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