Evidence for a Plasmid-Linked Restriction-Modification System in Lactobacillus helveticus

Area of Science:

• Microbiology
• Bacteriology
• Molecular Biology

Background:

• Restriction-modification (R/M) systems are crucial for bacterial defense against foreign DNA, including bacteriophages.
• Lactobacillus helveticus is a significant probiotic bacterium, and understanding its phage resistance mechanisms is important for industrial applications.

Purpose of the Study:

• To investigate the presence and nature of restriction-modification systems in Lactobacillus helveticus strains.
• To identify the genetic basis and location of the R/M system responsible for phage resistance.

Main Methods:

• Phage challenge assays were performed on Lactobacillus helveticus strains (CNRZ 1094, CNRZ 1095, CNRZ 1096) against bacteriophages 328-B1 and hv.
• Heat inactivation and chemical treatments (novobiocin) were used to assess R/M system stability and identify methods for generating non-restrictive variants.
• Plasmid DNA analysis, including electrophoresis and gene cloning into E. coli vectors, was employed to locate and characterize the R/M genes.

Main Results:

• Three Lactobacillus helveticus strains exhibited a restriction-modification system effective against phages 328-B1 and hv, reducing phage burst size.
• The R/M system was heat-stable and its phenotype could be eliminated by specific chemical treatments, correlating with the loss of a 34-kb plasmid.
• Genes conferring the R/M phenotype were confirmed to be located on the 34-kb plasmid, which was also present in other restrictive strains.

Conclusions:

• A plasmid-borne restriction-modification system confers phage resistance in specific Lactobacillus helveticus strains.
• The identified 34-kb plasmid is a key element in the phage defense mechanism of these bacteria.
• The R/M phenotype is not linked to lactose fermentation or proteolytic activity in the studied strains.