LESSON II. 



OCCURRENCE OF BACTERIA IN NATURE; THEIR 

 DESTRUCTION BY LABORATORY MEANS. 



Occurrence of Bacteria in Nature. Bacteria are found -very -widely distributed in 

 nature both as saprophytes and as parasites. 



The wide distribution of these microorganisms in nature can readily be shown by 

 placing in material free from bacteria (sterile), but known to be suited to their nutrition, 

 matter containing such organisms taken from a variety of sources such as may suggest 

 themselves. It is due to this almost universal presence of these and other low forms of 

 life that almost every organic substance, whether liquid or solid, is so apt to become 

 destroyed by "rotting," especially if the factors of temperature and moisture of the sur- 

 rounding medium be likewise favorable to the development of the germs. As illustra- 

 tive examples the following experiments may be suggested and may be added to at 

 almost any length at the will of the inquirer. 



Exercise i. With a knife blade which has at the time been sterilized 

 (cleansed) by careful heating in the smokeless flame of a Bunsen burner or 

 alcohol lamp (the blade having then been allowed to cool to a convenient 

 temperature without having been placed where it is likely to again become 

 contaminated), some of the superficial cells of the skin of the hand are 

 scraped from the surface and a bit of the scrapings transferred to a sterile 

 tube of any suitable culture medium, as nutrient bouillon. The tube, re- 

 sealed, is to be placed in an incubator holding a uniform temperature of 

 from 20 C. to 37 C. for from twenty-four to seventy-two hours. After a 

 time the presence of a living growth may be inferred by the clouding of the 

 bouillon or by the formation upon the surface or in the mass of solid media 

 of "colonies," appearing as drops or films of a colorless, or occasionally 

 tinted, material. Microscopic examination will confirm the statement that 

 such material is made up of countless organisms of some type. Note re- 

 sults at the close of twenty-four, forty-eight, and seventy-two hours. 



Exercise 2. In a like manner plant scrapings from the mucous mem- 

 brane of the mouth. Note results at the close of twenty-four, forty-eight, 

 and seventy-two hours. 



Exercise 3. Having sterilized the platinum loop in the naked flame and 

 allowed it to cool, a drop of water from the tap is similarly introduced into 

 a sterile tube of nutrient bouillon or other medium, placed into the incu- 

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