A Plasmid-Encoded Surface Polysaccharide Partly Blocks Ceduovirus Infection in Lactococci

Area of Science:

• Microbiology
• Dairy Science
• Bacteriophage Research

Background:

• Bacteriophages (phages) are a major cause of dairy fermentation failure, necessitating phage-resistant Lactococcus lactis and Lactococcus cremoris dairy starters.
• Experimental evolution of L. lactis strains to resist bacteriocin Lcn972 resulted in increased sensitivity to phage infection, with the underlying mechanism undefined.

Purpose of the Study:

• To identify phage defense mechanisms against ceduoviruses in wild dairy isolates L. lactis IPLA517 and IPLA1064.
• To elucidate the mechanism by which bacteriocin-resistant L. lactis derivatives became sensitive to phages.

Main Methods:

• Long-read sequencing to identify genetic differences in phage-sensitive derivatives.
• CRISPR-Cas9 technology to selectively remove a specific plasmid (p41).
• Phage infection assays and phage adsorption experiments.

Main Results:

• Loss of a 41 kb endogenous plasmid (p41), containing an exopolysaccharide (EPS) gene cluster, was common in phage-sensitive L. lactis derivatives.
• Selective curing of p41 from L. lactis IPLA1064 restored sensitivity to three ceduoviruses.
• Increased phage adsorption to p41-cured cells supported the hypothesis that EPS hinders phage receptor access.

Conclusions:

• Exopolysaccharides (EPS) play a significant role in protecting Lactococcus against ceduovirus phage infection.
• The loss of EPS production due to plasmid loss is a mechanism conferring phage sensitivity in L. lactis.
• Horizontal gene transfer of EPS genes between L. lactis and L. garvieae may occur in dairy environments.