110 LABORATORY EXERCISES IN BACTERIOLOGY. 



that it is absolutely necessary, in order to succeed and in order to avoid danger, that the 

 needle should be thoroughly sterilized in the flame (-v. exercise 6), both before taking up 

 the infectious matter and after inoculation is completed. 



If it is desired to transfer some growth from one tube to a tube of fresh solid 

 medium, the two tubes are grasped in the left hand as shown in the accompanying 

 figure (Fig. 33), the upper ends of the tubes being held well over the ulnar edge of the 

 palm. The cotton plugs may then be removed with a pair of dissecting forceps and 

 laid upon a fresh piece of paper (or may have been removed hy being grasped between 

 the fingers of the left hand before the tubes were placed in position, the loose ends 

 being held between the fingers and the deeper ends projecting beyond the dorsal surface 

 of the fingers). The tubes should be held in a horizontal position or slightly inverted 

 so as to prevent as much as possible the entrance of chance organisms from the atmos- 

 phere ; this position is not possible if one or other of the tubes contains liquid material, 

 in which case the tubes must be kept upright. After the stoppers have been disposed 

 of and the tubes properly held,the needle is taken in the right hand, held as one would 



FIG. 33. PROPER MODE OF HOLDING TUBES OF SOLID MEDIUM IN INOCULATION. 



hold a pen, and flamed until the whole length of the wire has taken on a cherry-red color, 

 and the lower end of the glass handle has been exposed to the flame. For a few moments 

 it is allowed to cool so that its heat will not prove fatal to the germs with which it 

 'is to be brought in contact (v. Fig. 2). As soon as safe the needle is thrust into the 

 infected tube and brought in light contact with the culture. Even though one cannot 

 see it, many of the individual germs adhere to the wire. It is at once withdrawn 

 and carried quickly and steadily into the second tube. The needle may now be drawn 

 in an even line over the surface of the medium, the growth which develops appearing 

 as a linear, more or less continuous colony, known as a "stroke" culture; or it may 

 be rubbed irregularly over the surface, when it gives rise to an irregular film or to 

 isolated growths, which is spoken of as a "smear" culture; or the plain, straight needle 

 may be thrust evenly down into the mass of the medium, when the growth following 

 the track of the puncture is spoken of as a "puncture" or "stab" culture (Fig. 34). The 

 loop is especially adapted for making a smear* culture. If the medium to be inoculated 

 be liquid, the needle is merely agitated slightly in it in order to set free some of the 

 adherent organisms in the liquid. 



