THE STUDY OF THE GROWTH OF BACTERIA 



123 



Another method is to inject some matter containing patho- 

 genic bacteria into a rabbit of guinea pig. The various juices 

 and the leucocytes of the animal destroy the non-pathogenic 

 bacteria and a pure culture, of a pathogenic form, may be isolated 

 from the blood or pathological lesions at 

 autopsy and transferred to culture media. 



By far the most useful and ingenious 

 method of procedure is the Koch, or plate 

 method. Koch was the first to employ solid 

 culture media for this purpose, and his 

 method depends upon the principle that a 

 liquid culture media may be inoculated with 

 bacteria and then spread out on sterile glass 

 plates or dishes where it quickly hardens, the 

 bacteria being uniformly separated from each 

 other, and for a time at least kept isolated 

 by means of the solid media, and after they 

 have developed into isolated colonies they 

 may be transplanted to tubes of media in 

 which they may be stored. In another way 

 if a man wanted to secure a pure lot of seed 

 of a single variety from a multitude of many 

 kinds, it would perhaps be impossible to 

 pick out by hand the seed wanted because 

 of their fewness and smallness, but if he 

 sowed them and waited until the plants 

 developed they could then be identified and gathered (Abbott). 

 Thus it is with plate cultures. 



To isolate a pure culture of bacteria, say the Bacillus pyocya- 

 neus from pus, the following procedure is adopted in this method. 



Three sterilized petri dishes, and three tubes of agar or gelatine 

 melted at 4oC. are used. A loopful of pus is taken up by a ster- 

 ilized platinum loop and mixed with the gelatine of the first tube. 

 To do this the tube is held across the left hand in a horizontal 



FIG. 30. Needles 

 used for inoculating 

 media. 



