78 BACTERIOLOGICAL METHODS 



up weight to 500 grams. Infuse 500 grams of lean meat in 500 

 cc. of distilled water for 24 hr. in ice chest. Make up loss by 

 evaporation, strain, weigh filtered infusion and add 2 per cent, 

 of peptone. Warm on water bath with constant stirring until all 

 of the peptone is dissolved. To 500 grams of the meat infusion add 

 500 cc. of the 3 per cent, agar solution, keeping the temperature 

 below 60° C. Boil for i min. and titrate to +1.0. Sterilize 

 in steam for 40 min. and restore any loss by evaporation. Re- 

 adjust if necessary and then boil for 5 min. with constant stirring. 

 Restore any loss due to evaporation and filter by passing it through 

 the filtering material (cotton and cotton flannel or perforated 

 filter paper) at least three times. Titrate and record the final 

 reaction. Tube, sterilize and store as for gelatin media. It 

 must be borne in mind that agar media are never as dear as broth 

 or gelatin media. 



5. Lactose Litmus Agar. — To make this medium add i per cent, 

 of lactose to nutrient agar just before sterilizing and make the 

 reaction neutral to phenolphthalein. 



If this medium is to be used in tubes the sterilized azolitmin 

 (i per cent, aqueous solution) is added just before the final mass 

 sterilization, that is, the sterilization before pouring into the 

 tubes. 



If the medium is to be used in Petri dishes, the azolitmin is 

 not added until ready to pour into the dishes. 



The azolitmin and the lactose should be sterilized separately 

 before adding to the agar medium, though it is permissible to mix 

 the lactose with the agar and sterilize together, preferably in the 

 autoclave (120° C. for 15 min.). 



It would appear that the azolitmin of the market varies con- 

 siderably and many bacteriologists prefer the pure litmus. A i 

 per cent, aqueous suspension of azolitmin should dissolve readily 

 when boiled for 5 min. 



This medium is much used in bacteriological work on pre- 

 sumptive sewage contaminations, as estimating the temperature 



