Insertional inactivation of determinants for Mg2+ and Co2+ transport as a tool for screening recombinant Lactococcus species clones

Sensitivity to the two peptide bacteriocin plantaricin EF is dependent on CorC, a membrane-bound, magnesium/cobalt efflux protein

Lactic acid bacteria produce a variety of antimicrobial peptides known as bacteriocins. Most bacteriocins are understood to kill sensitive bacteria through receptor‐mediated disruptions. Here, we report on the identification of the Lactobacillus plantarum plantaricin EF (PlnEF) receptor. Spontaneous PlnEF‐resistant mutants of the PlnEF‐indicator strain L. plantarum NCIMB 700965 (LP965) were isolated and confirmed to maintain cellular ATP levels in the presence of PlnEF. Genome comparisons resulted in the identification of a single mutated gene annotated as the membrane‐bound, magnesium/cobalt efflux protein CorC. All isolates contained a valine (V) at position 334 instead of a glycine (G) in a cysteine‐β‐synthase domain at the C‐terminal region of CorC. In silico template‐based modeling of this domain indicated that the mutation resides in a loop between two β‐strands. The relationship between PlnEF, CorC, and metal homeostasis was supported by the finding that PlnEF‐resistance was lost when PlnEF was applied together with high concentrations of Mg²⁺, Co²⁺, Zn²⁺, or Cu²⁺. Lastly, PlnEF sensitivity was increased upon heterologous expression of LP965 corC but not the G334V CorC mutant in the PlnEF‐resistant strain Lactobacillus casei BL23. These results show that PlnEF kills sensitive bacteria by targeting CorC.

Plasmids of lactococci – genetic accessories or genetic necessities?

Lactococci are one of the most exploited microorganisms used in the manufacture of food. These intensively used cultures are generally characterized by having a rich plasmid complement. It could be argued that it is the plasmid complement of commercially utilized cultures that gives them their technical superiority and individuality. Consequently, it is timely to reflect on the desirable characteristics encoded on lactococcal plasmids. It is argued that plasmids play a key role in the evolution of modern starter strains and are a lot more than just selfish replicosomes but more essential necessities of intensively used commercial starters. Moreover, the study of plasmid biology provides a genetic blueprint that has proved essential for the generation of molecular tools for the genetic improvement of Lactococcus lactis.