.8 TEXT-BOOK OF BACTERIOLOGY. 



vantages, and also to utilize them throughout the extent of their 

 applicability. 



With the culture of bacteria on the potato as his point of de- 

 parture, he was soon able to make improvements in solid food me- 

 dia, and thereby to give the start to the surprising progress made 

 in bacteriology within the last few years. 



The first place is given to the potato in treating of solid food 

 media, not alone to fulfil the duty of gratitude in a matter of his- 

 tory', but also because it still continues to be, in many cases, a 

 valuable means for the breeding of micro-organisms. It has, in- 

 deed, been found that the greater number of all the bacteria as yet 

 known to us (not alone the saprophytic ones) are capable of thriv- 

 ing upon it, and thereby displaying verj' characteristic appearances, 

 so that the potato culture sometimes offers the most essential ser- 

 vice in distinguishing species which, under other circumstances, 

 are easily confounded with one another. 



Take good medium-sized potatoes and cleanse them from the 

 dirt that cleaves to them by repeated vigorous brushing, for it is 

 evident that to prep3,re the potatoes for serving as nourishment 

 to pure cultures they must be freed from foreign germs. In the 

 upper strata of earth from which the tubers were taken there are 

 alwaj's great numbers of bacteria in the form of spores with a high 

 degree of tenacious vitality. These cling to the surface of the po- 

 tatoes and are particularly fond of hiding in those little recesses 

 called "eyes," whence the shoots proceed, or in the so-called "bad 

 spots " — i.e., portions of dead substance. These latter should be re- 

 moved. 



With the point of the knife dig out suspicious-looking portions, 

 taking care to go deep enough to reach the pure, unaltered " flesh " 

 of the potato. Only those parts of the skin should be removed from 

 which an undesired swarm of bacteria is feared, for the healthy, 

 smooth skin must be regarded as a valuable protection against ex- 

 terior pollution, and therefore should be left on. 



To insure the thorough destruction of all and any germs that 

 may remain, put the potatoes for half an hour into a solution of 

 ■^^i corrosive sublimajte. 



The potatoes are then boiled, for it has been found that in their 

 raw state they are not a suitable food for bacteria. This is best 

 accomplished by putting them in a tin vessel with a grating at the 

 bottom and placing this for about three-quarters of an hour in the 

 steam generator. The time required for thorough boiling will, of 

 course, differ somewhat according to the size and the kind of pota- 

 toes used. They are next halved with a heated and recooled knife 



