100 Report of the Department of Bacteriology of the 



which showed a plate count of 2,855,000, while the total microscopic 

 count was 10,620,000. In other words these two samples gave 

 widely different plate counts, while approximately the same number 

 of individual bacteria were seen with the microscope in each sample. 

 In the second sample referred to there were nearly twice as many 

 clumps as in the first. This indicates that for some reason the 

 clumps were broken apart to a greater extent in the latter case than 

 in the former, which of course would not change the total bacterial 

 count by the microscopic method but would result in an increased 

 number of sources from which colonies would grow on agar. The 

 same difference in count occurred in samples taken on February 

 18 and 19. In this case however the total number of clumps, and 

 the number of isolated bacteria and clumps together, are approxi- 

 mately the same. The difference between the plate and microscopic 

 counts here may also be due to the greater dissociation of the indi- 

 viduals of the clumps by shaking during the process of dilution in 

 the one instance (February 18) than in the other (February 19), 

 but a more probable explanation lies in the possibility that a certain 

 species which did not grow well on agar was present in the sample 

 taken February 19. 



Samples were likewise found which gave approximately the same 

 plate count but differed widely in the total number of individual 

 bacteria present. Such an instance occurred on March 28 when 

 a sample was taken which showed a plate count of 2,525,000 per 

 cubic centimeter with a total individual bacterial count of 10,920,000 

 per cubic centimeter by the microscopic method. A sample taken 

 March 31 gave a plate count of 2,600,000 per cubic centimeter, 

 while the total individual microscopic count was 5,340,000. In 

 this case, more than twice as many individual bacteria were seen 

 by the microscope on March 28 as on March 31, but approximately 

 the same number of colonies developed on the plates. This may be 

 explained by the fact that species occasionally appear which tend 

 to grow in more compact or larger clumps than others. These 

 resist separation when shaken. Such a condition increases the total 

 number of bacteria seen in the microscope but does not increase the 

 number of individual objects from which colonies develop on plates. 



In all of the samples studied (450) there were only three which 

 gave a plate count higher than the total number of individual bacteria 

 seen in the microscope. Although such counts are rare, they must 



