132 LABORATORY MANUAL OF MICROBIOLOGY 



Exercise 61 



Growth of Thiobacillus Thiooxidans in Liquid Medium 



Prepare six flasks of medium (Medium 68). 



Inoculate two flasks with 1-gram portions of compost from 

 Exercise 49 (100 days old), two flasks with 0.1 gram of same 

 compost. Two flasks are left as controls. 



Incubate flasks at 25°C. 



After 7, 14, and 30 days, determine pH of flasks. 



Fig. 15. — Thiobacillus thiooxidans (X 1,000). 



If there is an increase in acidity in the inoculated flasks above 

 the control, determine sulphates in solution in all flasks, and 

 examine culture microscopically. 



Repeated transfers of the culture upon fresh sterile medium will 

 result in a highly enriched culture of the organism. Final isola- 

 tion can be made by the use of the agar plate. ^ 



iWaksman, S. a., Jour. Bact., 7: 605-608, 1922. 



IRON BACTERIA 



Exercise 52 



Iron-precipitating Bacteria (Optional) 



Shake 20 grams of field soil with 200 cubic centimeters of 

 water. Dilute until 1 cubic centimeter equals 1 : 100,000. 



Pour plates of Medium 101 or similar formula. 



Incubate the plates for several weeks at 28°C. 



Note the precipitation of iron compounds around certain 

 colonies. 



