64 METABOLISM 



but an undetectable thougt essential trace of metal may also be present (cf. the 

 necessity for molybdenum in the fixation of nitrogen by Azotobacter ; Burk 1934). 



A particular ingredient may play no essential part in the metabolism of the 

 bacterium, but promote growth by neutralizing a toxic substance in the medium, 

 or by changing the physical state of the medium (see, for example, O'Meara 1937) to 

 one more suitable to essential metabolic processes. Thus the addition of charcoal 

 to a medium is said to improve its growth-promoting properties for gonococci 

 and meningococci (Glass and Kennett 1939) and for tubercle bacilli (Nassau 

 1942). According to GI9.SS and Kennett, the charcoal acts as an adsorbent of 

 inhibitory substances in the medium, perhaps toxic metabolic products, or by 

 catalysing certain oxidations. Substances even more inert than charcoal are 

 beneficial to growth. For example, Bact. coli will grow better in distilled water 

 containing CO2 and ammonia if talc is added, the talc acting presumably by reason 

 of a surface action that renders the gases more available as nutrients (Bigger and 

 Nelson 1943) ; and low concentrations of agar, by reducing the rate of diflJ'usion 

 of oxygen from the air, stabilize redox-potentials obtaining in fluid media at 

 levels suitable for the growth of anaerobic bacteria (Reed and Orr 1943 ; see also 

 Gould 1944). Zobcll (1943) in a study of the growth of- bacteria found in sea-water, 

 concluded that solid surfaces promoted growth by concentrating nutrients through 

 adsorption, by providing a resting-place for bacteria, and by retarding the diffusion 

 of exo-enzymes and hydrolysates away from the cell, thereby enhancing the assimila- 

 tion of substances hydrolysed outside the cell. Certain proof of the participation 

 of a given "essential nutrient" in bacterial metaboHsm is difficult to obtain, 

 though the marking of food substances with radioactive elements (see CO2 require- 

 ments, below) may increase the certainty. Assuming that a basal medium has 

 been established, of which separate ingredients are free of impurities, we are still 

 faced with various difficulties. Gladstone (1939), for examj)le, grew a strain of 

 the anthrax bacillus in a defined medium containing a number of amino-acids. 

 The medium ceased to support growth if either valine or leucine was removed, 

 and supported growth feebly if woleucine, glycine or cystine was absent. Glycine 

 and cystine were subsequently proved to be synthesized by the organism, but 

 the three remaining acids were apparently essential for growth. However, the 

 medium could be made to support growth by the removal of all three amino-acids ; 

 singly, each had a toxic effect ; that of valine was counteracted by leucine, vice versa, 

 and that of ^soleucine by valine and leucine together. None of them was an 

 essential nutrient. (In this connection see also McLeod and Wyon 1921, Wyon 

 and McLeod 1923, Euggierini 1933.) Again, the nutrients may be tested in too 

 complex a form, and a precursor of the substance prove equally effective. Thus 

 Staph, aureus requires thiamin (vitamin Bj, Knight 1937), but it can synthesize 

 thiamin if the two constituents, thiazole and pyrimidine, are substituted in the 

 medium (Knight and Mcllwain 1938). 



Considerable difficulty arises when we attempt to define the meaning of the 

 term " essential nutrients ". We may define them as essential for growth of a 

 bacterium, no matter how that growth may be achieved. In that case we must 

 to some extent ignore the changes that the experimental procedure may induce 

 in the bacterium, for its properties in the final medium may be markedly different 

 from those in the starting medium. We may illustrate this by the nutrition of 

 the typhoid bacillus. Braun and Cahn-Bronner (1921, 1922) grew Salm. typhi, 

 Salm. paratyphi B, and Sahn. enteritidis in media containing mineral salts, various 



