1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 379 



is entirely absent, since the addition of strong acid may not 

 only disintegrate any bacteria which may be present but may 

 also dissolve some of the nutrient medium. However, there is 

 no difficulty in obtaining a marked quantitative difference of 

 yellow precipitate when the same amount of reagents are used 

 in tubes possessing the white precipitate as against tubes which 

 do not, those tubes containing the precipitate always giving a 

 greater amount of yellow ammonium phosphomolybdate. In a 

 number of tests, where the bacterial growth was removed in toto, 

 often very difficult to do without breaking up the medium, and 

 where the acid was allowed to act only a short time, no yellow 

 precipitate occurred in tubes which did not possess the white 

 precipitate. 



It seems to be well established that beef extract contains phos- 

 phates, the following statement in Eyre's 'Bacteriological Tech- 

 nique,' p. 128, indicating this: "Meat extract ... is acid in 

 its reaction owing to the presence of acid phosphates . . . 

 The following appears to be a plausible explanation for the pro- 

 duction of a colorless zone plus a white precipitate on acid media. 

 It has been shown that the foxtail organism in its growth on 

 various media, including nutrient agar, produces alkali. The 

 alkali acts on the acid phosphates and causes them to precipitate 



>> 



f 



acid phosphates 



are precipitated out of solution when alkali is added.) Imme- 

 diately around the organism the phosphate is probably used up 

 as growth proceeds, while the phosphate beyond a certain dis- 

 tance of the bacterial growth, not having been used, is acted on 

 by the alkali which diffuses from the region of growth and is pre- 

 cipitated out of solution. There is also the possibility of an 

 action by a carbon dioxide gradient which, being greatest in the 

 region immediately surrounding the colony, might prevent pre- 

 cipitation by the acid reaction. No precipitate is produced on 

 alkaline media, probably because the substances from which the 

 white precipitate is derived were largely precipitated out when 

 the medium was rendered alkaline. This seems to be in keeping 

 with Eyre's observation (p. 150). In giving directions for mak- 

 ing nutrient agar he recommends the addition of sodium hy- 

 droxide until a reaction of +10 is obtained (this often is equiv- 

 alent to Ph 7.0 or P H 7.2 as previously shown) and further rec- 

 ommends the following as the next step after the addition of al- 



