140 EXCREMENTAL ORGANISMS 



Estimation of B. Coli. The methods in vogue for the esti- 

 mation of B. coli fall into two groups, (1) enrichment methods 

 and (2) plate methods. 



Enrichment Methods. In the enrichment methods, varying 

 quantities of the sample are inoculated into liquid media and 

 incubated, the media being subsequently examined as to the 

 presence or absence of B. coU. In this test a carbohydrate is 

 usually employed that is fermented by B. coli with the pro- 

 duction of gas and special tubes are used in which this gas is 

 trapped and retained as visible evidence of fermentation. On 

 account of the economy of space a small inverted tube con- 

 tained in a larger ordinary culture tube (Durham's tube) is 

 now in almost universal use in the fermentation process. As 

 in water examination, there are a number of points in connec- 

 tion with this test that require consideration. The first is the 

 composition of the medium to be employed. If the results are 

 to be based on the presence or absence of gas in the tubes, it 

 is evident that lactose and not dextrose must be the carbo- 

 hydrate employed as there are other groups than B. coli that 

 ferment the latter sugar. The nitrogen requisite for bacterial 

 reproduction is usually supplied by the addition of peptone, 

 although this may be partially displaced by sugar-free beef 

 infusion or extract. Potassium chloride is also a desirable con- 

 stituent (Chamot and Sherwood). Such a medium will give 

 gas formation even with attenuated B. coli, and, if only vigorous 

 forms are desired to be estimated the medium can be prepared 

 with a base of fresh ox bile instead of water. There is con- 

 siderable evidence, however, that the lactose ox-bile medium 

 inhibits the growth of a number of vigorous forms of B. coli in 

 addition to the attenuated ones and for this reason the fresh bile 

 medium is often regarded with disfavour. MacConkey's me- 

 dium, containing 0.5 per cent of bile salt, may also be used and 

 in this case the results will usually be intermediate between 

 those obtained with lactose broth and lactose bile. The main 

 objection to lactose broth is the excessive number of anomalies 

 caused by the overgrowth of other organisms. Aciduric bacilli 



