Bacteriophages in milk fermentations: Diversity fluctuations of normal and failed fermentations

In this study on lactococcal phages in Norwegian dairies producing Dutch-type cheese, virulent 936-like phages were identified as the predominant phage group. Twenty six unique phage sequence types were identified. Interestingly, open reading frames encoding conserved variable domains were found overlapping with the putative major structural protein gene. A growth inhibition assay employing arrays of different indicator isolates was used to study phage biodiversity, and large day-to-day variation was found within a dairy plant. A peak of phage diversity coincided with a fermentation failure. The effect of a starter culture rotation was reflected by a radical change in phage diversity, but phages infecting the starter culture rapidly re-emerged in the plant. Phage levels in raw milk were low and the phage contamination came from within the plant. The assay used for bacteriophage analyses may have a potential for predicting a fermentation failure.