BACTERIOLOGICAL TECHNIC. 



59 



may now be transferred to test-tubes by means of the straight platinum needle 

 or the loop, and the observations may thus be extended. Transfers can be 

 made to different kinds of media, as agar, gelatin, agar-gelatin, beef broth, 

 milk, prepared potato, etc. 



C. Making Bacterial Counts. In order to determine the number of bac- 

 teria in any given substance the same procedure as was just described is fol- 

 lowed, with the difference that a definite amount of the thoroughly mixed 

 contaminated substance is added to a definite amount of culture medium in 

 the test-tubes in which the dilution mixtures are made. For example, we will 

 suppose that it is desired to determine the number of bacteria (per c.c.) in 

 milk: Thoroughly mix the sample of milk by shaking it in the container. 

 Take o.i, 0.2, 0.5, or i c.c. of the milk (by means of a sterilized graduated 

 pipette) and add it to 9 c.c. of the liquefied culture medium in tube No. i; 

 i c.c. of tube No. i to tube No, 2, also with 9 c.c. of medium; i c.c. of tube 

 No. 2 to tube No. 3 (with 9 c.c. of medium), following the other directions 

 as already given. Plate out as already explained, and watch developments. 

 In Petri dish No. i the number of bacterial growths (colonies) will no doubt 



FIG. 36. Petri dish. 



These dishes are among the essentials in the bacteriological 

 laboratory. (Williams.) 



be so great as to make counting impossible. Petri dish No. 2 may contain 

 360 colonies, and dish No. 3 may contain not more than 40. An average 

 is obtained by repeating the test (using the same milk sample) a number 

 of times. In the above milk sample the average may be 42,000 microbes per 

 c.c. If the bacterial content is high, it is necessary to extend the attenuation 

 four and even five times. 



If it is desired to determine the number of bacteria per gram of dry soil, 

 it- will be necessary to carefully weigh a small quantity (i gm., more or less) 

 of average soil, triturate the entire sample with say, 100 c.c. of sterile distilled 

 water, and from this make the dilution cultures as above described, using 

 i c.c. or less of the soil triturate. To compute the number of bacteria per 

 gram of dry soil, it will now be necessary to determine the moisture percentage 

 in a sample of soil taken from the same place as the sample which was used 

 in making the triturate. The solution is simple. We will suppose the tritu- 

 rate sample weighed 0.856 gm. and the number of bacteria found was 



