NITRIFYING BACTERIA 459 



points, which under a low magnification appear dark gray, round, 

 with sharp margins. After growing up toward the surface the colonies 

 become larger, assume a yellowish-brown color and a dark centre and 

 slowly liquefy the medium. In gelatin stick cultures a white, confluent 

 mass is first formed along the stick canal. It soon leads to liquefaction 

 and the latter progresses until finally one-half or more of the medium 

 has become liquefied, while the bottom is covered with a white, 

 yellowish sediment. 



Both the cocci described ferment urea when present in the culture 

 media, and the decomposition continues until 13 per cent, carbonate 

 of ammonium has been formed. The optimum temperature of 

 development is between 30 to 33 C. The best artificial culture 

 medium, according to von Jaksch, is one containing: Urea, 3 gr. ; 

 tartrate of sodium and potassium, 5 gr. ; potassium monophosphate, 

 0.12 gr. ; magnesium sulphate, 0.06 gr., and water in sufficient quantity 

 to make 1000 c.c. 



Urobacillus Pasteuri. Miguel has isolated from air, soil, sewage, 

 and water 60 different bacteria, all of which possess the property of 

 fermenting urea. Of these 60 he has studied 17 species more par- 

 ticularly, and he distinguishes 3 types, namely, the urobacillus, the 

 urococcus, and the urosarcina. The organism which has the greatest 

 urea fermenting power was called Urobacillus Pasteuri by Miguel, 

 who found it frequently in sewage. This bacillus can split up 140 

 grams of urea in 1000 c.c. of bouillon. 



NITRIFYING BACTERIA. 



The nitrifying bacteria possess very peculiar properties differing 

 greatly from those of the bacteria considered in the preceding chapters. 

 They do not require organic material for their growth and multi- 

 plication, and, in fact, do not grow properly in its presence in artificial 

 culture media. The latter must contain simple chemical compounds 

 only. Nitrifying bacteria do not require the presence of light in their 

 synthetic metabolic processes. Nitrification, however, will take place 

 in the soil in the presence of small amounts of organic matter, but 

 any larger amount will stop it even in the soil, and it does not occur 

 in the manure as first existing in a concentrated form. After putre- 

 faction and decay in manure has decomposed most of its organic 

 matter, nitrification can occur. The process, likewise, does not take 

 place in an acid medium, and for this reason soils that have become 

 quite acid by the decomposition of a large amount of organic matter 

 must first be neutralized by carbonate of lime before there can be 

 any progress in nitrification. The nitrifying bacteria have, however, 

 a wide range of temperature, and nitrification occurs under otherwise 

 favorable conditions between 37 F. to 110 R ; it is best at 99 F., and 

 almost ceases at 122 F. Since nitrification is a process of oxidation, 

 it requires the presence of considerable quantities of air, and the 



