FIGURE 1.

Simplified representation of “selfish” and “cooperative” energetic behaviors in lactic acid bacteria. The key characteristics of “selfish” and “cooperative” energetic behaviors are indicated. The dotted red line separates the metabolic reactions that confer a “selfish” or a “cooperative” behavior. In lactic acid bacteria (LAB), mono- or disaccharides are catabolized: (i) to pyruvate via glycolysis, with the production of ATP and NADH, or ii) trough the exose monophosphate pathway (EMP), the heterofermentative pathway. The yellow arrow shows the regeneration of NAD⁺ that occurs during heme-dependent respiration. In Lactococcus lactis and in other LAB, the excess pyruvate can be metabolized by pyruvate formate lyase (Pfl), pyruvate dehydrogenase (Pdh) giving a mixed-acid fermentation with production of acetate. Ygf is the operon involved in heme homeostasis, and cydA is the gene for the cytochrome bd oxidase subunit I as described for L. lactis (Arioli et al., 2013). Arrows show the metabolic fluxes.