2 3 o AIDS TO BACTERIOLOGY 



Enumeration of ' Blood-Heat ' Organisms. Two sterile 

 Petri dishes are inoculated with 1 and O'l c.c. of the 

 sample respectively. Two tubes of nutrient agar are 

 melted in a bath of boiling water and allowed to cool to 

 45 C., when the tubes are wiped on the outside, the 

 mouths flamed, and the contents poured into the Petri 

 dishes. The agar and water are mixed by tilting, and 

 placed on a flat surface to cool. This needs to be done 

 with expedition (p. 42). The American committee 

 recommend the use of Petri dishes with ' porous earthen- 

 ware covers, in order to avoid the spreading of colonies 

 by the water of condensation.' Otherwise the agar plates 

 should be inverted in the incubator. Counting should 

 be done at the end of forty to forty-eight hours. 



The actual number of organisms capable of develop- 

 ment at blood-heat is subordinate in importance to the 

 ratio of organisms developing on the agar plates to those 

 developing on the gelatin plates. In a pure water this is 

 generally considerably less than 1 to 10 i.e., I to 20, 30, 

 or 40 while in an impure water the ratio becomes 1 to 8, 

 5, 3, or more. 



Estimation of B. Coli. Quantities of the sample amount- 

 ing in all to 50 c.c. for a shallow well or surface water, 

 or to 100 c.c. for a deep-well water, are added to tubes 

 of a medium containing some substance that is fermented 

 by the colon bacillus, and some other substance that, 

 while inhibiting the growth of most water organisms, 

 allows the growth of the colon bacillus. Savage prefers 

 neutral red glucose broth, but MacConkey's bile-salt broth 

 is generally used, and the carbohydrate incorporated is 

 usually lactose. The colon bacillus ferments this sugar 

 with production of acid and gas, a fermentation reaction 

 not given by a number of organisms that produce acid 

 and gas from glucose. The composition of the broth as 

 described on p. 237 is of ' single strength.' A quantity of 

 medium containing double the amounts of the constituents 

 is also prepared' double strength.' 



If the water from a shallow well is to be examined, 

 quantities of O'l and 1 c.c. of the sample are inoculated 

 respectively into two tubes containing 10 c.c. of single 

 strength medium. Tubes containing respectively 5 c.c., 

 10 c.c., 15 c.c., and 20 c.c. of double strength broth are 

 inoculated with 5 c.c., 10 c.c., 15 c.c., and 20 c.c. 



