170 GENERAL MICROBIOLOGY 



EXERCISE 5. TO DEMONSTRATE THE VARIATION IN 

 FOOD REQUIREMENTS OF BACTERIA AND THEIR 

 SELECTIVE POWER 



Apparatus. Two tubes of sterile fermented agar; two 

 sterile Petri dishes; potassium phosphate, di-basic; aspara- 

 gin; peptone; ammonium sulphate; sodium nitrate; dex- 

 trose; lactose; saccharose. 



Culture. B. prodigiosus. 



Method. 1. Melt the tubes of agar in the steam and, 

 when cool but still liquid (about 40 C.), inoculate each 

 heavily with B. prodigiosus and pour the plates. Allow to 

 stand twenty-four hours at room temperature before pro- 

 ceeding. Is there any visible growth on the plate? 



2. Mark on the bottom of each plate with drawing ink, 

 dividing it into three equal sectors. 



3. Use ink to indicate the places of chemicals, which 

 should be deposited at the center of each plate and of each 

 sector. 



4. Use very small quantities of the chemicals and be 

 very careful not to scatter them over the plate while convey- 

 ing them to their proper places, otherwise the purpose of the 

 experiment will be defeated. 



5. Incubate at room temperature and examine the plate 

 from day to day for growth. 



Does the fermented agar support growth of itself? 

 What explanation can you give for the action which 

 occurs? 



6. How is the variation in food requirements of B. pro- 

 digiosus shown? The selective action? Give another 

 example of the demonstration of the selective action of 

 bacteria. Which source of nitrogen is seemingly least 

 available? Which most available? Why? Which carbo- 

 hydrate is most easily digested? Which least? Why? 



Beijerinck, knowing that agar in and of itself is a food 

 for but very few microorganisms, reasoned that this sub- 



