EFFECT OF SUGAR IN MEDIA 119 



Heinemann and Glenn ^ investigated the action of dextrose 

 and lactose-litmus agar at 20° C. and 37° C. and concluded 

 that incubation at 20° C. for three days was the most preferable 

 technique as this temperature is less selective in its action than 

 higher ones and so yields more information as to the original 

 flora. After twenty-four hours incubation they found the 37° 

 count to be the higher, but this was reversed after a further 

 twenty-four hours incubation and the difference was still more 

 marked after seventy-two hours. Dextrose and lactose litmus 

 agar gave but insignificant differences in the total count but 

 the former showed a decidedly higher percentage of acid col- 

 onies, due, it is suggested, to colonies of the B. aerogenes type 

 becoming red only temporarily and finally assuming a blue 

 colour. For this reason Heinemann and Glenn prefer dex- 

 trose to lactose. The high counts obtained by these observers 

 seem to indicate that the samples had been kept for some time 

 and that considerable reproduction had taken place. This 

 possibly had an effect on the results obtained. For example: 

 Str. lacticus, which is usually abundant in stale milk, grows well 

 at 20°, but at 37° produces colonies in forty-eight hours that 

 are barely visible even with the aid of a low-power magnifying 

 glass and are usually overlooked when the medium is tinted with 

 htmus. 



The Committee on Methods of Milk Analysis appointed by 

 the American Public Health Association to investigate the 

 various details of the plate method using an agar medium 

 reported as follows (Am. J. of Pub. Hyg., 18, 431). 



Acidity (to phenol phthalein at boiling point). Of the 

 acidities-fO.5, +1.0, +1.5 and 2.0, an acidity of +1.5 per cent 

 gave the best results. 



Lactose. 0, 1, 2, 3, and 4 per cent of lactose was tried 

 at incubation temperatures of 20° C. and 37° C. At 37° C, 

 they found that the medium free from lactose was preferable, 

 but at 20° C. the one containing 1 per cent of sugar was the 

 best. 



Whey, Plain, and 4 Per Cent Lactose Agar media were com- 



