80 TEXT-BOOK OF BACTERIOLOGY. 



If Esmarch potatoes are not recommended entirely and exclu- 

 sively, the reason is that a few bacteria only display their most typ- 

 cial, characteristic manner of growth when cultivated on the fresh 

 halves prepared as previously described. 



There is a third method of preparing the potato for the use of 

 the bacteriologist, which is in many ways very useful, but which 

 has not proved reliable in all cases. This method, with slight vari- 

 ations, was recommended, almost at the same time as the former, 

 by Globig, Bolton, and E. Roux. It consists in boring out a cylin- 

 drical plug from the flesh of a good large potato (which requires no 

 previous preparation) w r ith a cork-borer or some similar instrument. 

 The skin at either end is cut off, and then, with an ordinary scalpel, 

 the cylinder is halved diagonally by an oblique longitudinal cut, each 

 half being then pushed into a test-tube that has been previously 

 sterilized. If the plug of potato is too thick it must be thinned 

 down with a knife. Next proceed to sterilize the test-tube with its 

 contents and cotton-wool stopper by the aid of the steam generator, 

 in which it remains one or two hours. This suffices to kill any 

 spores of potato bacilli that may have been left, and the culture 

 medium is now entirely ready. 



With a platinum needle or loop transfer the material for inocu- 

 lation to the oblique surface of the potato plug, and spread it out 

 properly. The further development must now be awaited. 



These cultures can also be preserved indefinitely without any fear 

 of pollution. The test-tube potatoes share this advantage with 

 those prepared in little dishes on Esmarch's plan, with which they 

 also have a second advantage in common, namely, that a great 

 quantity of them can be prepared at once and kept ready for use 

 without spoiling. 



The blood-red film made by a well-known bacterium the Micro- 

 coccus prodigiosus is a striking example of growth on the potato; 

 also the blackish-blue layer produced by a bacillus which is some- 

 times found in river-water, the dirty green culture of the bacteria 

 of green pus, the grayish-blue one of the blue-milk bacillus, etc., 

 etc. A hick white skin is found in a brood of Bacillus subtilis, and 

 a dull white, granulated mass is produced by the growth of anthrax 

 bacilli. Again, the typhus bacilli, when grown on potato, produce 

 a slightly moist, shining appearance. In fact, the growth is almost 

 invisible to the naked eye, and this peculiarity is taken advantage 

 of to distinguish them from all other species of bacteria. 



It is true that the growth of the individual micro-organisms on 

 the potato is not always quite uniform. The reaction of the culture 

 medium is of decided importance for the development of the bac- 



