LESSON HI. 



PREPARATION OF TUBES, FLASKS, DISHES, ETC. 



In the study of bacteria the first object before the investigator is to obtain, isolated 

 from all other forms of microorganisms, a sufficient growth of the particular type to 

 afford opportunity for observation of its characteristics. Such an isolated growth is 

 spoken of as a pure culture; where several varieties are more or less mingled in their 

 development, the term mixed or impure culture is used. The artificial cultivation of 

 these organisms is accomplished by affording them conditions for growth more or less 

 similar to those required by them in nature (nutrient material, a definite range of tem- 

 perature, moisture, and proper atmosphere). For the purpose of preserving the nu- 

 trient material free from other organisms than those purposely implanted upon it, and 

 to aid in maintaining perfect isolation of the various cultures, there is to be provided 

 no little apparatus, the principal forms of which may here be briefly considered. 



i. Test-tubes. A large number of test-tubes, the most commonly employed form 

 of container for culture media, should always be available. The ordinary chemical test- 

 tubes may be used for this purpose, but it is preferable that the tubes should be some- 

 what heavier and made of a glass which will not be corroded by the process of steriliza- 



FIG. ii. TEST-TUBE BRUSH. 



tion. They may be plain, or provided with the usual flange about the mouth, as suits 

 the fancy of the individual. A convenient size is five inches in length and five-eighths of 

 an inch diameter; and it will be found advantageous to have a small portion of the 

 outer surface of the upper part of each tube roughened by etching with "white acid," 

 for the purpose of receiving pencil marks in labeling. A larger size is usually provided 

 for the reception of potato cylinders sometimes employed as culture media. Moreover, 

 it is well to have a few very large tubes on hand for use in making anaerobic cultures and 

 as sedimentation tubes in the preparation of solidified blood-serum. 



Cleansing New Tubes. Before the nutrient media are introduced into the tubes the 

 latter are to be carefully cleansed, the mouth of each plugged with a cotton stopper, and 

 the whole sterilized. In preparing tubes which have not been previously used, ordinary 

 washing in water with a test-tube brush (Fig. 1 1) followed by soaking for from five to ten 

 minutes in a weak solution of one of the mineral acids (as a one per cent, solution of 

 hydrochloric acid) to neutralize any of the remaining alkali employed in manufacture, 

 and by rinsing in clean water, will be found sufficient. The tubes are then to be in- 

 verted on the draining board to dry. 



Cleansing Old Tubes, When tubes have been previously used for culture purposes 



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