PREPARATION OF CULTURE MEDIA. 33 



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. 43) can afterwards be performed at a higher temperature. 



THE PREPARATION OF CULTURE MEDIA. l 



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 natu- 

 rally. 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 transpar- 

 ent 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 spe- 

 cific differences which are valuable in the identification of bac- 

 teria. 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 

 become evident. Many organisms, however, grow best at a 

 temperature at which this nutrient gelatin melts, and therefore 

 another gelatinous substance of vegetable origin, called agar, 

 which does not melt below 98 C, was substituted. Bouillon 

 made from meat extract, gelatin, and agar media, and the modi- 

 fications of these, constitute the chief materials in which bacteria 

 are grown. 



1 The student is strongly advised to make himself familiar with the more elaborate 

 technique laid down by the Bacteriological Committee of the American Public Health 

 Association, which has been adopted in America as the standard of procedure in 

 bacteriological investigation. The technique as outlined in the following pages will 

 be found, however, to be more suitable to the necessities of the work of large classes, 

 or where facilities for the prosecution of advanced work are lacking. 



