TYPES OF MICROORGANISMS IN MILK 



267 



12. For careful quantitative work it is necessary to count 

 one hundred fields for each sample, i.e., fifty fields per square. 

 If n = the number of fields counted and m = the total num- 

 ber of bacteria found, the number of bacteria per cubic 

 centimeter is calculated by the following formula: 



z hundred thousand 



centimeter of milk. 



= number of bacteria per cubic 



In comparatively fresh milk where the bacteria are few, 

 count the whole microscopic field. 



An eye-piece micrometer having a large square ruled 

 into smaller squares is recommended where large numbers 

 of bacteria are present. The area of the large square is 

 different from that of the whole microscope field and con- 

 sequently the factor necessary for computation is different. 

 This factor can be determined by modification of the formula 

 given in 11. 



13. Draw a typical smear from different samples of milk. 

 Indicate the kinds of cells and the number found, also the 

 quality of the milk. 



14. What types of bacteria are found microscopically? 

 How do these compare with those found on plates as to 

 types and numbers? 



What are the advantages and disadvantages of the plat- 

 ing method? Of the microscopic method? For what type 

 of work is each best adapted? What other microscopic 



