Evidence for variability in the phenotypic expression of lysozyme resistance in Lactobacillus helveticus

Abstract

Summary - ln a cell population of Lactobacillus helveticus-sensitive strains, resistance to lysozyme of individual cells varied from 10 Ilg/ml to > SOOIlg/ml of lysozyme. The resistance to 100 Ilg/ml by successive transfers in milk with 100 Ilg/ml of lysozyme was apparenUy acquired by eliminating the less resistant cells and by increasing lysozyme resistance of the surviving cells. Resistance to 100 Ilg/ml of lysozyme was also obtained in one step by growth in MRS agar containing SOOIlg/ml of lysozyme. These results could be explained by a large variability of the phenotypic expression of resistance in each cellular line. Clones harboring a 11.S-kb plasmid were detected amongst resistant derivatives from sensitive strains which were plasmid-free. These clones exhibited properties (resistance to lysozyme, NaCI and antibiotic, carbohydrate fermentation and enzymatic activity patterns, phage resistance, restriction profile of total DNA) similar to those of the plasmid-free strains derived from the same sensitive parent strain. Two clones obtained alter curing by novobiocin exhibited a decreased resistance to lysozyme. These results suggest that the 11.S-kb plasmid could possibly contribute to the phenotypic expression of the resistance to lysozyme.