396 MILK AND MILK PRODUCTS ' 
ADVANTAGES AND DISADVANTAGES OF THE DIRECT MICROSCOPIC 
METHOD AS COMPARED WITH THE PLATE METHOD 
DISADVANTAGES 
Direct Microscopic Method Plate Method 
1. Difficult to measure accurately such 1. All bacteria do not grow on the plates 
a small quantity of milk. because of changes in food tempera- 
ture relations, or other conditions 
of environment. 
2. The sample measured is too small to 2. The difficulty of breakmg up the 
be representative. clumps in the milk affects the ac- 
curacy of the count. 
3. Dead bacteria may be counted. 3. Requires from two to five days’ in- 
cubation period. 
4. Error of count is great where bacteria 4. Different species require different in- 
are few or many. cubation temperatures. 
5. Cannot be used for quantitative 5. Gives no idea of the morphology of 
work when bacteria are few in the bacteria present. 
number. 
6. Many fields must be counted, because 6. More apparatus required, therefore 
of the uneven distribution, if an more expensive. Technique com- 
accurate count is desired. plicated and difficult for the trained 
bacteriologists to use in such a 
way to secure consistent results. 
7. Large compact clumps cannot be 
counted. 
8. Bacteria may be lost in the process of 
preparing slides. 
ADVANTAGES 
Direct Microscopic Method Plate Method 
1. Less apparatus required, therefore 1. Is necessary for isolation of pure cul- 
less expensive. Technique simple. tures. 
2. The results on a given sample may 2. Gelatin shows the liquefiers and if 
be reported in a few minutes. litmus is used, the acid producing 
bacteria. 
3. Shows the cell content, the presence 3. Shows the character of growth. 
or absence of streptococci and other 
important things necessary in esti- 
mating the sanitary quality of milk. 
4, Gives a better idea with regard to the 4. Shows living organisms only. 
actual number of germs present. 
Samples. Milk samples collected, as above described, may be pre- 
served by icing and handled as in the case of the plate method. All 
samples on which the cream has risen to the surface must be vigorously 
shaken before preparations are made from them. 
Apparatus. In addition to a microscope and ordinary microscopic 
slides, stains, etc., the only special apparatus required is a pipette 
