38 MANUAL OF MICROBIOLOGICAL METHODS 



Bacterial Growth Requirements 



All living organisms require a utilizable source of energy in order to grow. Those 

 using radiant energy are known as phototrophs, whereas those utilizing the chemical 

 energy liberated from oxidation-reduction reactions are referred to as chemotrophs . 

 In addition to an energy source, all living organisms require suitable carbon and nitro- 

 gen sources, as well as inorganic salts. The autotrophic organisms can utilize CO2 as 

 the sole carbon source, whereas heterotrophic organisms, although they may need CO2, 

 also require carbon sources more complex than CO2, either for the carbon skeleton 

 proper or for the hydrogen atoms linked to this skeleton, or both. The requirement 

 for nitrogen may be satisfied in the form of NH4'*', NOs", or N2, although many organ- 

 isms need complex organic nitrogenous compounds for this purpose. Such elements 

 as Na, K, Ca, Mg, Mn, Fe, Zn, Cu, S, P, CI, etc., are required for growth and are 

 utilizable gencrall}^ in the form of inorganic salts. In addition, many organisms 

 require growth factors — organic substances which the organisms cannot synthesize at 

 a significant rate and which are usually required in small amounts. 



Many bacteria require atmospheric oxygen for growth (obligate aerobes), whereas 

 others fail to grow in the presence of oxygen (obligate anaerobes) ; some can grow under 

 either set of conditions (facultative organisms), whereas still others can grow only under 

 low oxygen tension (rnicroaerophiles) . The relationship of a given organism to oxygen 

 is a manifestation of the oxidation-reduction potential range commensurate with 

 the physiological activity of that organism (see Chap. IV). The requirement for 

 oxygen by some bacteria can be satisfied by oxidizing agents such as NO3", S04~, etc. 

 On the other hand, obligate anaerobes can be grown in the presence of oxygen pro- 

 vided a sufficiently^ low 0/R potential is obtained by the addition of reducing agents 

 or by permitting the formation of reduced products (see Chap. V). Thus, although all 

 the physiological phenomena associated with the oxygen relationship have not been 

 explained completely, this relationship must be considered in order to obtain vigorous 

 bacterial growth. 



Most bacteria grow well only Vv^ithin a limited pH range. To maintain this range, 

 at least during the initial growth buffers are added to the medium; these buffers 

 may be of the types described in Chap. IV, or for the neutralization of acids, CaCOs 

 may be included in the medium. Finally, to be available to the organism, all the 

 components necessary for growth must be in aqueous solution. 



Bacteria apparently display all possible variations of the major nutritional require- 

 ments. The autotrophs and many heterotrophs can be grown in chemically simple 

 media of defined composition (synthetic media). By this means, qualitative and 

 quantitative assay of each ingredient of such media can be made, leading to the micro- 

 biological assay of vitamins, amino acids, carbohydrates, inorganic ions, etc. Because 

 of our imperfect knowledge of the exact nutritional requirements of most bacteria, the 

 comparatively few truly synthetic media have been devised chiefly for the cultivation 

 of autotrophs or in connection with specific research problems involving the growth 

 of certain heterotrophs. However, the large number of heterotrophic bacteria familiar 

 to most bacteriologists continue to be grown in rather complex media. To a great 

 extent this complexity is associated merely with the empirical manner in which bac- 

 teriology has been practiced since the days of Pasteur and probably can be eliminated 

 as exact nutritional information becomes available. A sort of tradition has developed 

 in regard to the complex growth media made from natural plant and animal materials, 

 which having been employed for many years remain as the familiar means of growing 

 bacteria. 



The purpose of this brief review of bacterial nutrition is to stimulate reflection on 

 the part of the laboratory investigator regarding the cultivation of bacteria. Growth 



