TEXT-BOOK OF BACTERIOLOGY. 81 



teria. Now the flesh of the potato is often slightly acid, though 

 usually decidedly alkaline, and this circumstance occasionally gives 

 rise to very striking variations in the appearance of one and the 

 same species of bacteria on the potato cultures. 



Even such bacteria as alwaj'^s show a preference for alkaline re- 

 action in their culture media will thrive on sour potatoes. It seems, 

 therefore, as if the vegetable acids, particularly the malic acid, 

 which exists in the potatoes are much less unpleasant to the bacte- 

 ria than the mineral acids and the lower acids of the sebacic group. 



The potato process can be used for dividing a mixture of bacteria 

 into its separate component parts. 



If an exceedingly small portion of a mixture of bacteria be spread 

 over the surface of a slice of potato, the germs would be too numer- 

 ous and lie too close together. A uniform layer, in which no dis- 

 tinction could be made, would cover the surface. Therefore with 

 continually-changed sterilized knives remove a little from the first 

 potato to a second, and from this again to a third, from the third 

 to a fourth, and so on to a fifth and sixth, always with smaller 

 quantities, so that at last, by this repeated thinning of the inocu- 

 lated matter, a very wide difl'usion of germs will be obtained. 



The different kinds of bacteria will develop in small pure cul- 

 tures each by itself, clearly distinguishable by their color. The germs 

 will be so far separated from each other that each will be able to 

 reproduce its own species on the solid medium without coming in 

 contact with other species. 



This process lies at the foundation of our whole present method 

 of resolving mixed masses of bacteria. 



Occasionally the potato is employed in a form which differs con- 

 siderably from those already described. The potatoes are peeled 

 as if for the dinner -table and boiled in the steam generator, with- 

 out any particular previous sterilizing. When they are sufficiently 

 boiled they are mashed to a stiff mass in an earthenware dish, 

 with a, little distilled water. The mash is then put into Erlenmeyer's 

 small flasks — a troublesome task — and thoroughly sterilized on 

 three successive days by an exposure for an hour each time to a 

 current of steam. ' 



The bacteria grow just as well upon this medium as upon the 

 slices, and being well guarded against pollutions from without by 

 the cotton- wool plug, this system is extremely well adapted for all 

 those cases in which it is desired to at once obtain large quantities 

 of some particular micro-organism, to get a so-called potato cul- 

 ture en masse. 



In a similar way another solid food medium is made which is 

 6 



