PCR detection of lactococcal bacteriophages in milk

A rapid PCR method capable of detecting low concentrations of lactococcal dairy phages in milk was developed. The PCR method initially comprised of a phage isolation and concentration step, in which Lactococcus lactis strains were added to the milk sample to be tested. Any lactocococcal phages present in the sample attached to the host strains, and following the addition of a milk clearing solution to the sample, the L. lactis cells, together with the attached phage were isolated by centrifugation from the milk. Phage DNA was extracted from the virions by boiling, and used as a template for PCR. The PCR results indicated that phages from the three main lactococcal dairy species (936, c2, and P335) were detectable at levels of 10(1) to 10(2) PFU/mL. This method could be used to test milk samples for the presence of lactococcal phages, in order to reduce fermentation failure due to phage contamination.