304 CLEAN MILK 



c. Wash the third and sterilize in steam. 



d. Leave fourth unwashed. 



Fill all four with fresh milk. Place all side by side at a room 

 temperature and notice the time of souring in each case. 



21. Bacteria in the Milk Pail. — Place ioo c.c. of sterile 

 water in a milk pail that has received a simple washing. With a 

 little absorbent cotton rub this water around thoroughly; allow to 

 stand a few moments, and, after another washing around the pail, 

 remove i c.c. of this water and plate in agar. Count the colonies. 



a. Repeat, using (i) a dirty pail; (2) a pail simply steamed 

 for a few seconds in a steam jet, and (3) a thoroughly sterilized 

 pail. 



22. Bacteria in Manure. With a sterilized platinum loop 

 transfer a small bit of either fresh or dried manure from a cow 

 to sterile water flask. Mix thoroughly. Transfer two loopfuls- 

 of the mixture to a tube of melted agar. Pour into petri dishes 

 and after growth count the number of colonies. To determine the 

 actual number in manure it is necessary to weigh the original bit of 

 manure and make subsequent calculations. 



23. Isolation of B. Coli. — Repeat No. 22, using litmus 

 agar. If any strong acid surface colonies appear, choose one that 

 shows a gas bubble if possible; isolate, inoculate on an agar slant, 

 and purify as usual. From the purified culture inoculate a glucose 

 fermentation tube. If gas appears, examine the culture and deter- 

 mine whether the bacterium is a short motile rod. If so, the species 

 is probably B. coli; if it is not motile, it is probably B. aerogenes. 

 Compare with page 296, f. 



24. Bacteria on Hair. — Melt a tube of agar and one of 

 gelatin, and pour into petri dishes. After they have hardened 

 place upon the surface of each one or two hairs from the flank of 

 a cow. Two or three days later examine, and note the numerous 

 bacterial colonies growing along the course of the hairs. 



25. Fore Milk. — Draw the first 3 jets of milk from one 



