Stability of plasmids in lactococci during extended incubation in growth media

Abstract

The stability of plasmids in Lactococcus lactis ssp. lactis strains C2 and ML3, and L. lactis ssp. cremoris strains ML1 and SC607, was investigated by extended incubation of bacterial cells in low nutrient media under acidic conditions. Strains were grown overnight (16–18 h) in skim milk and unbuffered medium (M17−) at 32 °C and subsequently held at that temperature for extended periods (≥ 96 h). Lac− variants were obtained from each strain in milk and (M17−) broth. The plasmid profiles of Lac− variants when compared with their parental Lac+ strains showed loss of one or more plasmid bands. None of the Lac− mutants showed loss of smaller plasmids (< 5 MDa) indicating that smaller plasmids in lactococci are more stable under these conditions than larger plasmids (> 10 MDa). Concomitant loss of the Lac+ phenotype and plasmids by the method used in the present investigation may have application for isolating mutants devoid of one or more plasmids.