22 



GENERAL BACTERIOLOGY 



EXERCISE 13. TEST-TUBE CULTUEES. 



EXPLANATORY. The extreme minuteness and slight variation in 

 the form of different bacteria render a thorough study of them by 

 direct microscopic observation a difficult and well nigh impossible 

 task. In their study, therefore, it is necessary to depart from the 

 usually accepted rules that govern the determination of the life 

 history of other forms of life and resort to special methods. The 

 most successful of these are those known as culture methods. Ac- 

 cording to these methods the bacteria are sown on various food sub- 

 stances and upon these they develop forming masses easily visible 

 to the naked eye. The manner of their growth and the changes 

 which they produce in these media make it possible to detect differ- 

 ences which would otherwise escape attention. The most common 

 culture media, bouillon, gelatin, agar, and potato have already been 

 prepared, and others will be described as needed. 



Cultures may be made either in test-tubes (streak or stab cult- 

 ures), or on glass plates, as plate cultures. The plate culture is 

 especially important and is used (a) to obtain pure cultures; and 

 (6) for ascertaining the character of the colonies as an aid to 

 their diagnosis. The tube-cultures are serviceable in giving oppor- 

 tunity for a further study of the characters as well as to furnish the 

 most convenient method of maintaining the cultures. 



GENERAL DIRECTIONS. Bacteria when obtained in "pure 

 culture" are usually grown in test-tube cultures. To make these 

 a small portion of a previous culture is transferred to fresh culture 

 media by means of the platinum needles. 



a. Stab Cultures are made in test-tubes containing solid, trans- 

 parent media, such as gelatin 

 and agar. The end of a ster- 

 ile needle is infected with the 

 material to be transferred. 

 The needle is then thrust into 

 the medium to the bottom of 

 the test-tube and withdrawn. 

 In this way the bacteria are 

 left along the entire length 

 of the needle track. For 

 method of holding tubes see 

 Fig. 9. They are held in an 

 inclined position to prevent 



the possibility of infection ^ g Method of holding test . tubes . 

 from the air. , cotton plugs. 



