94 TEXT-BOOK OF BACTERIOLOGY. 



teria which were to be separated into the different species contained 

 in it a small portion was taken and spread as evenly as possible 

 over the surface of a sliced potato. Then from the first potato to 

 a second, from the second to a third, and so on, the inoculating 

 material was transferred in continually smaller, " more diluted " 

 quantities, so that the germs were at length so widely separated, 

 so far isolated from each other, that they developed separately and 

 singly, yielding pure cultures which could not mix together on 

 their solid medium. 



In precisely the same way the gelatin was at first employed, 

 and Koch originally endeavored to imitate the form and other par- 

 ticulars of the successfully-tried potato culture by liquefying the 

 culture gelatin, pouring it into watch-glasses, on slides, etc., allow- 

 ing it to harden upon them, and inoculating the now solid surfaces 

 by dipping a platinum needle into the mixture of bacteria and then 

 drawing it repeatedly across the gelatin. 



Stroke by stroke the number of the germs planted becomes 

 smaller, and when later they develop along the strokes made by the 

 needle and form colonies, the separate species will appear isolated, 

 and it will not be difficult to divide them with certainty and precision. 



At the present day the " slide cultures " are employed in excep- 

 tional cases only, for it was soon observed that by this procedure 

 one of the advantages of gelatin was lost the advantage of its 

 permitting, before it assumes the solid form, the same complete 

 and equable distribution of germs as do the liquid media. 



We therefore perform the dilution of the inoculating material in 

 the dissolved gelatin, and after this the now universally-employed 

 method for obtaining pure cultures will at once be apparent. 



An experiment with human faeces, a substance extremely rich 

 in the most varied species of bacteria, can be made as follows: 



First dissolve the gelatin in a number of test-tubes. The best 

 way is to employ the water bath at about 35 C. Take one of the 

 test-tubes, taking care that the gelatin is completely dissolved, and 

 with the platinum needle introduce a trace of the material to be 

 inoculated. With such a tough consistency as that of the material 

 in question it is well to rub it and squeeze it against the glass, in 

 order to mix it well into the liquid ; in all cases, however, and under 

 all circumstances, endeavor, by repeated slight variations of the 

 slant of the test-tube, to produce a very thorough and equable dis- 

 tribution of bacteria throughout the gelatin. 



This being done, the "dilution" begins. Were the first tube 

 only to be employed, the number of germs that would develop 

 would certainly be far too great to think of separating them. 



