658 BACTERIUM 



reactions as are of value in identifying the different species within the genus, or 

 in distinguishing between them. 



Type of Growth. — The type of growth given by the various species within this 

 genus is very similar. When normal smooth strains are grown in broth a uniform 

 turbidity develops, increasing rapidly during the first 12 to 18 hours of growth, 

 and then more slowly up to 48 to 72 hours. Pellicle formation is rare and when 

 present is very slight. A slight deposit forms as growth increases, and this is 

 easily dispersed on shaking the tube. 



On agar, the colonies are relatively large, with an average diameter of 2-3 mm., 

 but vary considerably in size. They may be circular, raised and low convex, with an 

 entire edge and smooth surface; they may be flatter with a more irregular surface, 

 and a more effuse and irregular edge, or they may assume the typical vine-leaf 

 form which is commonly described as characteristic of Salm. typhi. Even with 

 freshly isolated strains the range of variation is wide ; and when old laboratory 

 strains are under examination the most varied colonial forms may be seen. Apart 

 from the possible appearance of rough variants, a single strain may show several 

 different types of colony, if successive subcultures in broth are interspersed with 

 platings and subculture of individual colonies. 



As an exception to this general rule we may note that members of the Fried- 

 lander group, when freshly isolated, give rise to typically mucoid colonies. The 

 differential value of this characteristic is diminished by the fact that certain other 

 members, particularly those belonging to the intermediate-aerogenes-cloacse group, 

 may similarly give rise to this type of growth. It is very common in strains 

 isolated from milk, and is often lost on subculture in the laboratory. 



There are a few other growth characters which possess some differential value. 

 Thus, we may note the classical difference between the growth of Salm. typhi 

 and Bad. coli on potato, the former being colourless and barely visible, the latter 

 displaying a characteristic yellowish tint (Fremlin 1893). Again there is the so- 

 called " nail-head " growth of Friedlander's bacillus when grown in stab culture 

 in gelatin, due to the raised, circular, convex growth which sometimes develops 

 on the surface above the inoculation-track ; but this phenomenon is inconstant. 

 Chromogenesis is not a common property of the group, but appears to be met 

 with occasionally in individual members, such as the organism described by Parr 

 (1937) under the name of Bact. aurescens. It is a moot point, however, whether 

 such organisms should not be classified in the genus Chromobacterium. 



Resistance to Heat, and to various chemical Substances. — Most members of 

 this group are killed by exposure to a temperature of 55° C. for about 1 hour, or of 

 60° C. for 15-20 minutes. So far as they have at present been studied, the various 

 species within the genus do not differ from one another in any significant way. 

 On the whole, the typical faecal coli strains tend to have a slightly higher resistance 

 to heat than the closely related members of theintermediate-aerogenes-cloacee group. 

 A small proportion of them are not completely destroyed by exposure to 60° C. 

 for 30 minutes in broth, or to pasteurization at 62-8° C. for the same time in milk 

 (Henneberg and Wendt 1935, Wilson et al. 1935). Towards chlorine in water the 

 aerogenes type tends to be slightly more resistant than the coli or intermediate 

 types (Bardsley 19386). In raw water stored under atmospheric conditions coli- 

 form bacilli may remain alive for weeks or months. On the whole Bact. aerogenes 

 tends to survive rather longer than Bact. coli, but the results are influenced, among 

 other things, by the temperature (see Piatt 1935, Raghavachari and Iyer 19396). 



