COLON BACILLUS IN WATER ANALYSIS. 113 



number of organisms developing at 38 C. at all approximates the num- 

 ber developing at 20 C., there is a strong suspicion that sewage or- 

 ganisms may be present. Normal waters give proportions of i to 25 

 or i to 50, while in sewage contaminated waters the proportion may be 

 as i to 4 or less. 



In addition, the appearance of pink colonies on the lactose litmus 

 agar is of great assistance in judging of a water. Both sewage strepto- 

 cocci and the colon bacillus give pink colonies those of the streptococci 

 are smaller and more vermilion in color. Microscopic examination 

 will differentiate the cocci from the bacilli. It is well to bear in mind 

 that the pink colonies after 24 hours may turn blue in 48 hours from 

 the development of ammonia and amines. Consequently the lactose 

 litmus agar plates should be studied after 24 hours. 



A good water supply will rarely show a pink colony, while in a 

 sewage contaminated one the pink colonies w r ill probably predominate. 



The diagnostic characteristics considered important by the Ameri- 

 can authorities in reporting the colon bacillus are: 



1 . Typical morphology, nonsporing bacillus, relatively small and 

 often quite thick. 



2. Motility in young broth cultures. (This is at times unsatisfac- 

 tory, as some strains of the colon bacillus do not show it even in young 

 bouillon cultures.) 



3. Gas formula in dextrose broth. Of about 50% of gas produced, 

 1/3 should be absorbed by a 2% solution of sodium hydrate (CO 2 ). The 

 remaining gas is hydrogen. (Later views indicate that the gas formula 

 is exceedingly variable and should not be depended upon. To carry 

 out this test one fills the bulb of a fermentation tube with the caustic 

 soda solution then, holding the thumb over the opening or with a 

 rubber stopper, the bouillon culture and the soda solution are mixed by 

 tilting the fermentation tube to and fro. The total amount of gas is 

 first recorded and then that remaining after the CO 2 has been absorbed 

 is reported as hydrogen.) 



4. Nonliquefaction of gelatin. 



5. Fermentation of lactose with gas production. 



6. Indol production. 



7. Reduction of nitrates to nitrites. 

 8 



