ISOLATION OF B. COLI 709 



lactose, and are therefore not typical B. coli, Houston has found 

 that a glucose medium is more delicate than a lactose one. For 

 general purposes, quantities of from 0-1 to 100 c.c. may be added 

 to tubes of the medium selected. Houston cultures quantities of 

 0-1, 1-0, 10 and 100 c.c. ; here the gaps are rather wide. Green- 

 wood and Yule consider that a series of quantities in geometrical 

 ratio should be used. In practice it is best to put up two series, 

 one of 100, 50, and 25 c.c., and a second of 40, 20, 10, 5, 2-5, and 

 1-0 c.c., with an additional tube of 0-1 c.c. The first series is 

 available if the water be of good to medium quality, the second 

 if it be medium to bad. 



If the medium shows changes (acid -f- 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. proteus, B. lactis ae'rogenes, 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 

 of either (a) litmus lactose agar, reaction -f- 10 ; (6) litmus 

 lactose bile-salt agar ; (c) Conradi-Drigalski agar, which 

 the author generally employs ; or (d) neutral-red lactose 

 bile-salt agar. ' (For composition of the media, see 

 p. 715.) 



The agar medium is melted, poured into the Petri dishes, and 

 allowed to solidify with the lid tilted so as to leave an aperture 

 through which the steam may escape. The plates with the lids 

 tilted may be dried in the warm incubator for half an hour. The 

 dilution is prepared by adding one 3 mm. loopful of the culture 

 (well shaken) to 8-10 c.c. of sterile water in a test-tube. Of this 

 dilution 1-3 drops are placed on the surface of a plate and spread 

 with a sterile Leaped glass rod. The plates are incubated 



