Characterization of Bacteriophages and Bacteria Indigenous to a Mixed-Strain Cheese Starter

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Abstract
To understand the types of phages and bacteria that coexist in a phage-carrying starter, we characterized isolates from milk fermented by a commercial mixed-strain culture. The bacterial population was 62% Lactococcus lactis ssp. cremoris, 19% Lactococcus lactis ssp. lactis biovar. diacetylactis, 2% Lactococcus lactis ssp. lactis, 16% Leuconostoc cremoris, and 1% unknown species. Plasmid analyses indicated 24 groups of L. lactis ssp. cremoris strains, 4 groups of L. lactis ssp. lactis biovar. diacetylactis, 1 group of L. lactis ssp. lactis, and 1 group of Leuc. cremoris. Fermentation times and sensitivity to outside phages varied between strains. Seven phages were isolated from the starter. Six attacked L. lactis ssp. cremoris strains, and one attacked L. lactis ssp. lactis biovar. diacetylactis. Three L. lactis ssp. cremoris phages crossreacted on starter hosts, and their DNA patterns were similar after restriction endonuclease digestion and agarose gel electrophoresis. Electron microscopy and DNA/DNA hybridization revealed two distinct small isometric phage types. One type attacked L. lactis ssp. cremoris and had a collar, whiskers, hexagonal tail-base structure, and 151-nm long tail. The other type attacked L. lactis ssp. lactis biovar. diacetylactis, had a tail fiber and a 215-nm long tail. Bacteria within and between plasmid groups demonstrated various degrees of resistance to these phages. Although lytic phages and sensitive hosts were both present, the mixed-strain culture functioned as a fast acid-producing starter.