74 APPLIED BACTERIOLOGY 



surface of the medium in the form of isolated drops, which 

 do not coalesce ; or they may form skin-like ridges, as is 

 the case in the growth of the tubercle bacillus. 



'Stab' Cultures. — A platinum wire inoculated with the 

 infecting material is thrust into the depth of an ordinary 

 culture-tube containing about 10 c.c. of nutrient medium, 

 care being taken to introduce the wire in a central line and 

 in a direction parallel with the sides of the tube. It is 

 best to always hold the tube during the inoculation in an 



FrG. 17. — Method of making a ' Stab ' Cultukb. 



inverted position to prevent the risk of contamination, and 

 to singe the plug before returning it to the tube. 



The characters of the growth in these ' stab ' or ' depth ' 

 cultures are very various. In the case of non-liquefying 

 organisms, the growth may be entirely on the surface or 

 only in the depth. In the first case, the organism is aerobic 

 — that is, it requires oxygen for its growth, and will only grow 

 in presence of this gas ; in the second case, the organism 

 is anaerobic, in which case it cannot grow in the presence 

 of oxygen or air, and consequently does not grow upon the 

 surface of the culture medium or along the upper portion of 

 the line of puncture. 



The growth, again, may grow both on the surface and 

 also along the line of puncture. In this case the organism 



