METHODS USED IN CULTIVATION OF BACTERIA 173 



touch the sides of the glass, and is passed into the tube which is 

 to be inoculated. If the tube contains a slanted medium, such as 

 agar, a light stroking motion from the bottom of the slant to its 

 apex will deposit the bacteria upon the medium evenly along a 

 central line. The needle may also be plunged downward into the 

 substance of the nutritive material so that in the same tube both 

 surface growth and deep growth may be observed. If a stab culture 

 is to be made in unslanted agar or in gelatin, the needle is simply 

 plunged straight downward as nearly as possible along the axis of 

 the medium. If a fluid medium is being inoculated, the wire should 

 be introduced only into the upper part of the liquid and the bacteria 

 gently rubbed into emulsion against the side of the glass. The 



FIG. 12. TAKING PLUGS FROM TUBES BEFORE INOCULATION. 



needle is then removed from the tube, the stopper carefully replaced, 

 and the platinum wire immediately sterilized in the flame. This 

 sterilization of platinum needles after they have been in contact 

 with bacteria should become second nature to those working with 

 bacteria, since an infraction against this rule may give rise to serious 

 and widespread consequences. In burning off platinum needles it 

 is well to remember that a part of the glass rod, as well as the wire 

 itself, is introduced into the tubes and may become contaminated, 

 and for this reason the rod itself, at least in its lower two or three 

 inches, should be passed through the flame as well as the wire. As 

 an extra precaution against contamination, the lips of test tubes and 

 flasks and the protruding edges of cotton plugs may be passed 

 through the flame and singed. 



