584 A MANUAL OF BACTERIOLOGY 



medium is more delicate than a lactose one. For general purposes, 

 quantities of from 0-1 to 25-0 c.c. may be added to tubes of the 

 medium selected. For the examination of an ordinary drinking- 

 water, the writer usually employs five tubes with 1 c.c. of the 

 water in each, two tubes (double strength) with 10 c.c. in each, and 

 one tube (double strength) with 25 c.c. For the larger amounts 

 large test-tubes and boiling tubes must be employed. 



If the medium shows changes (acid + gas) suggestive of the 

 presence of B. coli, it is only presumptive evidence of the presence 

 of this organism. Occasionally other organisms produce a similar 

 change, e.g. B. lactis aerogenes, B. cloacae. Hence the necessity for 

 the isolation and identification of the organism as recommended 

 in the next section. 



ISOLATION OF BACILLUS COLI, IF PRESENT. If indica- 

 tions of the presence of the Bacillus coli be obtained in 

 the preliminary cultivations (acid + gas), the organism 

 must be isolated and identified. If several tubes show 

 acid + gas, one or two of the tubes with the smallest 

 quantities of the water should be used for this purpose. 



This may be done by making surface cultures on plates 

 (sloping tubes generally suffice) of either (a) litmus lactose 

 agar, reaction + 10 ; (6) litmus lactose bile-salt agar ; 

 (c) Conradi and Drigalski agar, which the writer generally 

 employs ; or (d) ordinary nutrient gelatin. Agar media, 

 incubated at 37 C., have the advantage of saving time. 

 (For composition of media, see p. 590, et seq.) 



IDENTIFICATION OF, AND TESTS FOR, THE BACILLUS 

 COLI. Having obtained coli-like colonies on the plates 

 made from the preliminary cultivations of the water, 

 various tests must be used for identification. The organ- 

 ism should conform in morphology, motility and staining 

 reactions with the characters of the typical B. coli as given 

 at pp. 379-387, and must be subjected to various cultural 

 tests, e.g. the " flaginac " reactions of Houston (p. 384). 

 The writer generally employs these, with the addition of 

 the fermentation reactions given by dulcitol, mannitol, 

 and adonit litmus peptone water, and gelatin for absence 



