BACTERIA IN WATER 693 



tive, therefore, that plating of the water, if possible, shall not be delayed 

 for longer than one or two hours after collection. 



Bacteriological Examination of Water. In describing the meth- 

 ods of bacteriological examinations of water, we adhere strictly to the 

 recommendations of the Committee on Standard Methods of the 

 American Public Health Association, 1 taking the following paragraphs 

 with slight changes from their report of 1915: 



It should be remembered that quantitative estimations of bacteria 

 in water are of most value when repeatedly done and a "normal" for 

 the particular water supply has been established, so that deviation 

 from this "normal" can be easily recognized. Single isolated deter- 

 minations may easily lead to error. 



The following paragraphs are taken without change from the Public 

 Health Association's report: 



"Since gelatine does not give the total number of bacteria in the water, the 

 committee has thought it wise to use agar incubated at 37 C. as a standard 

 medium. This admits of counts in one day instead of two, and gives results 

 on the kind of bacteria growing at blood temperature and therefore more 

 nearly related to pathogenic types. 



"Media. The standard medium for determining the number of bacteria 

 in water shall be nutrient agar. All variations from this medium shall be 

 considered special media. If any medium other than standard agar is used, 

 this fact shall be stated in the report. 



"For general work the standard reaction shall be +1 per cent, but for long 

 continued work upon water from the same source the optimum reaction shall 

 be ascertained by experiment and thereafter adhered to. If the reaction used, 

 however, is different from the standard, it shall be so stated in the report. 



"Procedure. Shake at least 25 times the bottle which contains the sample. 

 Withdraw one c.c. of the sample with a sterilized pipette and deliver it into 

 a sterilized Petri dish, 10 cm. in diameter. If there be reason to suspect that 

 the number of bacteria is more than 200 per c.c., mix one c.c. of the sample 

 with nine c.c. of sterilized tap or distilled water. Shake 25 times and measure 

 one c.c. of the diluted sample into a Petri dish. If a higher dilution be required 

 proceed in the same manner, e.g., one. c.c. of the sample to 99 c.c. of sterilized 

 water, or one c.c. of the once diluted sample to nine c.c. of sterilized water, and 

 so on. In the case of an unknown water or a sewage it is advisable to use sev- 

 eral dilutions for the same sample". To the liquid in the Petri dish add 10 c.c. of 

 standard agar at a temperature of about 40 C. Mix the medium and water 

 thoroughly by tipping the dish back and forth, and spread the contents uni- 

 formly over the bottom of the plate. Allow the agar to cool rapidly on a hori- 



1 Amer. P. H. A., Stand. Meth. Exam. Water and Sewage, 1915. 



