TEXT-BOOK OF BACTERIOLOGY. Ill 



cannot even be recommended as being comparatively the best; all 

 have their defects, and it must be left to the operator to select one 

 from among- the possibilities which are about to be noticed. 



First of aU, for the culture of anaerobic micro-organisms it is 

 advantageous to add to the soUd media reducing substances, to 

 consume the oxygen which may be present. Of such additions 

 the commonest are: 1^ to 14. grape sugar, 1^ to 2^ formate of 

 sodium, 1^ to 10^ resorcin, etc. 



Proceed as follows: put the material in which anaerobic germs 

 are suspected into liquid gelatin or agar, and pour it out in the 

 usual manner over glass plates. But before the food medium is 

 fully hardened, cover it with a sterilized scale of mica. This will 

 stick to the viscid surface, and prevent the entrance of oxygen. In 

 order to keep out the oxygen still more fully, seal the open edges of 

 the mica with melted paraffin. 



But this only removes the oxygen partially. We obtain much 

 better results when we employ a larger mass of the food medium, 

 using the medium itself as a means to keep out the oxj-gen. Libo- 

 rius has ^\Tought out a systematic method of culture in deep la^-ers 

 of solid food media. A depth of 15 or 20 cm. of gelatin or agar in a 

 test-tube is, as far as possible, freed from air and oxygen by 

 thorough boiling, then cooled down to 40° C, and the material to 

 be inoculated carefully and equally distributed throughout the 

 liquid by the aid of a strong platinum loop. The hquid must now 

 be quickly hardened — iced water is the best means — and in the 

 short time but little air can re-enter, while the lower portions of 

 the food medium are, to a certain extent, protected by the upper 

 ones from the outside air and the penetration of oxygen. 



In fact, even strictly anaerobic bacteria develop in such cultures. 

 When care has been taken by suitable dilutions, by transferring 

 small quantities of infected gelatin into a second and third tube, 

 with a deep layer of food medium, to bring about a suiHcient dis- 

 tribution of germs, the colonies arise separately and in very char- 

 acteristic forms, and display the peculiarities of the different spe- 

 cies with great clearness. This method has, indeed, one advantage 

 over all others, which makes it particularly valuable. We know 

 that a great number of the bacteria familiar to us (in particular all 

 the pathogenic species) are semi-anaerobic — i.e., they can thrive 

 when oxygen is present. In all breeding processes in which the 

 oxygen is entirely excluded from the culture vessel there is no 

 means of distinguishing the semi-anaerobic from the strictly aerobic 

 species, which can thrive only when the oxygen is excluded. In the 

 deep cultures, on the other hand, the part nearest the surface re- 



