REPORT OF COMMITTEE ON DESCRIPTIVE CHART 139 



In case the fault seems to lie in poor growth, search should be 

 made for a nitrate medium in which the organism in question 

 does make good growth by means of the following modifications : 

 increasing or decreasing the amount of peptone; altering the 

 reaction; adding some readily available carbohydrate. Presence 

 of nitrite or gas in any nitrate medium whatever should be re- 

 corded as nitrate-reduction. Unless the routine formula is used, 

 the exact composition of the medium must always be given. 



If the organism grows well and yet produces no nitrite or gas, 

 the determination must be recorded as doubtful unless the 

 organism can grow well in some synthetic medium containing no 

 nitrogen except nitrate. It is recommended that such an organ- 

 ism be tested in a medium containing small quantities of phos- 

 phate, calcium, chlorine, etc., with KN0 3 as a source of nitrogen 

 and sucrose as a source of energy and of carbon. 3 Such a medium 

 generally allows good growth with an organism capable of 

 utilizing nitrate and sucrose. Unfortunately neither glucose 

 nor lactose can be used in this medium as a source of carbon 

 and energy, for the ordinary "c.p." preparations of these sugars 

 contain much ammonia. If the organism in question grows 

 (even but slightly) on a synthetic medium of this sort, it should 

 be tested for nitrite by the usual method and for ammonia by 

 means of Nessler's reagent (comparing with an uninoculated 

 tube as a check). The presence of nitrite, of ammonia (i.e., a 

 more pronounced ammonia reaction than in check tube), or of 

 gas indicates nitrate-reduction. 



The production of gas (free N) from nitrate is not a very 

 common one; but a considerable number of soil organisms have 

 this power, and one should be on the lookout for it in studying 

 soil bacteria. The agar slant test is ordinarily a sufficiently 

 delicate test; but, if liquid media are used, more reliable results 

 may be obtained by the use of fermentation tubes. 



Chromogenesis. Color production should be recorded if ob- 

 served in broth, on beef-extract agar, gelatin or potato, or if 



3 An illustration of such a medium which has proved satisfactory for some bac- 

 teria is: K2HPO4, 0.5 gram, CaCl 2 , 0.5 gram, KN0 3 , 1 gram, sucrose 10 grams, agar 

 12 grams, water 1000 cc. 



