1 92 Cultures, and Their Study 



media, and that cultures planted in them may not be con- 

 taminated through improper technic, it is important habit- 

 ually to practise certain manipulations by which as much 

 latitude can be given the operator as is consistent with 

 thorough defense against contaminating organisms. To this 

 end the containers of stored culture media should be kept in 

 an upright position, that the cotton stoppers are not mois- 

 tened or soiled. If moistened with the culture media, molds 

 whose spores fall upon the surface of the stoppers may 

 gradually work their mycelial threads between the fibers 

 until they appear upon the inner surface and drop newly 

 formed spores into the contained media, or the cotton stop- 

 pers may be glued fast and further successful manipulations 

 prevented. 



In handling tubes care must be taken to stand them up in 

 tumblers, racks, or other contrivances, and not lay them 

 upon the table so that the contents touch the stoppers. 



o 



Fig. 38. Capillary glass tubes. 



When the cotton plugs are removed in order that the 

 contents of the tubes or flasks may be inoculated or other- 

 wise manipulated the removal and replacement should be 

 done as quickly as convenient, and the mouth of the tube 

 should be flamed before removal. They must be held 

 between the fingers, by that part which projects above the 

 glass, not laid upon the table, from which dust, and inci- 

 dentally bacteria, may be taken up and subsequently 

 dropped into the medium; nor must they be touched with 

 the fingers at that part which enters the neck of the container 

 lest they take up micro-organisms from the skin. The 

 stoppers thus require careful consideration lest they become 

 the. source of future contamination. 



So soon as the cotton stopper is removed, the medium is 

 left without protection from whatever micro-organisms 

 happen to be in the air, so that it should be replaced as 

 soon as possible, and every manipulation requiring its 

 removal performed expeditiously. During the time the 



