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.

Probiotic characterization of potential hydrolases producing Lactococcus lactis subsp. lactis isolated from pickled yam.

The aim of this study was to characterize potential probiotic strain co-producing α-amylase and ß-galactosidase. Sixty-three strains, isolated from pickle samples were screened for their hydrolase producing capacity by utilizing different starches as carbon source. One out of 63 strains, isolated from traditionally fermented pickled yam showing maximum hydrolase activity (α-amylase (36.9 U/ml) and ß-galactosidase (42.6 U/ml)) within a period of 48 hours was identified as Lactococcus lactis subsp. lactis. Further, it was assessed for the probiotic characteristics under gastrointestinal conditions like acidic, alkaline, proteolytic enzymes, bile stress and found to exhibit tolerance to these stresses. The therapeutic potential of the isolate is implicated because of its antagonistic effect against enteric foodborne pathogens (Salmonella typhimurium, Escherichia coli 0157:H7, Staphylococcus aureus, Yersinia enterocolitica and Aeromonas hydrophila). The results of this study entail a potential applicability of the isolate in developing future probiotic foods besides the production of industrially significant hydrolases.