PREPARATION OF CULTURE MEDIA 



35 



formation of indol ; it is also used as a basis for other special 

 media. 



Indicators. To any of the ordinary media, substances may be 

 added which serve to show any difference in reaction taking place 

 during bacterial growth ; in other words, they indicate the ability 

 of the microorganism present to produce fermentative or putre- 

 factive changes. Litmus is perhaps the most generally used. 

 After filtration of the medium, which should be slightly alkaline, 

 sufficient of a reliable solution such as the " Kubel-Tiemann Solu- 

 tion " is added to give a distinctly bluish tint. A deepening of 

 the blue color will indicate increased alkalinity; a change from 

 blue to a pink color will reveal the presence of an acid. Neutral 

 red is an indicator frequently used; in the presence of acid it 

 becomes a deep rose color, 

 in an alkaline medium it is 

 yellow with sometimes a 

 green fluorescence. 



9. Milk. Fresh milk is 

 placed overnight in the ice 

 box so that the cream may 

 rise ; in the morning the 

 milk is siphoned off from be- 

 neath the cream and sufficient litmus added to give it a purplish 

 blue color. It will generally be found to be alkaline ; if not, a little 

 sodium hydroxide solution should be added to give it the required 

 color. It is then ready for tubing and fractional sterilization. Litmus 

 milk is a convenient medium for observing the ability of certain bac- 

 teria to ferment lactose, or to coagulate the soluble albumin present. 



10. Neutral-Red Lactose Peptone Medium. 



Peptone solution (8) 100 c.c. 



Lactose 1 gm. 



Saturated aqueous solution of neutral red ... 1 c.c. 



The medium is filtered into fermentation tubes (Fig. 11) and steri- 

 lized by the fractional method. This medium is used largely for 

 the examination of water and shellfish to determine by the presence 

 of the colon bacillus whether sewage pollution has occurred. 



FIG. 11. Fermentation Tubes. 



