Plasmids from Lactobacillus helveticus: distribution and diversity among natural isolates

To investigate the distribution and the level of diversity of extrachromosomal molecules in Lactobacillus helveticus strains in relation to their different ecological niches.

The plasmid profile of 22 Lact. helveticus strains, isolated from five different Italian cheeses, was determined. Among the tested strains, there was a variable presence of plasmids: eight plasmid-free strains and the remaining with several plasmids that could be differentiated on the basis of number and molecular weight. The profiles showed between one and five plasmid bands, which size ranged between 2.3 and 31 kb. Four of these plasmids were further analyzed by restriction digestion and compared with the plasmids from Lact. helveticus ATCC 15009(T). Analyses and comparison of their primary structures and hybridization experiments revealed the presence of different DNA homology groups.

This study indicates that within Lact. helveticus species, there is a high degree of variability in relation to the presence of plasmid molecules. Moreover, the structural diversity found among some of these plasmids allows to hypothesize the presence of different evolutionary lineages.

Studies on plasmid distribution and diversity should be considered as an essential component in a continuing effort to explore microbial diversity as well as to understand the real role of plasmids in the flow of genetic information in natural bacterial communities.