Gene-Trait Matching and Prevalence of Nisin Tolerance Systems in Lactococus lactis.

Lactococcus lactis cheese starter cultures typically contain a mix of many strains and may include variants that produce and/or tolerate the antimicrobial bacteriocin nisin. Nisin is well-established as an effective agent against several undesirable Gram-positive bacteria in cheese and various other foods. In the current study, we have examined the effect of nisin on 710 individual L. lactis strains during milk fermentations. Changes in milk acidification profiles with and without nisin exposure, ranging from unaltered acidification to loss of acidification, could be largely explained by the type(s) and variants of nisin immunity and nisin degradation genes present, but surprisingly, also by genotypic lineage (L. lactis ssp. cremoris vs. ssp. lactis). Importantly, we identify that nisin degradation by NSR is frequent among L. lactis and therefore likely the main mechanism by which dairy-associated L. lactis strains tolerate nisin. Insights from this study on the strain-specific effect of nisin tolerance and degradation during milk acidification is expected to aid in the design of nisin-compatible cheese starter cultures.

Protein costs do not explain evolution of metabolic strategies and regulation of ribosomal content: does protein investment explain an anaerobic bacterial Crabtree effect?

Protein investment costs are considered a major driver for the choice of alternative metabolic strategies. We tested this premise in Lactococcus lactis, a bacterium that exhibits a distinct, anaerobic version of the bacterial Crabtree/Warburg effect; with increasing growth rates it shifts from a high yield metabolic mode [mixed-acid fermentation; 3 adenosine triphosphate (ATP) per glucose] to a low yield metabolic mode (homolactic fermentation; 2 ATP per glucose). We studied growth rate-dependent relative transcription and protein ratios, enzyme activities, and fluxes of L. lactis in glucose-limited chemostats, providing a high-quality and comprehensive data set. A three- to fourfold higher growth rate rerouted metabolism from acetate to lactate as the main fermentation product. However, we observed hardly any changes in transcription, protein levels and enzyme activities. Even levels of ribosomal proteins, constituting a major investment in cellular machinery, changed only slightly. Thus, contrary to the original hypothesis, central metabolism in this organism appears to be hardly regulated at the level of gene expression, but rather at the metabolic level. We conclude that L. lactis is either poorly adapted to growth at low and constant glucose concentrations, or that protein costs play a less important role in fitness than hitherto assumed.

Lactococcus lactis†YfiA is necessary and sufficient for ribosome dimerization.

Dimerization and inactivation of ribosomes in Escherichia coli is a two-step process that involves the binding of ribosome modulation factor (RMF) and hibernation promotion factor (HPF). Lactococcus lactis MG1363 expresses a protein, YfiA(L) (l) , which associates with ribosomes in the stationary phase of growth and is responsible for dimerization of ribosomes. We show that full-length YfiA(L) (l) is necessary and sufficient for ribosome dimerization in L. lactis but also functions heterologously in vitro with E. coli ribosomes. Deletion of the yfiA gene has no effect on the growth rate but diminishes the survival of L. lactis under energy-starving conditions. The N-terminal domain of YfiA(L) (l) is homologous to HPF from E. coli, whereas the C-terminal domain has no counterpart in E. coli. By assembling ribosome dimers in vitro, we could dissect the roles of the N- and C-terminal domains of YfiA(L) (l) . It is concluded that the dimerization and inactivation of ribosomes in L. lactis and E. coli differ in several cellular and molecular aspects. In addition, two-dimensional maps of dimeric ribosomes from L. lactis obtained by single particle electron microscopy show a marked structural difference in monomer association in comparison to the ribosome dimers in E. coli.