Abstract
Lactococcus lactis is the most widely exploited microorganism in global dairy fermentations. Lactococcal strains are described as typically harboring a number of prophages in their chromosomes. The presence of such prophages may provide both advantages and disadvantages to the carrying host. Here, we describe the deliberate generation of three distinct lysogens of the model lactococcal strain 3107 and the impact of additional prophage carriage on phage-resistance and anti-microbial susceptibility. Lysogen-specific responses were observed, highlighting the unique relationship and impact of each lysogenic phage on its host. Both homologous and heterologous phage-resistance profiles were observed, highlighting the presence of possible prophage-encoded phage-resistance factors. Superinfection exclusion was among the most notable causes of heterologous phage-resistance profiles with resistance observed against members of the Skunavirus, P335, P087, and 949 lactococcal phage groups. Through these analyses, it is now possible to identify phages that may pursue similar DNA injection pathways. The generated lysogenic strains exhibited increased sensitivity to the antimicrobial compounds, nisin and lysozyme, relative to the parent strain, although it is noteworthy that the degree of sensitivity was specific to the individual (pro)phages. Overall, the findings highlight the unique impact of each prophage on a given strain and the requirement for strain-level analysis when considering the implications of lysogeny.
Highlights
Industrial dairy fermentations rely heavily on the repeated and reliable application of desirable starter cultures
The identified prophages bear some sequence similarity to prophages of several lactococcal strains including those of the prototypical lactococcal strains IL1403 and MG1363 (TP712; Table 1)
All genes between the integrase- and repressor-encoding genes, and orf12Dub35A, were cloned and expressed in L. lactis 3107, but no significant phage-resistance was observed. This suggests that additional gene(s) are present on the genome of Dub35A that encode such functions or that a combination of such genes may be required for functionality. It suggests that there remain some novel and, as yet, unidentifiable, phage-resistance genes, and these may be a powerful tool in developing strategies to limit phage proliferation in the dairy fermentation industry
Summary
Industrial dairy fermentations rely heavily on the repeated and reliable application of desirable starter cultures. Strains of Lactococcus lactis are among the most intensely applied starter cultures in mesophilic fermentations. Most genomes of lactococcal strains are reported to harbor one or more prophages [1,2,3,4]. (partial) culture lysis releases intracellular enzymes involved in flavor and aroma development and accelerated cheese ripening [5]. This culture lysis may be mediated by autolysis via AcmA (and its homologues), the major lactococcal glucosaminidase that hydrolyses N-acetylmuramyl-1,4-β-N-acetylglucosamine bonds in peptidoglycan [6,7], or via induction of resident prophages by ultraviolet light or thermal shock. Environmental stressors including pH or nutritional starvation may induce prophage excision and release
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