BACTEEIOLOGICAL EXAMINATION 219 



Much of this information may be obtained by the use 

 of selective media, selective temperatures, or by a 

 proper combination of the two. 



" By the use of litmus-lactose-agar in place of agar 

 or gelatin we obtain similar counts of total bacteria, 

 and in addition are able to separate those bacteria into 

 two groups, which do and do not produce acid fermenta- 

 tion of lactose, and the numbers of the two classes of 

 bacteria so obtained indicate more completely the 

 character of the water than would the numbers of either 

 class alone. By incubating our plates at temperatures 

 of 30 or 40° C. we are able to obtain counts in 12 to 18 

 hours, which counts, while smaller than those on plates 

 incubated for a longer period at a lower temperature, 

 appear to be fully as significant. If we increase our 

 number of determinations by incubating duphcate 

 plates at two or more temperatures, the various results 

 and the ratios between them furnish a check upon one 

 another in addition to increasing the available data 

 upon which to base an interpretation." (Gage, 1907.) 



Finally, the search for the Bacillus coli furnishes the 

 most satisfactory of all single tests for faecal contamina- 

 tion. This organism is preeminently a denizen of the 

 alimentary canal and may be isolated with ease from 

 waters to which even a small proportion of sewage has 

 been added. On the other hand, it is never found 

 in abundance in waters of good sanitary quality, and 

 its numbers form an excellent index of the value of 

 waters of an intermediate grade. The streptococci 

 appear to be forms of a similar significance useful as 

 }ielding a certain amount of confirmatory evidence. 



