352 PUBLIC HEALTH BACTERIOLOGY 



added, and the whole sterilized. Thereafter, the sample, 

 having been well shaken, is added direct by sterile pipette, 

 and always without concentration by filtration. If the 

 sample is too strong, suitable dilutions are made, and i 

 c.c. of the dilution is added. In the case of an unknown 

 water the following tubes would be put up : 



o-oooi c.c. of sample to 10 c.c. of single strength medium. 



of double strength. 

 25 c.c. of triple ,, 



30 c.c. of quadruple ,, 



The tubes are put in a nest or basket, and incubated at 

 37 C. for twenty-four hours. The possible results are 

 four : 



(a.) Acid and gas. 



(b.) Acid, no gas. 



(c.) No acid, no gas ; turbidity. 



(d.) No visible change. 



Interest lies in (a) and (b), and they are commonly 

 associated ; but the absence of (a) from all the tubes 

 should not be held as precluding the necessity for further 

 investigation. Note the tubes showing acid and gas, or 

 acid alone ; say that the tube containing least amount of 

 sample, and which shows acid and gas, is that to which 

 o- 1 c.c. of sample was added, then the result is stated thus : 

 Sample showed acid and gas formation down to o-i c.c. 

 As a rule all the higher tubes will show acid and gas too. 



Some workers call this the " presumptive B. coli test ; '* 

 but as the following paragraphs (from Notter and Firth) 

 show, the test is only a step on the way towards the isolation 

 of B. coli. 



MacConkey's Neutral-red, Bile-salt, Peptone, Glucose Water. 

 Reaction of certain bacteria with : 



Group i. Bacteria producing acid -f- gas. 



B. coli communis, B. enteritidis (Gaertrier), B. 

 paracolon, B. paratyphosus, B. pneumoniae, B. lactis 



