92 



LABORATORY EXERCISES IN BACTERIOLOGY. 



feared that filtration will be slow, the filtration funnel and receiving 

 flask, a cover being adjusted to the funnel to prevent the water of con- 

 densation from dripping into its contents, may be placed in the auto- 

 clave, where the temperature is raised to 110 to 115 C. Or the hot- 

 water filtration bucket suggested by Matlock may be employed to keep 

 the preparation well liquefied during the period of filtration (Fig. 27). 

 After filtration the weight should be exactly corrected (weight of bouillon 

 plus five grams of agar) by evaporation of excess by further boiling or 

 by addition of boiled water if deficient; after which the reaction is to 

 be determined and adjusted to standard. Should the medium now 

 become turbid, it is to be reheated and again filtered, after which half 



of it may be transferred to tubes and the re- 

 mainder retained in the stock-flask, and steril- 

 ized either in the autoclave or in the ordinary 

 steam-bath (by fractional steaming in the lat- 

 ter case). Those tubes intended for surface 

 inoculations are to be allowed to solidify in 

 a slanting position, to increase surface expo- 

 sure, the remainder allowed to cool in erect 

 position, for use in stab cultures and for 

 plate cultures. 



Modifications of Agar. 



(a) Gelatine-agar .- For plate cultures agar is 

 not as well suited as gelatine, as it does not spread 

 as evenly as the latter unless heated to a tempera- 

 ture fatal to bacteria which it is desired to have 

 diffused through the melted mass; and on the other 

 hand gelatine cannot be used for either tube or plate 

 cultures if it be desired to subject the cultures to the 



body temperature in the incubator (37.5 C.), because it becomes liquid at 30 C. 

 In order to adjust these difficulties and to provide a medium capable of indicating 

 the liquefying power of bacteria in cultures grown at incubator temperature, a 

 mixture of gelatine and agar has been proposed, which serves the desired end 

 fairly. It may be made by adding the liquefied agar to liquefied gelatine in equal 

 proportion ; or in the manufacture there may be added to bouillon but one per 

 cent, of agar, after this has been melted adding five or six per cent, of gelatine. In 

 other respects the preparation follows the steps indicated in the preparation of either 

 agar or gelatine. The medium retains its solid consistence at incubator temperature ; 

 and while liquefaction does not take place with the same readiness and to the same 

 degree as in case of the pure gelatine, it occurs sufficiently to be indicative. 



(6) Glycerine Agar. This medium is especially valuable as a nutrient in the cul- 

 tivation of tubercle bacilli and a few other organisms. It is prepared by adding five 

 per cent, of glycerine to ordinary agar after it has been completed, all but the steriliza- 

 tion. 



FIG. 27. HOT- WATER FILTRA- 

 TION BUCKET. 



