CHAPTER V. 

 THE CULTIVATION OF MICRO-ORGANISMS. 



Avoidance of Contamination. Micro-organisms are so numer- 

 ous on the body of man and in his environment that they are likely 

 to be present on all articles about us unless special precautions 

 are taken to remove or destroy them. The dust blown about 

 in the air contains bacteria and spores of molds. The primary 

 essential in all bacteriological culture work is the exclusion of 

 these extraneous micro-organisms. The unskilled or careless 

 worker may quickly add some of these chance organisms to the 

 material which he is attempting to study, introducing an element 

 of almost hopeless confusion unless it is recognized. Another 

 essential of great importance, especially when working with patho- 

 genic microbes, is the complete destruction of all living bacteria 

 before they are allowed to pass beyond strict and absolute con- 

 trol. The unskilled or careless worker in the laboratory, who 

 allows micro-organisms to escape from him while he is attempt- 

 ing to study them, is a serious menace not only to himself but to 

 all others in the laboratory. These two primary essentials must 

 be mastered by practice in handling harmless forms. 



Every instrument with which bacteria are handled should be 

 sterilized before it is used, and again after use. In the case of 

 the commonly used platinum wire, this sterilization is accom- 

 plished in the flame. The wire is heated to a glow and allowed 

 to cool before handling bacteria, and immediately after its use, 

 before it leaves the hand, it is brought close to the flame so as to 

 dry the material on it and then again heated to redness. Care- 

 ful drying in this way avoids sputtering and consequent scattering 

 of bacteria, which is almost certain to occur if moist material, 

 especially fat or protein, is immediately thrust into the flame. 



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