BACTERIOLOGY OF WATER. Ill 



tainkig .2 c.c. (1/5) of the water. The number of organisms would be 

 200 per c.c. Ten colonies were counted on the agar plate containing 

 .2 c.c. and incubated at 38 C. Number of bacteria developing at body 

 temperature equals 50 per c.c. 



There is no strict standard as to the number of bacteria a water 

 should contain per c.c. Koch's standard of 100 colonies per c.c. is 

 generally given. It is by the qualitative rather than the quantitative 

 analysis that one should judge a water. 



If there should be very many colonies on a plate, the surface can be 

 marked off into segments with a blue pencil. If very numerous, cut 

 out of a piece of paper a space equal to i square centimeter. By 

 counting the number of colonies inclosed in this space at different 

 parts of the plate, we can strike an average for each space of i square 

 centimeter. To find the number of such spaces contained in the plate, 

 multiply the square of the radius of the plate by 3.1416. Then multi- 

 ply this number by the average per square centimeter, and we have the 

 total number of colonies on the plate. This is the principle of the 

 Jeffers disk. 



The relative proportion between the bacterial count at 20 C. and 

 that at 38 C. is of great importance from a qualitative stand-point, as 

 will be seen later. 



2. Deliver into a. series of Durham fermentation tubes containing 

 glucose bouillon and into another series containing lactose bouillon 

 varying definite amounts of the water to be examined. In tubes show- 

 ing the presence of gas in both glucose and lactose bouillon the evidence 

 is presumptive that the colon bacillus is present. For the positive 

 demonstration plates must be made from such tubes as show gas. 



It is sufficient to deliver from graduated pipettes in each series 

 quantities of water varying in amount from .1 c.c. to 10 c.c. In our 

 laboratory we inoculate with .1 c.c., .2 c.c., .5 c.c., i c.c. and 10 c.c. 

 of the suspected water. If the .1 c.c. tubes show gas, we have reason 

 to assume that the water contained at least 10 colon bacilli per c.c. If 

 only the 10 c.c. tubes showed gas those with less amounts not having 

 gas we would be in a position to state that the water contained the 

 colon bacillus in quantities of 10 c.c., but not in quantities of i c.c. or 

 less Many authorities regard water as suspicious only when the colon 



