CHAPTER X 



METHODS OF SECURING PURE CULTURES OF BACTERIA 



BACTERIA must be studied in pure culture if one is to deter- 

 mine with certainty their cultural, physiological, or pathogenic 

 characters. One of the first efforts made in the study of a disease 

 or any other process brought about by bacteria is to separate its 

 causal organism from all others. Many methods have been devised 

 for this purpose, not any one of them applicable to every case. 



Dilution Method. This method of securing pure cultures is 

 of historic interest only. In the beginnings of the cultivation of 

 microorganisms the culture-media commonly used were liquids, 

 such as infusions from meat and vegetables, and beerwort. This 

 method was used most commonly in securing pure cultures of 

 yeasts. A long series of flasks was prepared with sterile media. 

 The impure culture or mixture of organisms was mixed thoroughly 

 with the contents of the first flask, and a definite amount transferred 

 from this to another flask, from this to each of several others, from 

 each of these into another group, and so on. The last dilution 

 would, in general, remain sterile, but among some of the dilutions 

 would be a group in which some flasks would show growth and 

 others of the same dilution would not. The inference was that 

 such a flask had been planted with but a single organism, and the 

 flask contents, therefore, constituted a pure culture. This method 

 is cumbersome, uncertain, and is rarely used. 



Isolation by Smearing. If a loopful of a mixed culture of 

 microorganisms be drawn across the surface of a solid medium in 

 parallel streaks, the first portion will generally show a solid line 

 of mixed growth, but farther along the growth is discontinuous. 

 Many of the isolated colonies here will be found upon examination 

 to consist of pure cultures. This method is used for the isolation 

 of bacteria from the mouth and throat in some cases. 



Direct Isolation. Barber has devised a capillary pipette method 

 whereby it is possible to pick up a single bacterial cell and transfer 

 it to a nutrient medium without any other organisms being carried 



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