18 A MANUAL OP BACTEEIOLOGY 



Exercise 21. Effect of Drying upon Bacteria 



1. Prepare five clean cover glasses and sterilize by quickly- 

 passing them several times through a flame. 



2. Heavily inoculate 10 cc. of distilled water with B. subtilis 

 and mix by shaking thoroughly. 



3. With the sterile platinum loop place an approximately 

 equal quantity of this bacterial suspension on each cover glass. 



4. Place the cover glasses in a sterile Petri dish and dry them 

 in the incubator at a temperature not higher than 28° C. 



5. On the next day and every twenty -four hours later transfer 

 one cover glass with sterile pincers to a sterile Petri dish and 

 flood it with a tube of melted agar. Rock the dish gently to 

 distribute the bacteria before the agar hardens. 



6. Determine the effect of drying upon this organism by 

 counting the colonies as they develop on the agar plates. 



7. Duplicate the experiment, using B. coli. 



8. To what difference between these two organisms can you 

 refer the difference of results ? 



Exercise 22. The Effect of Different Disinfectants 



Pour beef bouillon into a series of test tubes, filling each about 

 one third full. Inoculate all tubes from a hay infusion. To the 

 different tubes add the following: a, nothing; b, \ g. NaCl; 

 e, 1 g. NaCl ; d, 2g. sugar ; e, 5 g. sugar ; /, 1 drop mercuric 

 bichloride solution (1 : 1000) ; g, 6 drops mercuric bichloride 

 solution ; h, 1 drop formalin ; i, 3 drops formalin ; j, 1 drop 

 carbolic acid solution (1 : 20) ; k, 10 drops carbolic acid solution; 

 I, 150 mg. borax ; m, 300 mg. borax. Tubes A, i, j, and k should 

 be corked. Why? Place all test tubes in the incubator and 

 examine at intervals to see which of them undergo putrefac- 

 tion and which are thoroughly disinfected. Note how very 

 much more efficient some disinfectants are than others. How 

 do you explain the results of h, c, d, and e ? Which proves to 

 be the most efficient? For a discussion of the relative value 

 of these and other disinfectants the student is referred to the 

 Appendix. 



