Antibiotic-resistant Bacteria in Raw Milk and Ability of Some to Transfer Antibiotic Resistance to Escherichia coli

Abstract

Raw milk samples were examined for number and percentage of bacteria resistant to seven antibiotics: penicillin, ampicillin, chloramphenicol, neomycin sulfate, polymyxin B sulfate, tetracycline and streptomycin sulfate. A significant negative correlation was found between the total aerobic count of the milk sample and the concentration (above 5 or 10% of the total count) of bacteria in each milk resistant to each of the antibiotics tested. Three of 42 gram-negative isolates were capable of transferring their antibiotic resistance to Escherichia coli. Substantial numbers of antibiotic-resistant bacteria in raw milk were found and some survived pasteurization. Inspection of farms failed to indicate a relationship between farm practices or use of antibiotics in feed or as pharmaceuticals and number of antibiotic-resistant bacteria in the raw milk.