Regulatory phenotyping reveals important diversity within the species Lactococcus lactis

The diversity in regulatory phenotypes among a collection of 84 Lactococcus lactis strains isolated from dairy and nondairy origin was explored. The specific activities of five enzymes were assessed in cell extracts of all strains grown in two different media, a nutritionally rich broth and a relatively poor chemically defined medium. The five investigated enzymes, branched chain aminotransferase (BcaT), aminopeptidase N (PepN), X-prolyl dipeptidyl peptidase (PepX), alpha-hydroxyisocaproic acid dehydrogenase (HicDH), and esterase, are involved in nitrogen and fatty acid metabolism and catalyze key steps in the production of important dairy flavor compounds. The investigated cultures comprise 75 L. lactis subsp. lactis isolates (including 7 L. lactis subsp. lactis biovar diacetylactis isolates) and 9 L. lactis subsp. cremoris isolates. All L. lactis subsp. cremoris and 22 L. lactis subsp. lactis (including 6 L. lactis subsp. lactis biovar diacetylactis) cultures originated from a dairy environment. All other cultures originated from (fermented) plant materials and were isolated at different geographic locations. Correlation analysis of specific enzyme activities revealed significantly different regulatory phenotypes for dairy and nondairy isolates. The enzyme activities in the two investigated media were in general poorly correlated and revealed a high degree of regulatory diversity within this collection of closely related strains. To the best of our knowledge, these results represent the most extensive diversity analysis of regulatory phenotypes within a single bacterial species to date. The presented findings underline the importance of the availability of screening procedures for, e.g., industrially relevant enzyme activities in models closely mimicking application conditions. Moreover, they corroborate the notion that regulatory changes are important drivers of evolution.