CONDITIONS OF GROWTH 657 



is far too irregular and variable a character in the Bacterium group to serve as 

 a criterion for specific separation. 



Some species, such as Bact. friedldnderi, are normally capsulated, and this 

 character, when it occurs, has some differential value ; but those bacilli which 

 are normally capsulated, and form mucoid colonies when first isolated from the 

 tissues, frequently lose the property of capsule formation during subculture on 

 artificial media, while other normally non-capsulated species may acquire a capsule 

 under particular conditions. It may be noted that many coliform strains isolated 

 from milk are normally capsulated. Most of these strains belong to the inter- 

 mediate-aerogenes-cloacse group, but some undoubted strains of Bact. coli form a 

 capsule, and give rise to a mucoid growth on solid media. It may further be 

 noted that the presence of a capsule is not incompatible with active motility. 



Conditions of Growth. — The members of this group grow readily on the ordinary 

 nutrient media of the laboratory, without the addition of any accessory substances. 



Fig. 140. — Bact. coli. Fig. 141. — Bact. coli. 



Colonies on agar plate after 24 hours Larger and flatter type of colony on agar 



( X 8). plate after 24 hours ( X 8). 



They are aerobic, and facultatively anaerobic, though the growth is usually far less 

 copious under the latter conditions. The optimum temperature is, for most species, 

 in the neighbourhood of 37° C, and the range over which growth occurs is fairly 

 wide, extending for most species from about 42° C. as an upper limit to 18° C. or lower. 



There are, however, certain differences in behaviour that are of significance from 

 the systematic point of view. Thus, Bact. aerogenes grows very poorly, or not at all, 

 at a temperature of 44° C, and differs in this respect from the closely related Bact. 

 coli. Moreover, many strains of Bact. aerogenes have their optimum growth 

 temperatures nearer 30° C. than 37° C. 



It happens that the nutritional requirements of several species within this genus, 

 and the enzymic mechanisms that they employ in their attack on various substrates, 

 have been studied in considerable detail by the methods that have recently been 

 introduced in the investigation of the biochemical activities of bacteria. Many 

 of the results obtained in these studies have been described and discussed in 

 Chapter 3. In the present chapter we may therefore confine our attention to such 



