A growth factor is an essential organic molecule, such as a vitamin, amino acid or peptide, that an organism cannot synthesize and must obtain from its environment to support growth or, in animals, a signaling protein that regulates cell proliferation and differentiation.
Explanation
In microbiology, growth factors describe specific organic compounds that some bacteria, fungi or protozoa require because they cannot synthesize the compound themselves. These include vitamins (thiamine, riboflavin), purines, pyrimidines or amino acids; organisms that need them are called auxotrophs. Media formulated with these supplements enable the cultivation of fastidious microbes. Haemophilus influenzae, for example, is dependent on hemin (factor X) and nicotinamide adenine dinucleotide (factor V). Legionella pneumophila requires L-cysteine and iron, and lactic acid bacteria often need B vitamins. Growth factors differ from carbon or energy sources like glucose because they are required in small amounts as coenzymes or precursors. In multicellular eukaryotes, the term also refers to secreted peptides or glycoproteins that stimulate cell division, survival or differentiation. Epidermal growth factor binds cell‑surface receptors to activate pathways that drive epithelial proliferation. Platelet‑derived growth factor recruits fibroblasts during wound healing, while nerve growth factor supports neuronal survival. Dysregulation of these signaling molecules contributes to cancers and fibrotic diseases, and recombinant growth factors are used clinically to stimulate blood cell production.
Examples and important facts
Haemophilus influenzae fails to grow on standard blood agar because it needs both hemin and nicotinamide adenine dinucleotide; chocolate agar provides these by lysing red cells. Legionella species grow on buffered charcoal yeast extract because it supplies L‑cysteine and iron. In mammals, transforming growth factor‑β modulates embryonic development and tissue repair, while insulin‑like growth factors mediate the effects of growth hormone. Folic acid and niacin serve as growth factors for lactic acid bacteria used in fermented dairy products. Overactive epidermal growth factor receptor signaling has been linked to certain lung and colon cancers, leading to the development of targeted inhibitors.
Growth factors can describe both nutritional requirements of microorganisms and the proteins that direct cell proliferation in animals. Recognizing these distinct but related concepts is essential for culturing fastidious microbes and for understanding how cell signaling controls development, regeneration and disease.
Related Terms: Auxotroph, Essential nutrient, Vitamin, Hemin, Cytokine