Growth factor is a kind of organic matter necessary for normal metabolism of microorganisms and cannot be synthesized from simple carbon or nitrogen sources. Its demand is generally small. In addition to vitamins, growth factors in a broad sense also include alkali, porphyrin and its derivatives, sterols, amines, branched or straight-chain fatty acids from C4 to C6, and amino acids in great demand. Growth factors in a narrow sense generally refer only to vitamins.

Although growth factor is an important nutrient element, it is different from carbon source, nitrogen source and energy source, and not all microorganisms have to absorb it from the outside. The relationship between various microorganisms and growth factors can be divided into the following categories:

(1) Growth factor autotrophs Most fungi, actinomycetes and many bacteria, such as E.coli, are growth factor autotrophs without external supply of growth factors.

(2) Growth factors nourish heterogeneous microorganisms. They need a variety of growth factors, such as lactic acid bacteria, various animal pathogens, protozoa and mycoplasma. For example, general lactic acid bacteria need a variety of vitamins; Many microorganisms and their auxotrophs (mutants) need different purine and pyrimidine bases; Haemophilus needs porphyrin and its derivatives as its growth factor; Mycoplasma often needs sterols; Haemophilus parahaemolyticus -cus (Haemophilus parahaemolyticus) needs amines; Some rumen microorganisms need C4 ~ C6 branched or straight chain fatty acids; Some anaerobic bacteria, such as Bacteroides melanogenesis, need vitamin K and hemin.