40 METHODS OF CULTIVATION OF BACTERIA 



finely and then ground in a mortar ; 8 oz. of tap water are 

 added and the mixture is heated slowly so as to cook the meat 

 thoroughly ; normal sodium hydrate is added until the reaction 

 is alkaline to litmus. The medium is divided into tubes and 

 autoclaved. It can be adapted for growing cultures of anaerobes 

 in the ordinary atmosphere by running a little sterile liquid 

 paraffin on to its surface. 



1 Media containing an Indicator. 



Litmus Media. — To any of the ordinary media litmus (French, 

 tournesol) may be added to show change in reaction during 

 bacterial growth. The litmus is added, before sterilisation, as 

 a strong watery solution {e.g., the Kubel-Tiemann solution, vide 

 p. 50) in sufficient quantity to give the medium a distinctly 

 bluish tint. During the development of an acid reaction the 

 colour changes to a pink, and may subsequently be dis- 

 charged. 



Neutral-Red Media. — This dye was introduced by Griinbaum 

 and Hume as an aid in determining the presence or absence of 

 members of the b. coli group, especially in the examination of 

 water. The media found most suitable are agar or bouillon con- 

 taining '5 per cent, of the sugar to be tested, to which - 5 per cent, 

 of a 1 per cent: watery solution of neutral-red is added. The alka- 

 line medium is of a yellowish brown colour which in the presence 

 of acid passes into a deep rose red. Sometimes there subsequently 

 occurs a change to a fluorescent : green, caused apparently by a 

 change in the composition of the dye, as the fluorescence is not 

 discharged by addition of alkali. (See also p. 356.) 



Blood Serum Media. 



Solidified Blood Serum. — Koch introduced this medium for 

 the cultivation of the tubercle bacillus and in order to obtain it 

 in a comparatively clear state, adopted the method of inspissation 

 at 65° C. after sterilising by the intermittent method at low 

 temperature — B (4) (p. 30). The procedure is somewhat tedious, 

 and for all ordinary purposes opaque coagulated serum, sterilised 

 by the usual methods, can be substituted. A sufficient quantity 

 of serum is placed in a series of sterile test-tubes ; these are then 

 placed in a sloped tray, put in the steam steriliser, and steamed 

 for an hour. We have found the following method, however, 

 to give the best results. The serum in test-tubes is first 

 thoroughly inspissated, in the sloped position, at 65° C. 



