142 



BIOLOGY A3W- TECHNIQUE 



fh 



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 hquid and the bac- 

 teria gently rubbed into emulsion 

 against the side of the glass. The 

 needle is then removed from the tube, 

 the stopper carefully replaced, and the 

 platinum wire immediately sterihzed in 

 the flame. This steriUzation of plati- 

 num 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 re- 

 member that a part of the glass rod, 

 as well as the wire itself, is introduced 

 into the tubes and may become con- 

 taminated, 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 contamina- 

 tion, the lips of test tubes and flasks and the protruding edges of cotton 

 plugs may be passed through the flame and singed. 



Fig. 27. — Platinum Wiees. 



THE ISOLATION OF BACTERIA IN PURE CULTURE 



It is obvious that in many cases where bacteria are cultivated from 

 water, milk, pathological material, or other sources, many species may 

 be present in the same specimen. It is likewise obvious that scientific 

 bacteriological study of any bacterium can be made only if we obtain 



