SPECIAL CULTURE MEDIA 39 



SPECIAL CULTURE MEDIA. 



An enormous variety of different media has been brought 

 forward for use in cases either where special difficulty is ex- 

 perienced in getting an organism to grow, or where some special 

 growth characteristic is to be studied. It is impossible to do 

 more than give the chief of these. 



Peptone Solution. 



A simple solution of peptone (Witte) constitutes a suitable 

 culture medium for many bacteria. The peptone in the propor- 

 tion of 1 to 2 per cent., along with '5 per cent. NaCl, is dissolved 

 in distilled water by heating. The fluid is then filtered, placed 

 in tubes, and sterilised. The reaction is usually distinctly 

 alkaline, which condition is suitable for most purposes. For 

 special purposes the reaction may be standardised. In such a 

 solution the cholera vibrio grows with remarkable rapidity. It 

 is also much used for testing the formation of indol by a 

 particular bacterium ; and by the addition of one of the sugars 

 to it the fermentative powers of an organism may be tested 

 (p. 80). Litmus may be added to show any change in reaction. 



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. 48) 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 has been introduced 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 containing '5 per cent, of 

 lactose, to which '5 per cent, of a 1 per cent, watery solution 

 of neutral red is added. The alkaline medium is of "a yellowish 

 brown colour which on 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 com- 

 position of the dye, as the fluorescence is not discharged by 

 addition of alkali. 



