Bacteriophages in Food Safety: Fermented Dairy Products

Microbiological indicators make it possible to reveal potential safety risks in the dairy industry. Bacteriophages affect the lysis of starter cultures because they can disrupt fermentation processes in dairy production. This study featured the seasonal factors that affect the phage status during dairy fermentation, the newly isolated bacteriophages, and the defense systems used by lactococci strains. The research featured raw milk, cream, and skim milk; whole and skim milk powders; curd and cheese whey; strains of lactococci from different species with different phage resistance (Uglich Biofabrika Ltd; Bioresource Center of All-Russian Collection of Industrial Microorganisms); two new bacteriophages ph. 1622 and ph. 1623. The research relied on a number of standard microbiological, genetic, and mathematical methods. The mesophilic aerobic and anaerobic microbial count was performed by inoculation on a dense nutrient medium (State Standard GOST 32904-2014) while the two-layer inoculation method revealed the bacteriophage titer. The genomic DNA analysis involved a phenol–chloroform extraction followed by precipitation with isopropanol and electrophoretic separation in agarose gel. The experiments yielded reliable data on the quantitative change of phage particles in the raw material, the seasonal variability of mesophilic lactococci phages, and the genetics of the new industrial bacteriophages. The highest count of phage particles belonged to the samples obtained in the summer whereas the lowest was associated with the winter samples. The count of phage particles correlated with the bacterial contamination of the samples. The phage resistance index in Lactococcus lactis subsp. lactis, L. lactis subsp. cremoris, and L. lactis subsp. lactis biovar. diacetylactis had a seasonal character, the highest variability being recorded in L. lactis subsp. lactis, i.e., an acid former of starters. The DNA and amino acid sequences of phage proteins in phages ph. 1622 and ph. 1623 isolated from industrial samples made it possible to create panels of phage alternative strains. The seasonal variability in lactic acid bacteria cultures and bacteriophage activity may affect the quality and safety of dairy products. The DNA of ph. 1622 and ph. 1623 differed in restriction patterns, which means they were distinct phages. Comparative genomics revealed their similarity to the well-known L. lactis-infecting c2 phage. The new phages exhibited different lactococcal cell infection mechanisms. The ph. 1623 genome insertion encoded an orphan DNA methyltransferase that could potentially suppress bacterial immune systems. Further research may reveal lactococcal phage sensitivity and defense mechanisms.