56 A MANUAL OF BACTEEIOLOGY 



media, and ferment dextrose and lactose, forming acid but no 

 gas. On the litmus-lactose-agar plate, made from a polluted 

 water, the streptococcus colonies may be distinguished from 

 other acid-forming colonies by their small size, compact structure, 

 and permanent deep red color. The organisms grow feebly on 

 the surface of beef agar, but flourish better under partial anaerobic 

 conditions, such as those prevailing in the lower part of the stab 

 canal. 



While it seems entirely reasonable to look upon the strep- 

 tococci as evidence of pollution, still there is no such well- 

 established connection with sewage as in the case of the colon 

 bacteria. 



Houston regards streptococci as indicative of recent and objec- 

 tionable pollution. Horrocks, however, found that B. coli gradu- 

 ally disappeared from jars of sewage kept in the dark for three 

 months, and that those forms which survived were mainly varie- 

 ties of streptococci and staphylococci. He believes that strepto- 

 cocci indicate pollution with old sewage which is not necessarily 

 dangerous. 



1. Transfer from colonies of streptococci to tubes of ster- 

 ile media. Cultivate on beef gelatin, beef agar, and dextrose 

 bouillon. 



2. Make stained preparations from bouillon cultures. 



Exercise 81. Isolation of Bacillus coli 



Much remains to be desired in the way of methods for iso- 

 lating the colon bacillus. With the most careful work it is 

 not possible to isolate this germ from all samples of polluted 

 waters. 



1. Prepare an " enriching culture " by inoculating fermenta- 

 tion tubes of sterile dextrose bouillon each with 1 cc. of the 

 sample under investigation. Mix by tilting the tubes. Incubate 

 at 37.5° C. for ten to twenty-four hours. Watch the tubes and, 

 upon the first appearance of gas, plate out the organisms on 

 litmus-lactose agar. 



2. Watch the plates for acid-forming colonies. Transfer from 

 them to agar-slope cultures. If the culture resembles B. coli. 



