76 PRACTICAL BACTERIOLOGY. 



advantageous to all bacteria^ it may be always added 

 to the gelatine or agar-agar. 



This method of cultivating anaerobes is the best at 

 present knowD, and indeed it leaves nothing to be 

 desired. Even those bacteria which are affected by 

 the smallest trace of oxygen, can be very easily culti- 

 vated in this manner. The development of the colonies 

 proceeds very much in the same manner as that of 

 aerobes in the dishes or in the ordinary coated tubes, 

 though of course anaerobic bacteria generally present 

 a somewhat different appearance. 



Side by side with the colonies of strictly anaerobic 

 bacteria, numerous other forms develop, namely, the 

 facultative anaerobes, or the bacteria which flourish 

 equally well with or without oxygen; for between 

 strictly anaerobic and strictly aerobic bacteria, all 

 imaginable intermediate grades occur. In order now 

 to classify the different species according to their 

 need of oxygen, and to separate strictly anaerobic 

 from aerobic bacteria, there is an excellent method, 

 which is called cuUivafdon in deep layers of solid 

 nutrient media. Test-tubes about 18 cm. long are 

 filled with gelatine or agar-agar till the surface of the 

 medium is only a few millimetres from the cotton-wool 

 plug. The contents are then boiled, in order to expel 

 all traces of air, and are allowed to cool until they 

 just begin to solidify. Some of one of the colonies, 

 grown, as described above, under hydrogen, is then 



