70 BACTERIOLOGICAL CHEMISTRY 



C. Oxidise Ferrous or Manganotjs Compounds 

 (Iron Bacteria). 



1. Simple bacteria — 



(a) Long sheathed filaments, e.g. Didymo- 



helix [Gallionella) . 

 (6) Coccoid masses, e.g. Sideromonas. 



2. Filamentous bacteria, e.g. Leptothrix, 



Grenothrix. 



p. Hydrogen Bacteria, Hydrogenamonas. 



We will now consider some of these in more detail. 



The Nitrifying Organisms. — Winogradsky has contri- 

 buted much of our knowledge of these organisms. They 

 are divided into two groups, those which oxidise ammonia 

 to nitrite (Nitrosomonas) and those which oxidise nitrite 

 to nitrate {Nitrobacter). A species has been reported 

 which can oxidise ammonia directly to nitrate and 

 which can use nitrate as a source of nitrogen. They 

 are strict autotrophs and must have ammonia or nitrite, 

 as the case may be, for their continued existence. Nitro- 

 somoims has an optimum pE. of 8-3 to 8-8 for oxidation 

 of ammonia to nitrite. Nitrobacter oxidises nitrite to 

 nitrate between pH 8-3 and 9-3. The presence of organic 

 substances, when tested in vitro, inhibits their growth 

 and respiration. They are aerobic, non-sporing cocci 

 and short rods which occur almost universally in soils. 

 The nitrifying organisms have a considerable agricul- 

 tural importance in that between them they are largely 

 responsible for maintaining the supply of nitrate used 

 in plant metabolism . 



The Sulphur Bacteria. — The sulphur bacteria form a 

 very heterogeneous group. They include obligate and 

 facultative autotrophs and may be aerobes, facultative 

 anaerobes or strict anaerobes. There are two important 

 aerobic obligate autotrophs. One of these, Th. thioparus, 

 was isolated from soil as small non-sporing rods by 



