18 LABORATORY EXERCISES IN BACTERIOLOGY. 



bator, and the results noted at the close of twenty-four, forty-eight, and 

 seventy-two hours. 



Exercise 4. In a similar manner a bit of dust from the floor or other 

 exposed surface in the laboratory is obtained by drawing the sterilized 

 platinum needle over it, which is then brought in contact with a prepared 

 sterile tube of some nutrient medium, the latter placed in the incubator, 

 and results noted at the close of twenty-four, forty-eight, and seventy-two 

 hours. 



Exercise 5. Expose for fifteen minutes, to the atmosphere of the room, 

 a dish preparation of gelatine or agar; close the dish and at room tempera- 

 ture for the former, or at incubator temperature for the latter, note at the 

 close of twenty-four, forty-eight, and seventy-two hours the results of the 

 probable inoculation of the medium from the air of the room. 



STERILIZATION. 



By this term is meant the cleansing of any substance so as to free it from such living 

 organisms as may exist within or upon it. Just as it is necessary, in the preparation of a 

 garden intended for the cultivation of useful vegetables, to free the soil from weeds 

 which may interfere with the growth and excellence of the desired plants, so it is essen- 

 tial that all substances upon which it is intended to grow some given form of bacteria be 

 first freed from all contaminating organisms. So, too, lest such contamination occur 

 subsequently and more or less continuously, all containers and such other apparatus as 

 in the necessary manipulations will come in contact with the nutrient media employed, 

 must likewise be subjected to sterilization. Thus alone can one hope to so isolate his 

 specimen as to make the study of its individual characteristics a possibility and to avoid 

 a hopeless confusion and indistinguishable mixture with any number of other types of 

 bacteria and other protophytes. 



(A) To accomplish this end heat is one of the most available means at hand. It is 

 employed in a number of different manners, each suitable for some particular substance 

 or for some particular object in connection with the general purpose of sterilization. 



1. Flaming. This consists in exposing the object to be sterilized to the naked, 

 smokeless flame of a Bunsen burner or of an alcohol lamp until it may be fairly presumed 

 that any living matter upon its surface has been destroyed. It is usually practised for 

 the rapid cleansing of such objects as the platinum inoculating needle, blades of knives 

 (where the preservation of the temper of the blade is no object), glass rods, and other 

 metallic or glass apparatus of suitable size, and such as are unlikely to be injured by 

 the exposure. The object to be sterilized should be held in the upper, hottest, part of 

 the flame, the time of exposure being usually one or twe minutes, according to the size of 

 the surface to be exposed to the heat, and the probable degree of contamination to be de- 

 stroyed. With metallic objects, like the platinum needle, exposure is usually main- 

 tained until a distinct glow of the metal is attained. Having presumably thus burned 

 all vestiges of life from the object, it is commonly allowed to cool to a harmless tempera- 

 ture before being further used, lest contact with the heated surface be destructive to 

 such substances or organisms desired to be preserved with which the sterile object must 

 subsequently be brought into close relation. As much care as possible should be taken 

 to prevent contamination of the sterilized object after it has been flamed in the 



