TEXT-BOOK OF BACTERIOLOGY. 115 



With a short platinum wire transplant a colony as deep as 

 possible into the solid gelatin or agar. If we transplant a strictly 

 anaerobic micro-organism, only the lower part of the puncture will 

 develop. There will be a certain frontier mark, showing how deep 

 the influence of atmospheric oxygen extends. The development is 

 stronger by far in the deeper portions. In somewhat older cul- 

 tures the growths advance slowly upward, and at last there re- 

 mains only a narrow ring of medium close to the surface, where no 

 growth takes place. This is caused by the gaseous excretions of 

 the anaerobic bacteria themselves, which probably consist chiefly 

 of hydrogen, and collecting, expel the air which had penetrated 

 downward from above. Thus the micro-organisms grow in their 

 own gases and gradually rise toward the air. 



That the anaerobia in particular distinguish themselves by 

 generating gases is already known. With some species we per- 

 ceive masses of gas collected under the solid bridge which sepa- 

 rates the liquefied culture from the outer world from the oxy- 

 gen-containing atmosphere. If this barrier be punctured with a 

 platinum needle and the food medium be stirred up a little, numer- 

 ous bubbles of gas arise and seek to gain the open air. 



Lastly, one more food medium will be mentioned in which we 

 can also develop pure cultures of anaerobic bacteria. It is neither 

 solid nor is it transparent, yet it is so undoubtedly useful in some 

 cases that it deserves notice. It is the raw hen's egg, as recom- 

 mended by Hueppe. The shell of the egg must be cleaned and 

 sterilized with sublimate, alcohol, and water free from germs. 

 Then the surface is dried with sterilized cotton wool, and an open- 

 ing pierced in one end with a platinum needle. Through this open- 

 ing the inoculation is performed, after which a small piece of ster- 

 ilized blotting-paper is fastened over the hole with collodion. The 

 small quantities of oxygen which were already in the egg or which 

 entered it through the hole are insignificant, unless we have to deal 

 with the very strictest anaerobic species, and are, besides, soon 

 absorbed by the new gases which develop, in particular by the 

 sulphuretted hydrogen. Some bacteria thrive very luxuriantly in 

 this medium so full of unaltered albuminous substance, and pro- 

 duce their excretions in rich abundance. 



Now that we have learned the special processes which are nec- 

 essary for the culture of anaerobic micro-organisms, it remains to 

 say a few words about those by which we are enabled to breed the 

 strictly parasitical kinds, which need a high degree of heat. 



As food media we can employ agar and blood-seiuim ; plates 

 can only be prepared with agar; they are kept continually in the 

 incubator like the test-tube cultures. 



