Bias in the Listeria monocytogenes Enrichment Procedure: Lineage 2 Strains Outcompete Lineage 1 Strains in University of Vermont Selective Enrichments

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Abstract
The effects of various growth conditions on spontaneous φLC3 prophage induction in Lactococcus lactis subsp. cremoris IMN-C1814 was analyzed with a half fraction of a 44 factorial experimental design. The four factors included in the study were nutrient availability, acidity, osmolarity, and temperature, each applied at four levels. These environmental factors are related to the fermentation processes in the dairy industry, in which bacteriophage attacks on sensitive starter strains are a constant threat to successful fermentation processes. The frequency of spontaneous φLC3 induction was determined by quantitative analyses of restored DNA attachment sites (attB) on the bacterial chromosomes in a population of lysogenic cells. Statistical analysis revealed that all four environmental factors tested affected φLC3 prophage stability and that the environmental factors were involved in interactions (interactions exist when the effect of one factor depends on the level of another factor). The spontaneous φLC3 induction frequency varied from 0.08 to 1.76%. In general, the induction frequency remained at the same rate or decreased when level 1 to 3 of the four environmental factors was applied. At level 4, which generally gave the least favorable growth conditions, the induction frequency was either unchanged, decreased, or increased, depending on the type of stress. It appeared that the spontaneous induction frequency was independent of the growth behavior of the host. It was the environmental growth conditions that were the decisive factor in induction frequency.

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