Retail milk samples (231) of pasteurized, whole milk powder (WMP), and ultrapasteurized (UP) were collected in Hermosillo, Sonora, Mexico, to determine the incidence of multiple antibiotic resistance. Pathogens such as Escherichia coli, Staphylococcus aureus, and Salmonella were determined as well as nonpathogens such as Pseudomonas, Klebsiella, Serratia, and Hafnia alvei. Drug resistance of each isolated culture was tested using the Kirby-Bauer technique. The following antibiotic discs and concentrations were used: amikacin 30 mcg, ampicillin 10 mcg, carbenicillin 100 mcg, cefamandole 30 mcg, chloramphenicol 30 mcg, erythromycin 15 mcg, gentamicin 10 mcg, kanamycin 30 mcg, nitrofurantoin 300 mcg, penicillin 10 U, polymixin B 300 U, tetracycline 30 mcg, and tobramycin 10 mcg.
Sixty-seven percent of the pasteurized milk samples contained fecal coliforms with multiple drug resistance. Four percent contained Salmonella which was 100% resistant to penicillin and showed some resistance to polymixin B and chloramphenicol. S. aureus which was drug resistant was present in 2.9% of the samples. The WMP and UP samples were relatively pathogenic free; however, nonenteric coliforms were detected showing multiple drug resistance. E. coli isolated in 1 UP showed resistance to ampicillin, chloramphenicol, gentamycin, polymixin B, and tetracycline.
A high incidence of drug-resistant organisms in milk products represents a public health hazard in that food poisoning outbreaks would be difficult to treat and this pool of multidrug-resistant organisms in the food supply represents a reservoir for transferable resistant genes. An active surveillance and monitoring program should be mandated to minimize this risk for the consumer.