686 BACTERIOLOGY OF MILK 
which interfere somewhat with the staining, and stained (after drying), 
with aqueous methylene blue. Alkaline methylene blue should not 
be used because the alkali tends to loosen the film of casein. 
The bacteria are counted with an oil immersion lens. It is necessary 
to adjust the optical combination of lens and eye-piece so that the 
diameter of the microscopic field is exactly 0.16 mm., corresponding 
to an area of 0.005 sq. cm. This can be readily accomplished with a 
stage micrometer. 
Each organism in a microscopic field corresponds to one-five- 
hundred-thousandth the number in a cubic centimeter of the original 
sample of milk (0.0002 X 0.01 = 0.000002) , because 0.01 cc. of milk was 
spread on an area of 1 sq. cm. and ~^\j^ of the volume is viewed in the 
microscopic field. In other words, the microscopic field contains the 
bacteria of 2^o"o'Fo"o7 cc. of the original sample of milk and it is poten- 
tially equivalent to an agar plate culture of the milk in a dilution of 
iT'oTrooo" 0"- 
If the bacteria were uniformly distributed, the number of bacteria 
observed in one field multiplied by 2,000,000 would give directly the 
number of bacteria per cubic centimeter in the milk; usually, however, 
the organisms are somewhat irregularly distributed and in practice 
several fields are counted and the average munber of organisms per 
field is multiplied by 2,000,000. Duplicate determinations should 
always be made. The results obtained are fairly uniform when the 
exact details of the method are closely followed. 
The advantage of the direct microscopic count are: a very material 
reduction in the time necessary to obtain results; milk which con- 
forms to the standard may be quickly passed. Badly contaminated 
milk can be detected by simple inspection without even the formality 
of a count. There are also certain disadvantages. All bacteria which 
are stainable wuth methylene blue are visible by this method and 
dead organisms as well as those which are viable appear in the count. 
This is a decided source of error in pasteurized milk, where a relatively 
large proportion of bacteria are killed by heat; the method also does 
not distinguish sharply between different types of organisms. 
On the whole, the advantages very materially outweigh the disad- 
vantages and employed judiciously the method is of great practical 
value in the bacterial control of dairies and milk supplies. 
The information obtained by the bacterial count is of importance 
chiefly from the viewpoint of the past history of the milk. Milk pro- 
duced in cleanly surroundings, handled carefully in sterile utensils, 
kept cool and delivered promptly, should contain relatively few bac- 
teria. If the milk is handled properly but not ke])t cool, the mmibers 
of organisms usually increase greatly, but as a rule the variety of 
organisms present will be limited. Improperly handled milk kept cool 
will frequently exliibit several types pf bacteria, but not necessarily a 
high total count. A consistent low count with but few types of bac- 
teria usually indicates a satisfactory milk supply. 
