Number of viable bacteria and presumptive antibiotic residues in milk fed to calves on commercial dairies

Abstract

Objective To assess the number of bacteria and presumptive antibiotic residues in milk fed to calves and to identify those bacteria and the antibiotic susceptibility of selected bacterial strains. Design Cross-sectional prospective study. Sample Population 189 samples obtained from 12 local dairies. Procedure Samples of waste milk and milk-based fluids (eg, milk replacer, colostrum, bulk-tank milk) were obtained. Cumulative number of viable bacteria was determined. Bacteria were cultured aerobically, and antibiotic susceptibility testing of selected strains was performed. Presumptive antibiotic residues were detected by use of test kits. Results Geometric mean of the cumulative number of bacteria for waste milk samples was significantly higher than for other types of milk or milk-based products. Streptococcus sp (84/165 samples) and Enterobacteriaceae (83/165 samples) were the predominant bacteria identified, followed by Staphylococcus sp (68/165 samples). Escherichia coli was the gram-negative species most commonly isolated (52/165 samples; 32%); however, none were strain O157. Salmonella sp or Mycoplasma sp were not isolated. Of 189 samples, 119 (63%) were positive when tested for β-lactams or tetracycline by use of 2 commercially available assays. In vitro, some bacteria were resistant to commonly used antibiotics. Clinical Implications Waste milk that has not been effectively treated (eg, pasteurization) to reduce microbial load prior to use as calf feed should be used with caution, because it may contain a high number of bacteria that may be pathogenic to cattle and human beings. Antibiotic residues that would constitute violative amounts and existence of multiple antibiotic resistant bacterial strains are concerns in calf health management and dairy food safety. (J Am Vet Med Assoc 1997;211:1029–1035)