BACTERIOLOGICAL TECHNIC 87 



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

 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 cc. If the bacterial content is high, it is necessary to extend the dilu- 

 tion four and even five times. Usually boiled distilled water is used 

 with which to make desired dilutions (i-io, i-ioo, 1-1,000, i--io,ooo, 

 etc). 



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 cc. of sterile dis- 

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

 using i cc. 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 per- 

 centage 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 triturate sample weighed 0.856 gm. and the number of bacteria found 

 was 3,000,000; and the percentage of moisture was 10. From these data 

 it would be found that i gm. of dry soil will contain 3,855,011 microbes. 



The above is sufficient to make clear how one might proceed to deter- 

 mine the number of microbes in and upon old pills, tablets, powders; 

 on one ivory vaccine tip, in one glycerinated vaccine tube, in i cc. of bac- 

 terial vaccine, antitoxin, syrup, tincture, fluidextract, camphor water, 

 distilled water, sewage, drinking water, etc. Naturally, great caution and 

 care must be observed to avoid errors and faulty conclusions. In fact, 

 no one should attempt such work in actual practice until after considerable 

 preliminary laboratory experience. 



It is not practicable nor is it necessary to give fuller information regard- 

 ing bacterial cultures. We have not touched upon the various methods 

 for determining whether or not the microbes under investigation are 

 essentially aerobic or essentially anaerobic; the manner of determining 

 the thermal death-point; relationship of rate of growth to temperature, 

 etc. We have said nothing of the use of indicators added to culture media, 

 as litmus, rosolic acid, and phenolphthalein, nor have we explained the 

 special use of special culture media in determining the nature and identity 

 of bacteria. These and many other details we must omit, merely stating 

 that, should it become desirable to make such investigations, the necessary 

 information must be secured elsewhere, as in some standard laboratory 

 guide in bacteriological technic. 



The following outline of special methods will serve as a guide in making 

 bacteriological examinations of soils, air, pharmaceuticals, liquids, etc. 



