130 GENERAL MICROBIOLOGY 



the straight needle. Then, still using the straight needle, 

 from the freshly prepared broth culture, inoculate lightly 

 one tube of melted agar (at 40 to 50 C.) and pour into a 

 sterile Petri dish. If the organism shows only a slight growth 

 on the stock culture, transfer directly to melted agar. 



7. Moisten a strip of lead acetate paper and insert with 

 cotton plug in tube of Dunham's solution. Blackening of 

 this paper shows the formation of H2S. 



Between what substances does a chemical reaction take 

 place? What are the resulting products? 



8. Draw and describe twenty-four-hour cultures of the 

 first four bacteria in all media. If at any time presence 

 of growth is doubtful, compare with a tube . of sterile 

 medium. In the absence of growth, reinoculate. 



9. Record macroscopical changes only, in litmus milk; 

 and in fermentation tubes note only, the place of growth, 

 presence and percentage of gas; also the formation of H2S 

 in Dunham's solution. 



10. Make a permanent stained preparation of each organ- 

 ism (following directions under Exercise 28). Young 

 (twenty-four to forty-eight hour) cultures must be used. 

 Use either aqueous-alcoholic fuchsin or aqueous-alcoholic 

 methylen blue. 



11. Make a flagella stain of the largest motile organism 

 among your cultures. 



It is absolutely necessary that a young (eighteen to twenty- 

 four hour, not older) culture be used for this purpose. Fol- 

 low the directions under Exercise 31. 



12. Make further drawings and descriptions from day 

 to day if any change in the growth from that of the preceding 

 day is observed. Three drawings of a culture will be suf- 

 ficient. Endeavor to illustrate typical growth by careful 

 drawings. 



13. State whether the agar plate colony described is a 

 surface or a subsurface colony. How do these two types of 

 colonies differ? Why? 



