Limitations of standard antibiotic screening assays when applied to milk for cheesemaking

A range of antibiotics registered for therapeutic use on lactating cattle was assessed for their inhibitory effect on starter lactic acid bacteria. A number of non-β-lactam antibiotics, novobiocin, lincomycin, oleandomycin and oxytetracyline HCI inhibited acid production of one or more L.lactis strains grown in reconstituted skim milk. The inhibitory concentrations of the antibiotics were in some instances below their detectable limits using either or both the Bacillus stearothermophilus (var. calidolactis) disk assay and the Delvo SP assay. The presence of specific antibiotic residues in milk may inhibit acid production by starter bacteria and significantly affect the cheesemaking process, leading to longer make times and the disruption of cheesemaking schedules.