CULTURE MEDIA 59 



sterilise. Solidify in the upright or oblique position as 

 required. 



In the case of bar or stick agar, first steep the agar in 

 1 per cent, acetic acid for a quarter of an hour, then drain 

 and wash it so as to thoroughly remove the acid. The 

 further procedure is the same as detailed above. This 

 yields a very clear, pale product, and is perhaps preferable 

 when an autoclave is not available. 



Glycerin agar. Add 4 to 6 per cent, of glycerin to 

 the nutrient agar after filtration and proceed as before. 



Glucose agar. One or two per cent, of grape sugar is 

 added to the nutrient agar after filtration. 



Litmus media. The addition of neutral litmus to the 

 various culture media is a useful method of demons- 

 trating the production of acid or of alkali by organisms. 

 To prepare the litmus solution take the lump litmus, 

 powder finely, and boil with distilled water so that a 

 saturated solution is obtained. Filter, and preserve in 

 a flask stoppered with cotton-wool, after sterilising by 

 boiling for half an hour on two successive days. For 

 some purposes a special solution of litmus, the Kubel- 

 Tiemann solution, which can be procured ready for use, is 

 employed. It must not have any antiseptic added to 

 it (as is sometimes done to preserve it for use in the 

 chemical laboratory). 



Sufficient of this litmus infusion is added to the nutrient 

 media, after filtration, to tinge them a distinct purplish 

 colour. After steaming the colour has usually disappeared, 

 but returns as the tubes cool. 



Milk. Use separated milk, but failing this, centri- 

 fugalise ordinary new milk, or place it in a tall cylinder 

 and allow it to stand overnight in a cool place, preferably 

 in an ice safe. Then pipette off the milk from the bottom, 

 rejecting the cream. Introduce the separated milk into 

 test-tubes to the depth of about an inch to an inch and 



