Researches on Nitrifying Bacteria 419 



duration of this period we can classify our curves into the follow- 

 ing categories: lag phase less than ten days; lag phase between 

 ten and fifteen days; and lag phase more than fifteen days. This 

 lag phase corresponds to the adaptation of bacteria to the medium, 

 and to the quantity of bacteria seeded. It thus expresses the po- 

 tential activity of tlie sample studied, since all possible conditions 

 of environment are similar, and only the nitrogenous substrate is 

 added in excess. 



The period of active nitrite production which begins at the 

 end of the lag phase is very fast; the slope of the curve at the 

 point of maximum derivation is steep. The slope of this curve 

 is still related to the activity of the bacteria, but, even though 

 the slope of the curve is often steeper when the latent period is 

 short, we found some exceptions, and it is difficult to interpret 

 these curves in great detail. The maximum points of the ordinates 

 generally have similar values, and, with two exceptions, they cor- 

 respond to the percentage of nitrogen obtained through total 

 transformation of ammonium or nitrite nitrogen. These points 

 seem, therefore, of no interest to us for judging the potential of 

 our specimens. 



The period of decrease is alwavs rapid, often quite abrupt, 

 the nitrite nitrogen generally disappearing after two or three 

 days. This period has no relation to the nitrite-producing bacteria, 

 but depends on the activity of bacteria which rapidly "con- 

 sume" nitrite. 



Nitrate Curve 



The nitrate curve reproduces the shape of nitrite curve. The 

 lag phase is necessarily a function of the production of nitrite ni- 

 trogen, since this is the source for nitrate production. Thus the 

 nitrate activity cannot be foreseen from the lag phase. In our 

 measurements, the lag phase extends from 8 to 27 days. 



The slope of the curve of nitrate production is the only point 

 susceptible of analysis for the purpose of determining the activity 

 of this group of bacteria. However, the general shape of the 

 curve is less regular than that of the nitrite cur\^e, undoubtedly 

 due to the interference of de-nitrification phenomenon. 



Whatever the case, the nitrification should be considered as 



