226 GENERAL MICROBIOLOGY 



18. After all agar slant cultures have grown sufficiently 

 (twenty-four hours at least), make sub-cultures from each 

 pure culture into litmus milk, gelatin stab, nutrient broth, 

 dextrose fermentation tube, Dunham's peptone solution 

 and nitrate peptone solution for corroborative tests, and 

 record the characteristics of growth according to the descrip- 

 tive chart of the Society of American Bacteriologists, on 

 the form on p. 235. 



19. Determine to which group of water organisms each 

 pure culture belongs. (Consult the table in Savage's Bac- 

 teriological Examination of Water Supplies, pp. 192-1S3.) 



20. Compare pure cultures isolated from acid colonies 

 with a pure culture of B. coli in each case. 



21. Keep the plates at room temperature after forty- 

 eight hours of incubation and count at the end of seven 

 days. Record the counts as above. 



22. Keep the agar shake and fermentation tubes for 

 seven days and record any changes that take place. 



23. Examine gelatin plates after forty-eight hours and 

 then every day or so for seven days. 



24. Count before the liquefying colonies get so numerous 

 or large as to render counting difficult. 



25. Record the total number of organisms per cubic 

 centimeter; also the proportion of the liquefying to the 

 non-liquefying organisms. Deep-well water should contain 

 none or but very few liquefying organisms. Why? 



26. If liquefying organisms are present in large propor- 

 tion or in great numbers on the plates from deep-well 

 water, re-plate from the same source (not from the original 

 sample) to determine whether the liquefying colonies 

 came from the original sample or from some fault of 

 technic. 



27. Fish each type of colony and determine to which 

 group of water organisms it belongs. Are the same organ- 

 isms found on gelatin as on agar plates? 



28. Record and compare the number and types of organ- 



