NUTRITION OF HETEROTROPHIC BACTERIA 83 



practically no carbon dioxide is produced, large inocula 

 are necessary to establish growth. For the satisfactory 

 groA\i:h of Brucella abortus about 10 per cent, of carbon 

 clioxide is necessary. 



It has recently been shown that many heterotrophic 

 organisms, for example yeasts, Esch. coli and the pro- 

 pionic acid bacteria, can assimilate carbon dioxide by a 

 mechanism similar to that of autotrophic bacteria (see 

 pp. 77, 78 and Chap. XII). 



The next step in the loss of synthetic power by 

 bacteria is probably that of the ability to utilise ammonia 

 as nitrogen source. The nitrogen must be supplied in 

 the form of organic compounds, usually as amino -acids. 

 This would appear to be the step in nitrogen metabolism 

 analogous to the loss of ability to use carbon dioxide in 

 the change from autotrophic to heterotrophic bacteria. 

 Just as there is an intermediate group of organisms 

 between the autotrophs and the heterotrophs, the faculta- 

 tive autotrophs, so there is a group of organisms which 

 can utilise either ammonium salts or amino -acids for their 

 nitrogen supply. An example of this group is Eberthella 

 typhosa (B.. typhosus), which is capable of growth on either 

 source of nitrogen but develops better on amino -acids. 

 Usually strains of any one species in this intermediate 

 group vary in their ability to use ammonium salts. For 

 example, the Salmonella, Proteus, the dysentery and 

 typhoid bacilli and the Vibrios, as a general rule, comprise 

 two types of strains : (f?) " exacting " strains which 

 cannot utilise ammonium salts but need amino -acids, 

 and (b) " non-exacting " strains which will grow on 

 ammonium salts as well as on amino -acids. The 

 " exacting " strains are usually pathogenic. Organisms 

 of this group may be contrasted with those of the coli 

 group and Serratia rnarcescens which can grow on 

 ammonium salts or amino-acids, but of which no 

 " exacting " strains, using only amino-acids, are known. 



