TEMPERATURE 



17 



growth of the organism can be observed. Of the nutrient elements required, 

 nitrogen is obtained from albumens and proteids, carbon from milk-sugar, 

 cane-sugar, or the splitting up of proteids; salts (particularly phosphates and 

 salts of potassium) are readily obtain- 

 able from those incorporated in the f~ 



media ; and the water which is required ^^ V_ 



is obtainable from the moisture of the 

 media. 



FIG. 5. INOCULATING NEEDLES. 

 Platinum wire fused into glass handles.. 



There are two common forms 



of test-tube culture, viz., on the 



surface and in the depth of the 



medium. In the former the 



medium is sloped, and the inocu- 

 lating needle is drawn along its 



surface ; in the latter the needle 



is thrust vertically downwards 



into the depth of the solid 



medium. Plate cultures and anaerobic cultures will be described at 



a later stage. 



Temperature. -When the medium, has been inoculated the 



culture is placed at a temperature which will be favourable. For 

 every species of bacteria there is a favour- 

 able temperature, termed the optimum 

 temperature. This is usually the tempera- 

 ture of the natural habitat of the organism. 

 Two standards of temperature are in use in 

 bacteriological laboratories. The one, room 

 temperature, varies from 18-22 C. ; the 

 other is Hood-heat, and varies from 35-38 C. 

 (Plates 1 and 2). It is true some species will 

 grow below 18 C., and others above 38 C. 

 The pathogenic (disease-producing) bacteria 

 thrive best as a rule at 37 C., and the non- 

 pathogenic at the ordinary temperature of 

 the room. The different degrees of tempera- 

 ture are obtained by means of incubators. 

 For the low temperatures gelatine is chosen 

 as a medium, for the higher temperatures 

 agar. Most bacteria grow well at room 

 temperature (about 60 F.), but they will 

 grow more luxuriantly and speedily at 

 blood-heat. 

 Whilst these are the ordinary limits of temperature affecting 



bacteria, they do not by any means include the extremes of heat and 



cold which micro-organisms can withstand. The average thermal 



death-point is about 55 C, but certain species, termed thermophilic, 



E 



FIG. 6. Media for 'Surface 

 and Depth Culture. 



