1890.] MICROSCOPICAL JOURNAL. 115 



Tlie Prepanitioii of Nutritive Agar. 



By V. A. MOORE, M. D., 



ASSI^TANr IN THE LABORATORY OF THE BUREAU OF ANIMAL INDUSTRY, DF.rARTMENT OF AGRI- 

 CULTURE, WASHINGTON, V>. C. 



The extent to which nutritive agar is employed in the cultivation of 

 bacteria renders it of much imj^ortance that its method of preparation 

 should be made as perfect as possible. When it is prepared after the 

 method recommended in works on Bacteriology (which is practicallv 

 the same as that first formulated by Koch for the preparation of solid 

 culture media), a medium is obtained that favors the growth of most 

 germs. In this respect the method is desirable, but in regard to the 

 other requisites of a satisfactory solid medium it is quite deficient. The 

 objections to the method with reference both to the process itself and 

 the character of the resultant agar are three in number, (i) The difK- 

 culties attending the filtration of the agar. This process alone often 

 requires a very considerable length of time besides the use of a hot fil- 

 tering apparatus that must be provided especially for this purpose. (3) 

 The presence in the sterile agar of a flocculent precipitate that is inva- 

 riably thrown down during the process of its sterilization and which 

 greatly interferes with its usefulness, especiallv in making roll and 

 plate cultures. (3) The variation in the consistency of the agar. Tt 

 is impossible to obtain this material of the same consistency as the agar 

 is only partially dissolved even after long boiling in the simple beef- 

 infusion. The coagulation of the albumen ensheathes the stems of agar, 

 floats them to the surface where the}- remain imbedded in the firm, al- 

 buminous coagulum. This property of the agar is worthy of consid- 

 eration, for with the varying consistency^ of the medium a consequent 

 change follows in the character of the growth of most germs. 



For the purpose of securing a process for the preparation of nutritive 

 agar that was free from the above-mentioned difficulties I have reviewed 

 carefully the methods of Jacobi,* Von Freudenreich,t and Cheesnian|, 

 in all of which I found difiiculties that were equally as objectionable as 

 those possessed bv the original method. 



The use of a solution of beef-extract in distilled water instead of the 

 simple beef-infusion, made directly from the fresh meat, was also tried, 

 but the agar thus prepared did not favor as vigorous a growth of many 

 germs as when piepared from the fresh meat-infusion. So feeble was 

 the growth of many germs upon this agar that the method was aban- 

 doned, although very satisfactory in other respects. 



In the course of this experimental work it was found that when the 

 stems of agar were cut into small pieces and boiled in a fluid containing 

 no coagulable material, that it was entirely broken up and the soluble 

 portion dissolved. The insoluble particles that remained suspended 

 in the licjuid were easily and completely removed by the addition of 

 egg albumen, and subsequent boiling and filtering. From these facts 

 a method for the preparation of nutritive agar was derived, which con- 

 sists in first preparing the neutralized beef-infusion-peptone, and thus 

 getting rid of all coagulable material before the agar is added. This 

 process is eftective in greatly diminishing the time and attention re- 



*CeiUralblatt f. Bacteriologie u Parasitcnkund III (1888), p. 538. 



i //'/d, p. 797. 



t American Naturalist, XXII (1888), p. 472. 



