Bacterial Multiplication 147 



forms in the samples will aid the technician in determining the 

 degree of pollution. 



If a bacterial suspension is diluted 1:10, 1:100, 1:1000, 1:10,000, 

 1:100,000, 1:1,000,000 and 1:10,000,000, etc., in nutrient broth 

 tubes, these tubes may be incubated for a suitable time at the 

 optimum temperature and examined for evidence of growth. 

 Should the tubes up to and including a dilution of 1:100,000 show 

 growth, but the 1:1,000,000 tube fail to show growth for example, 

 it would indicate that the original bacterial suspension probably 

 contained about 100,000 bacteria but not as many as one million 

 organisms per ml. The difference between these two figures is 

 considerable, and a single viable organism either carried over or 

 left behind in the serial dilution could make a ten-fold difference 

 in the result. 



THE PLATE COUNT 



An improved modification of the dilution technic is the agar 

 plate count. By substituting a liquefiable-solid medium, such as 

 nutrient agar, for the nutrient broth, the contents of the tubes can 

 be poured into petri dishes, the organisms trapped in the solidified 

 medium can grow, and the resulting colonies can be counted. If 

 the dilutions are carefully prepared, if strict attention is paid to the 

 temperature of the agar medium in the tubes, and if the samples 

 being treated are carefully shaken according to the standard 

 method previously described under the Breed Smear technic, the 

 plate count affords a fairly reliable index of the number of living 

 organisms in a suspension capable of growing under the particular 

 set of conditions provided. 



Rather than prepare the dilutions in measured amounts of agar, 

 which must be kept within a limited temperature range during the 

 dilution procedure, it is customary to prepare dilutions in bottles 

 of sterile water. Aliquot portions are pipetted into sterile petri 

 dishes, and sterile nutrient agar at a temperature of between 45- 

 50° C. is poured into the dishes and mixed thoroughly to insure 

 even distribution of the bacteria. Each cell, or clump of cells, will 

 theoretically develop into a bacterial colony, visible to the unaided 

 eye. It is important to have the test material sufficiently diluted 



