174 GENERAL MICROBIOLOGY 



EXERCISE 8. TO DEMONSTRATE THE PRODUCTION 

 OF H 2 S BY BACTERIA 



Apparatus. Three tubes ordinary gelatin; tube ordinary 

 agar; sterile Petri dish; lead carbonate, 0.1 gm. 



Cultures. B. coli communis; B. mycoides; B. mesen- 

 tericus vulgatus. 



Method. 1. Make stabs of all organisms in gelatin and 

 place these at a temperature not exceeding 20 C. 



2. Melt a tube of agar and while hot add 0.1 gm. of 

 lead carbonate to the tube and mix well by rolling it vigor- 

 ously between the hands (avoid air bubbles) . 



3. Pour into the sterile Petri dish and when cold make a 

 streak (2.5 cm. long and 3 cm. apart) of each organism on 

 the plate in the order named. Invert and place at 25 C. 



4. Examine the gelatin stabs from day to day for lique- 

 faction; examine the plate culture at the same time. Note 

 the action on lead carbonate (Beijerinck's test). 



5. Write chemical equations for the action of sulphur- 

 eted hydrogen on lead carbonate. 



/\ 



Lead carbonate = Pb<Q >C = 



x cr 



6. Is there any relation between the power of organisms 

 to liquefy gelatin and to produce " lead-blackening " sul- 

 phur? Explain. 



From what compounds is the H^S produced in this 

 experiment? What type of organisms can be detected by 

 this test? Where do they occur in the largest numbers in 

 nature? 



Which of the ordinary laboratory media offer the greatest 

 source from which this gas may be produced? Explain. 



By what other means may H 2 S production by bacteria 

 be demonstrated? 



