VARIETIES OF COLON BACILLI 181 



the boiling of 1-4 c.c. of a 48-hour dextrose broth 

 culture with 5 c.c. of a 10 per cent caustic soda solu- 

 tion, is a quick method of obtaining the Voges and 

 Proskauer reaction. A yellow color is produced under 

 these conditions by the sugar alone, and a pinkish 

 eosin-like color when the acetyl-methyl-carbinol is 

 present. The reaction is hastened by shaking or 

 blowing into the tube to promote oxidation. West 

 confirms the conclusion of MacConkey and Bergey 

 that this reaction is characteristic of the B. lactis- 

 aerogenes and B. cloacae types (saccharose positive, 

 dulcite negative organisms). 



The characters of colonies on various solid media, 

 such as Endo agar or Conradi-Drigalski agar appear 

 to be of minor importance, usually depending on one 

 of the simple fermentative reactions for their differential 

 value. The aesculin test and the Eijkman test (fer- 

 mentation at 46) have been discussed in connection 

 with their use as enrichment procedures in Chapter VI. 



Biological Significance of Variations in the Colon 

 Group. The general view among water bacteriologists 

 has been that forms differing from the " typical " B. 

 coli in one or more respects represented original intes- 

 tinal types weakened by a prolonged sojourn in an 

 unfavorable environment. As Whipple says (Whipple, 

 1903), " The type form of Bacillus coli is one which 

 can be defined within reasonably narrow limits, but 

 when the organism has been away from its natural 

 habitat for varying periods of time, and has existed 

 under abnormal conditions, its ability to react nor- 

 mally to the usual tests appears to be greatly im- 



