200 ELEMENTS OF WATER BACTERIOLOGY 



but raffinose, 41 per cent attacked dextrose and lactose 

 only, and i per cent dextrose alone. 



Rogers, Clark and Davis (Journal of Infectious 

 Diseases, 19 14, XIV, 411) have made a most impor- 

 tant contribution to our knowledge of the colon group. 

 The most striking point is perhaps the fact that when 

 the gas produced by these organisms in dextrose broth 

 is collected under a vacuum and analyzed with proper 

 precautions, two groups may be clearly distinguished 

 by the ratio of hydrogen to carbon dioxide produced. 

 One well-defined group including half the B. com- 

 munior strains, all the B. communis strains, one-fifth 

 of the B. aerogenes strains and four-fifths of the B. 

 acidi-lactici strains studied gave a constant ratio of 

 about one part of carbon dioxide to one part of hydro- 

 gen, while the rest gave a ratio of over 1.4 to i.o and 

 usually 2 to I or more. The high-ratio group was usually 

 saccharose-positive, dulcite-negative. The low-ratio 

 group included strains of more widely differing reactions. 



In a subsequent study, Rogers, Clark and Evans 

 (Journal of Infectious Diseases, 19 14, XV, 100) present 

 a most significant comparison of the types of colon 

 bacilli present in milk and in bovine faeces, respectively. 

 Of 125 strains isolated from milk, about half belonged 

 to the high-ratio group defined above (presumably 

 corresponding to B. aerogenes), while in 150 cultures 

 from bovine faeces this high-ratio group was foutid 

 only once. The organisms of the low-ratio group in 

 both milk and bovine faeces were divided rather sharply 

 into a saccharose-positive dulcite-positive type (presum- 

 ably B. communior) and a saccharose-negative generally 

 dulcite-negative type (presumably B. acidi-lactici). 



