3. Amino acids
Microorganisms with complex nutritional requirements may need small amounts of organic nitrogen
compounds for growth because they are substances that the microorganism is unable to synthesize from
available nutrients. Such essential nutrients, called growth factors, include purines and pyrimidines
(necessary for synthesis of nucleic acids), amino acids (required for synthesis of proteins) and vitamins
(needed as coenzymes and functional groups of certain enzymes). The growth factors are not
metabolized directly as sources of carbon or energy; rather they are assimilated by cells to fulfill their
specific role in metabolism. Mutant bacterial strains that require growth factors not needed by the wild
type (parent) strain are referred to as auxotrophs. Amino acid catabolism could play an important role in
the ability to obtain energy in nutrient-limited environments; however, the catabolic pathway of many
amino acids in LAB is still not clear. Some LAB degrade arginine to citrulline, ornithine and ammonium
via the ADI pathway to produce additional energy.