48 
Samples collected for bacteriological examinations are usually taken in 
sterile, wide-mouthed, glass-stoppered bottles, packed in ice, and examined as 
soon as possible. 
Ordinary milk contains so many bacteria that it has to be diluted greatly in 
order that such a number of colonies will be present on the plates, that the 
counting may be done easily and accurately and that the results may be of 
value. 
1. Prepare lactose-agar plates from high-grade milk and ordinary market 
milk. Make duplicate dilutions according to the outline in Figs. 17 and 18. 
Be sure that all dilutions are well shaken. Make duplicate plates of the high 
grade milk with 1/1000 cc. and 1/10.000 cc. and of the market milk with 
1/1000 cc., and 1/10,000 and 1/100,000 cc. Make one blank for a control. 
2. Label each dish with the kind of milk, dilution, and the desk number be¬ 
fore making the dilutions. 
3. Melt the agar and cool it to 45 degrees C. before pouring the plates. 
4. Place the plates, inverted, in the incubator for 48 hours or one week at 
room temperature. 
5. Count the colonies, using a plate counter if necessary. When giving the 
final count per cc. of the milk keep in mind the number of colonies allowable 
on agar plates. Arrange the results in tabular form as shown in Table VI. 
Place the table on the regular note paper. 
TABLE VI.—Bacterial content of milk 
Name of 
sample 
Dilution 
Colonies 
on plate 
Bacteria 
per cc. 
Number 
of kinds 
Remarks 
I 
II 
I 
II 
I 
II 
I 
II 
6. (a) How do the total number and the kind of colonies compare in 
the two milk samples? 
(b) Does each colony represent a single organism? 
(c) Why was lactose agar used for the plates? 
