EFFECT OF FREEZING ON BACTERIA 199 



What inter-relations have the optimum, minimum and 

 maximum temperature requirements of one species of 

 microorganism? 



What influence will the reaction of the medium have 

 upon the extremes of temperature at which microorganisms 

 will grow? 



5. Discuss your results in detail and draw any conclu- 

 sions permitted. Point out any practical applications. 



REFERENCES 



MARSHALL: Microbiology, pp. 153-157. 

 LAFAR: Technical Mycology, Vol. I, pp. 58-60. 

 JORDAN: General Bacteriology, 4th Ed., pp. 70-72. 



EXERCISE 22. THE EFFECT OF FREEZING UPON 

 SPORE - FORMING AND NON - SPORE - FORMING 

 BACTERIA 



Apparatus. Small ice dish; thermometer; three tubes 

 each of sterile cider, sterile milk, sterile broth and sterile 

 wort; coarse salt; ice. 



Cultures. Sacch. cerevisice; Bad. lactis atidi; B. 

 megaterium; Aspergillus niger. 



Method. 1. Heavily inoculate the cider tubes with the 

 yeast, the milk tubes with Bad. ladis acidi, the broth tubes 

 with B. megaterium and the wort tubes with Aspergillu niger. 



2. Incubate one set of cultures at 25 C. 



3. Make sufficient freezing mixture of ice and salt 

 to nearly fill the ice dish. 



4. Carefully insert the two remaining sets of cultures 

 in the freezing mixture and keep the freezing mixture at or 

 below C. for two hours. 



5. Remove one set of tubes and incubate at 25 C. 



6. Then place the ice dish containing the third set of 

 cultures in the refrigerator (note the temperature) . 



7. Examine both sets of cultures at the end of twenty- 

 four hours and forty-eight hours for growth. 



