100 



BACTERIOLOGY. 



their own species. In this way colonies are formed each possessing 

 its own biological characteristics and morphological appearances. 

 When an adventitious germ from the air falls upon the culture, it also 

 grows exactly upon the spot upon which it fell, and can be easily 

 recognised as a stranger. To maintain the individuals isolated from 

 one another daring their growth, and free from contamination, it is 

 only necessary to thin out the cultivation, and to protect the plates 

 from the air. The slower growth of the micro-organisms in solid 

 media, affording much greater facility for examining them at various 

 intervals and stages of development, is an additional point in favour 

 of these methods ; and the characteristic macroscopical appearances 

 so frequently assumed are, more especially in the case of morpho- 

 logical resemblance or identity, of the greatest importance. The 

 colonies on nutrient gelatine (examined with a low power) of micro- 

 organisms such as Bacillus anthracis and Proteus mirabilis, the 

 naked eye appearances in test-tubes of the growth of the bacilli 

 of anthrax and tubercle, and the brilliant growth of Micrococcus 

 prodigiosus, may be quoted as examples in which the appearances are 

 often very striking and sometimes quite characteristic. 



SOLID MEDIA. 



(A) PREPARATION OF NUTRIENT GELATINE AND NUTRIENT 

 AGAR-AGAR. 



Nutrient Gelatine is prepared as follows: Take half a kilo- 

 gramme of beef (one pound), as free as possible from fat. Chop it 

 up finely, transfer it to a flask or cylindrical 

 vessel, and shake it up well with a litre of 

 distilled water. Place the vessel in an ice- 

 pail, ice-cupboard, or in winter in a cold 

 cellar, and leave for the night. Next morn- 

 ing commenca with the preparation of all 

 requisite apparatus. Thoroughly wash and 

 rinse with alcohol about 100 test-tubes, and 

 allow them to dry. Plug the mouths of the 

 test-tubes with cotton-wool, taking care that 

 the plugs fit firmly but not too tightly. 

 Place them in their wire cages in the hot-air 

 steriliser, to be heated for an hour at a temperature of 150 C. In 

 the same manner cleanse and sterilise several flasks and a small 

 glass funnel. In the meantime the meat infusion must be again 

 well shaken, and the liquid portion separated by filtering and 



FIG. 29. WIRE CAGE 

 FOR TEST-TUBES. 



