208 GENERAL MICROBIOLOGY 



EXERCISE 28. TO SHOW THE INFLUENCE OF DIRECT 

 SUNLIGHT UPON THE GROWTH OF MICRO- 

 ORGANISMS 



Apparatus. Two tubes of sterile agar; two sterile Petri 

 dishes; two sterile 1 c.c. pipettes; two tubes of sterile 



FIG. 45. Action of Direct Sunlight on Bacteria. These plates were 

 heavily inoculated with R. coli and B. prodigiosus, respectively, 

 and then were exposed bottom side up to the direct rays of the 

 January sun for four hours. At the moment of exposure the 

 figure 0, cut from black paper, was pasted to the plate, shading 

 the bacteria underneath. After one, two and three hours the 

 corresponding figures were pasted to the plates. The above 

 picture was taken twenty-four hours after exposure, proving that 

 three or four hours' exposure to direct sunlight weakens and may 

 even kill bacteria. B. prodigiosus proved more sensitive than 

 B. coli. (From Marshall.) 



distilled water, salt solution or broth for dilution purposes; 

 black paper; glue. 



Cultures. Ps. campestris; B. typhosus. 



Method. 1. Inoculate a tube of sterile liquid heavily 

 with Ps. campestris. Mix the contents well. 



2. Place 1 c.c. of this suspension in a sterile Petri dish 

 and pour the plate. 



