294 WATER ANALYSIS. 



If the water is suspected to be bad, smaller amounts of water 

 may be taken, 1 c.c. of the diluted water being added to 9 c.c. 

 of sterile water. If it is supposed that the water is good, the 

 amounts taken may be increased. The quantities given will, 

 however, usually serve. Many of the colonies on the gelatine 

 will be found to have liquefied the medium, and, if the counting 

 is delayed, the liquid may run down and contaminate the other 

 portions. The author has found that if the colonies are counted 

 in two and a-half days, no practical inconvenience is found from 

 this source. 



The Search for Bacillus Coli Communis. Add 100 c.c. 

 of water to one of the tubes containing triple strength M'Conkey 

 medium. 10 c.c. to one of the tubes containing double strength 

 medium, and 1 c.c. each of the water and 

 of the diluted water to tubes containing 

 the ordinary medium ; mix well by 

 shaking, and incubate (Fig. 39) at 40 C. 

 (104 F.). Examine after one and two 

 days. If acid and gas are not produced 

 B. coli communis is absent ; if acid and 

 gas are produced there is presumptive 

 evidence of this organism. 



Dip a sterile iron wire slightly bent at 

 one end, into each of the tubes showing 

 Fig. 39. Bacteriological acid and gas, and plunge the wire into a 

 peptone water tube, stirring well. Eub 



the wire over the surface of M'Conkey agar, and incubate for 

 twenty-four hours at 40 C. If red colonies are formed the 

 evidence for the presence of B. coli communis is strong. 



Inoculate five of the red colonies into peptone water tubes, 

 and into tubes of the three sugar media. Place the sugar media 

 in the incubator at 40 C. for three hours, and then for half an 

 hour in an ice chest, and incubate at 22 C. (72 F.) for twenty- 

 four hours. Incubate the peptone water tubes at 40 C. for 

 twenty-four or, better, forty-eight hours. 



B. coli communis produces indole in peptone water, and ferments 

 both glucose and lactose with the formation of gas bubbles and 

 acid, but does not ferment cane sugar ; a variety of this organism, 

 however, ferments cane sugar. 



Test for Indole. To the peptone water add a few drops of an 

 alcoholic solution of ^-dimethyl amino-benzaldehyde, and a 

 few drops of a saturated solution of potassium persulphate ; 

 warm slightly, and in the presence of indole a cherry-red colour 

 is produced. 



The Search for Bacillus Sporogenes Enteritidis. Add 

 a little alum to 500 c.c. of the water, and if a turbidity is 



