H. (I. Thornton 



245 



filtration of the media prepared from it is greatly facilitated. Most 

 workers, however, have washed the agar either with water or dilute 

 acid and have then dried it, while in the shred condition. To test the 

 advantages of washed agar in counting technique, some agar was Avashed 

 and filtered by Cunningham's method, and a second quantity was 

 washed while in the shred condition in 0-1 per cent. H2SO4 tor 10 

 minutes at 15° C, rinsed free from acid by continued changes of water, 

 and dried. A medium, having the following composition, was made up 

 with the two samples of agar and with unwashed agar as a control. 



Distilled water 

 Agar 

 K2HPO4 

 MgSO^.THaO 



1000 c.c. 

 15 gm. 



1 „ 

 0-2 „ 



CaCl 

 KCl 

 Dextrose 



Asparagine 



0-1 gm. 

 0-1 „ 

 0-5 „ 

 0-5 , 

 ering, (B) after 



Each of the three media was tested (A) without filt 

 filtering at 100° C, (C) with the salts filtered before adding the agar, 

 which was not filtered. The acidity was adjusted before autoclaving. In 

 each medium the change of reaction during sterilisation in the autoclave 

 was tested, and platings of a single diluted suspension of Kothamsted 

 soil were made to test its capacity for allowing colony development. 

 The data obtained are shown in Table II. 



Table II. 

 Effect of Acid washed Agar. 



Treatment 



pH value pH value 



before pH value after 



adjust- adjusted auto- Bacterial colonies 



ment to claving on each plate 



Agar unwashed, medium 



not filtered 

 Agar unwashed, medium 



filtered ... 

 Agar unwashed, salts fil- 

 tered 

 Cunningham treatment, 



medium not filtered ... 

 Cunningham treatment, 



medium filtered 

 Cunningham treatment, 



salts filtered ... 

 Acid washed and dried, 



medium not filtered ... 

 Acid washed and dried, 



medium filtered 

 Acid washed and dried, 



salts filtered 



6-5 



7-0 90, 87, 85, 82, 78, 73 



7-0 74, 74, 72, 69, 65, 63 



7-0 95, 80, 72, 70, 68, 68 



6-7 53, 52, 52, 48, 47, Sp.* 



6-7 69, 63, 54, Sp. Sp. Sp. 



6-7 62, 61, 55, 53, Sp. Sp. 



6-8 82, 75, 70, 68, 65, 58 



6-9 83, 77, 73, 71, 69, 58 



6-8 80, 76, 75, 64, 60, 59 



Mean 

 no. of 

 colonies 



82-5 

 69-5 

 75-5 

 50-4 

 62-0 

 57-8 

 69-6 

 71-8 

 690 



* Sp. : platings lost through spreading organisms. 



