30 METHODS OF CULTIVATION OF BACTERIA 



FIG. 5. Steriliser for blood 

 serum. 



temperature above that point. Such a 

 medium is sterilised on Tyndall's prin- 

 ciple by exposing it for an hour at 57 

 C. for eight consecutive days, it being 

 allowed to cool in the interval to the 

 room temperature. The apparatus shown 

 in Fig. 5 is a small hot-water jacket 

 heated by a Bunsen placed beneath 

 it, the temperature being controlled 

 by a gas regulator. To ensure that 

 the temperature all around shall be 

 the same, the lid also is hollow and 

 filled with water, and there is a special 

 gas burner at the side to heat it. 

 This is the form originally used, but 

 serum sterilisers are now constructed 

 in which the test-tubes are placed in 

 the sloped position, and in which 

 inspissation (vide p. 40) can after- 

 wards be performed at a higher 

 temperature. 



THE PREPARATION OF CULTURE MEDIA. 



The general principle to be observed in the artificial culture 

 of bacteria is that the medium used should approximate as 

 closely as possible to that on which the bacterium grows naturally. 

 In the case of pathogenic bacteria the medium therefore should 

 resemble the juices of the body. The serum of the blood 

 satisfies this condition and is often used, but its application is 

 limited by the difficulties in its preparation and preservation. 

 Other media have been found which can support the life of all 

 the pathogenic bacteria isolated. These consist of proteids or 

 carbohydrates in a fluid, semi-solid, or solid form, in a trans- 

 parent or opaque condition. The advantage of having a variety 

 of media lies in the fact that growth characters on particular 

 media, non-growth on some and growth on others, etc., constitute 

 specific differences which are valuable in the identification of 

 bacteria. The most commonly used media have as their basis 

 a watery extract of meat. Most bacteria in growing in such an 

 extract cause only a grey turbidity. A great advance resulted 

 when Koch, by adding to it gelatin, provided a transparent solid 

 medium in which growth characteristics of particular bacteria 



