Bacteriophages represent the largest threat to dairy fermentations, causing lysis of starter culture cells which interrupts fermentations and reduces product quality. In dairy processing plants, sanitisation and disinfection using purpose-made biocidal solutions is a critical step in controlling phage contamination. Relatively little is currently understood about the virucidal mode of action of biocides on bacteriophages and, despite the reported efficiency of many sanitisers, phages persist in dairy facilities. Varying phage resistance to these biocides could contribute to their persistence, with some phages surviving biocidal concentrations previously proven to be lethal to others.
Numerous studies have been performed on the efficacy of biocides in the inactivation of dairy phages, including phages of Streptococcus thermophilus, Lactobacillus, and Lactococcus lactis. This study focuses on variations in resistance to biocidal activity within a single phage species, the 936-type lactococcal phage species, as these are the most frequently encountered phages in the dairy processing environment. The susceptibility of thirty-six phages to biocidal inactivation was examined using a range of commercially available sanitisers. The virucidal modes of action of a number of these biocides were characterised, employing a range of techniques including electron microscopic imaging and SDS-PAGE analysis of phage proteomes before and after exposure in parallel with mass spectrometry and Western hybridisation analysis. Genomic sequences of the phages were also explored to identify the genetic and molecular basis of the identified biocide resistance.
The results of this study indicate that significant variations in phage resistance to biocides are present within the 936-type phage group. Furthermore, rather than possessing resistance to specific biocides or biocidal types, resistant phages tend to possess tolerance to multiple classes of antimicrobial compounds. We have identified structural targets for a number of frequently employed biocides via SDS-PAGE and mass spectrometry analysis, while also visualizing the damage caused using electron microscopy.