Pathways for α-ketoglutarate formation by Lactococcus lactis and their role in amino acid catabolism

The production of α-ketoglutarate, which is required for amino acid transamination, by lactic acid bacteria is often a limiting factor in the conversion of amino acids to aroma compounds in cheese. In theory, there are three main pathways for its formation. The direct pathway, involving a glutamate dehydrogenase (GDH), has already been demonstrated to be functional for certain Lactococcus lactis strains. In this work, we explored two other pathways for the formation of α-ketoglutarate by L. lactis and evaluated their impact on amino acid catabolism. The pathway that requires citrate permease (CitP), citrate lyase (CitL) and aspartate aminotransferase was operative for L. lactis subsp. diacetylactis and stimulated the conversion of amino acids. However, the pathway requiring aconitase and isocitrate dehydrogenase activities was never operative; none of the 13 strains tested had detectable aconitase and only one had detectable isocitrate dehydrogenase. Therefore, only the GDH and CitP/CitL pathways can stimulate flavour development during cheese ripening.