86 BACTERIOLOGICAL METHODS 



some very objectionable features, chief of which is that the residue 

 remaining in the tube after pouring retained a certain percentage 

 of the organisms, thus interfering with the accuracy of the results. 

 It must, however, be admitted that the method has some ad- 

 vantages, chief of which is the more uniform mixing of the bacteria 

 with the medium and their more uniform distribution in the 

 plate, making accurate counting of the colonies easier. 



5. Making the Dilutions. Whether or not making dilutions 

 is necessary depends upon the number of organisms present in 

 the substance to be analyzed. The number of colonies in a 

 Petri dish must not exceed 200 in order to make counting fairly 

 easy and accurate. In fact with the method of direct planting, 

 as usually recommended, which generally results in a some- 

 what irregular distribution of the bacteria (hence also the colonies 

 to be counted) it would be desirable to make the dilutions such 

 that the number of colonies in each plate shall not exceed 100. 

 If o.i cc. quantites are to be plated or planted, as above recom- 

 mended, it would follow that dilution would not be necessary 

 as long as the number of bacteria per cc. does not exceed 

 1000. 



However, since most food and drugs contain more chan that 

 number of bacteria per cc., it becomes necessary to make dilu- 

 tions. The standard dilutions are made by tens, as i-io, i-ioo, 

 i-iooo, and 1-10,000. The dilutions are made by adding i cc. 

 of the substance to be analyzed to 9, 99, 999 and 9999 cc. of 

 sterile distilled water, or other desirable sterile diluent, and 

 shaking thoroughly. In practice it is desirable to plate three 

 of the graded dilutions, so that the second higher dilution will in 

 all probability yield about 100 bacteria in the o.i cc. of the 

 material plated. Thus with fairly pure drinking water, the plant- 

 ings would be made from the undiluted water, the i-io and the 

 i-ioo dilutions, presuming that there are about 10,000 bacteria 

 per cc. present. In case of unusually pure drinking water, that is 

 water in which the number of bacteria is probably not more 



