EXAMINATION OF MILK 157 



SECTION 5 



DETERMINATION OF B. COLI AND STREPTOCOCCI IN 

 MILK 



I. Inoculate a series of ten dextrose broth fermen- 

 tation tubes with i c.c. of market milk diluted i : 10, 

 another series of ten fermentation tubes with a dilution 

 of 1:100, another series 1:1,000. 



2. Incubate these tubes at 37" C. 



3. Measure the gas formed after 24 and 48 hours 

 and determine the composition of the gas after 48 

 hours. Directions for this work are given on p. 81. 



4. Determine the number of B. coli present in i c.c. 

 of the milk according to the formula on p. 82. 



5. Make smears from each fermentation tube and 

 stain with Gram stain. 



6. Examine these smears for streptococci and 

 determine the number according to the same formula, 

 substituting the number of tubes containing strepto- 

 cocci for the letter N in the formtila. 



7. Plate out one of the fermentation tubes contain- 

 ing both B. coli and streptococci and carry some of the 

 colonies through all media. 



Note. — ^The work of this exercise should be repeated with 

 pasteurized and certified milk. For pasteurized milk the undi- 

 luted sample and dilutions of i : 10 and i : 100 should be used, for 

 certified milk the undiluted sample and a dilution of i : 10. 



Estimation of numbers of B. coli in milk may be made by 

 inoculation of fermentation tubes containing dextrose broth 

 with falling amounts of milk, i.e., i c.c. o.i c.c. o.oi c.c. etc. 



