PREPARATION OF NUTRIENT AGAR. 



35 



r' 



occasional additions of small quantities of water until it is thoroughly cooked in 

 the form of a thick mush. It is then put into the remainder of the water or bouil- 

 lon and subjected to streaming steam for two hours, after which, if the first heating 

 was sufficient, it filters readily without the use of a hot-water filter, or the necessity 

 of keeping it in the steamer during the filtering. The stirring rod must touch all 

 parts of the bottom of the dish exposed to the flame, every few seconds during the 

 preliminar)- heating, otherwise the agar will burn on and be spoiled. On some 

 ^--^ ^-^ accounts it is best to begin 



operations with beakers rather 

 than the enameled iron dishes. 

 In this way all likelihood of 

 using burned agar is avoided, 

 since the moment the agar 

 burns on the beaker cracks and 

 the agar is spilled. For bacte- 

 riological use agar should be 

 clear, not cloudy or filled with 

 unremoved precipitates. 



The writer now employs an 

 autoclave and uses an agar flour 

 procured from L,autenschlager or 

 Merck (fig. 33). If one has an au- 

 toclave the preliminary heating 

 of the agar in an open dish with 

 a minimum quantity of water 

 and all the subsequent stages 

 may be dispensed with and the 

 entire process carried on in the 

 autoclave, unless it is known 

 or suspected that media heated 

 in the autoclave are less well 

 adapted to the growth of par- 

 ticular organisms than those pre- 

 pared at 100° C. The amount 

 of agar added to the culture 

 fluid is usually i per cent. On 

 the making of nutrient agar 



F.g. 33.* 



consult "Formuke," and the various standard text-books. 



Is there any difference in the appearance of colonies when grown at 5° to 10°, 

 15° to 20°, and 30° to 37° C? Observe the amount of precipitate that collects in 

 the fluid in the V. For other observations as to growth on this substratum see 

 "Gelatin." Every organism should be studied in numerous Petri-dish poured-plate 



*FiG. 33 

 agar flour. 



-Agar-agar flour as received from European manufacturers. Package of Merck's 



