70 METABOLISM 



Happold, Priestley and Wheatley 1934, Aitkin, Barling and Miles 1936, Nye and Lamb 

 1936, Khairat 1939, Fleming 1941, Kempner and Schlayer 1942). 



In some cases the gas stimulates, not growth, but the appearance of some particular 

 metabolite. On artificial media, for example. Staph, aureus (Chap. 25) produces significant 

 amounts of its characteristic toxin in air containing 20 per cent, of COg, and the anthrax 

 bacillus its capsular substance (Ivanovics 1937) in the presence of an excess of COj. 



The use of defined media lias made the study of CO2 requirements more exact. Walker 

 (1932) found that Bad. coli failed to grow in a liquid synthetic medium when incubated 

 in a current of COj-free air. Walker concluded that COj was necessary for growth, and 

 that the lag phase (see Chapter 4) represented the time taken by the organism to produce 

 in its immediate vicinity a growth-promoting concentration of CO2. Gladstone, Fildes 

 and Richardson (1935) demonstrated a similar inhibition for Salm. typhi, Ps. pyocyanea, 

 B. subtilis, B. anthracis, C. diphtherice, CI. sporogenes and CI. welchii by the passage of 

 COg-free gas through culture media. 



The nature of the stimulating action of CO2 is not clear. Carbon dioxide 

 is utilized, even by heterotrophic bacteria. Wood and Werkmau (1936, 1938, 

 1940) and Krebs and Eggleston (1941) demonstrated its fixation by propiono- 

 bacterja and its probable role in the synthesis of a four-carbon chain from pyruvic 

 acid. Wood and his colleagues (1940), by using COg made from the radioactive 

 carbon isotope C13, was able to trace the isotope as far as succinic acid. Bad. coli 

 and some Clostridia utilize CO2 (Ruben and Kamen 1940). Hes (1938), on the other 

 hand, suggests that the CO2 acts as a catalyser in several oxido -reduction reactions. 

 The possible interrelationship of CO2 with enzyme systems is evident from the work 

 of Pappenheimer and Hottle (1940), who found that adenylic acid was a growth 

 stimulant for Str. pyogenes if the CO2 concentration was as low as 0-25 per cent., but 

 was dispensable if the concentration was raised to 2-5 per cent. Similarly, McCul- 

 lough and Dick (1942) found that the incapacity of forty-one strains of jBr. abortus to 

 grow in the absence of a high concentration of COg in the atmosphere (see Chapter 

 34) was associated with incapacity to grow in a certain medium containing amino- 

 acids and four vitamins. After the strains had been trained to grow in air, thirty 

 of them grew in the basal medium. 



Aerobiosis and Anaerobiosis. 



It is customary to divide bacteria into three categories in regard to their 

 behaviour towards molecular oxygen : (1) the obligatory anaerobes — or anaerobes 

 without the qualifying adjective — which will grow only when oxygen is rigorously 

 excluded (2) the facxdtative anaerobes, which will grow both aerobically and anaero- 

 bically, i.e. in the presence or absence of oxygen, and (3) the obligatory aerobes, 

 which will grow only when suppHed with molecula^' oxygen. 



Most of the organisms in Class (2) — to which incidentally the great majority 

 of the bacteria with which we are concerned belong — are able to grow over a 

 very wide range of oxygen pressures, but some species prefer a relatively restricted 

 range, lying well below that of ordinary atmospheric conditions. 



It should be noted that in the fluid cultures commonly employed in bacteriology, 

 the degree of oxygenation in cultures of aerobic bacteria is by no means neces- 

 sarily optimal. The oxygen content in the depths of a broth culture may be 

 very low (Rahn and Richardson 1941). In many cases, forced aeration of the 

 culture greatly increases growth (see Wilson 1930, Rahn and Richardson 1942), 

 though the effect may in some cases be due as much to removal of COg as to the 

 increased oxygen supply. 



In a large number of aerobic and facultatively anaerobic bacteria, the capacity 



