GROWTH FACTORS OF LOWER ORGANISMS
Authors: PESKETT. GEOFFREY LEWIS 
Published: January 2008

Summary.The foregoing article is an attempt to give a general account of the growth factors of lower organisms,i.e. the substances which must be supplied in addition to food materials of known composition in order that normal growth can occur. As regards the growth factors required by lower organisms, the number of cases to which such a strict definition can be applied is limited to a few, and it has been necessary to include discussion of factors of which the indispensable nature is uncertain. In the case of growth factors used by higher organisms, discussion has been limited to a consideration of vitamins which are all, by definition and common usage of the term, essential. In most instances the chemical composition of growth factors is unknown.In order to give the biological reader as general a view as possible the author has included an account of some of the more important experimental details which may influence growth—not necessarily growth factors in the strict sense—though these should be already well known to specialists in the field under review. These influences may be connected with chemical substances in the culture medium,e.g. inhibitory substances, carbon dioxide, amino acids, salts; or with its physicochemical properties,e.g. surface tension,pH, oxidation‐reduction potential. Further, a discussion is included of two important points connected with the organism itself, viz. size of seeding and cultural characteristics (Section II).There is evidence for the existence of growth‐stimulating factors in the case of most of the organisms which have been studied. Often proof has been lacking of the absolute necessity of such factors for growth, and it is true to state that much of the work that has been carried out, beyond showing that growth of an organism can be improved by addition of certain materials, has added little to our real knowledge of the problem. More detailed knowledge of indispensable factors has been obtained in the case of some of the Coccaceae, Bacteriaceae (tribe Haemophileae), Mycobacteriaceae, Chlamydobacteriaceae and Fungi. In the Bacteria (Schizomycetes) the organisms which have attracted most attention have done so on account of their medical interest, most of them being pathogenic. Effort has therefore been directed towards obtaining a simple method of culturing the organisms rather than solving the fundamental problems of growth which are involved. As a consequence much of the work is disjointed and cannot be co‐ordinated in review (Section III).The growth factors of lower organisms, especially those which are essential, are obviously similar to the vitamins required by higher organisms. Advance of our knowledge of vitamins has led to multiplication of the number of factors that are known to be required by animals. In spite of this and the increased information now available, none of these vitamins has been definitely shown to be identical with any of the factors required by lower organisms, though frequently the resemblance has proved to be very close (Section IV).The question of production of growth factors by lower organisms involves (a) factors for higher organisms, and (b) factors for other lower organisms. It has been shown that production of both these types of growth factor is a widespread occurrence. A more difficult problem awaiting solution is mentioned, viz. the production of factors required for growth of a given species by that organism itself. This may occur in the case of yeasts, for example, which do not grow from a small seeding on a synthetic medium if bios is absent, though if a large seeding is used a sufficient crop may be obtained to show that bios is present therein (Section V).