TO COUNT BACTERIA IN WATER 263 



percent agar, to which has been added nutrose and crystal violet. 

 In the following order add two litres of water to three pounds of 

 beef, straining and boiling for an hour; after filtering, add twenty 

 grams each of nutrose and peptone, and ten grams of salt. Sixty 

 grams of agar are then added and the mixture boiled and filtered 

 after being rendered alkaline. Boil 300 c.c. litmus solution with 

 thirty grams of lactose, mix with the foregoing and alkalinize with a 

 soda solution, and then add to this 4 c.c. of a 10 percent soda 

 solution, and 20 c.c. 1-1,000 crystal violet (Hochst B.). Mix these 

 solutions together, tube, and pour on plates, spreading the feces 

 or water over the agar, dry and invert in the incubator twelve to 

 twenty-four hours. 



The typhoid colonies in this medium appear less granular and 

 dark than do the colon colonies. 



Typhoid colonies 1-3 mm. in size appear blue, colon colonies 

 red, all other bacteria are temporarily inhibited by the crystal violet. 

 Transfer the colonies to bouillon and test with a highly diluted 

 serum from a rabbit artificially immunized, by the agglutination test. 



To Count Bacteria in Water. 



The sample must be collected in a sterile bottle, and the plates 

 poured immediately, since bacteria multiply enormously after a few 

 hours. 



Take ^ c.c. or J c.c. or i c.c. of the water in sterile pipettes and 

 mix with a tube of melted gelatine or agar, pour quickly into cool 

 sterile petri dishes and place in a cool dry place. The American 

 Public Health Association also recommends the use of + i percent 

 agar plates grown both at room and body temperature. The counts 

 for the two are averaged. After forty-eight hours count the colonies 

 and the result (after multiplication where y 1 ^ or J c.c. of water was 

 used) will be the number of bacteria per cubic centimeter. It may 

 be necessary to dilute the water 5 or 10 times before pouring plates. 

 A glass plate ruled into squares, known as a Wolffhiigel plate, should 

 be used for counting. The number of bacteria in potable waters 



