74 BACTERIOLOGICAL CHEMISTRY 



that they are responsible, in part at least, for the decay 

 of stonework and concrete. 



The Iron Bacteria. — The iron bacteria are found 

 associated with deposits of ferric hydroxide around 

 mineral springs, mines and similar localities. The deposit 

 is usually in the form of a sheath round chains of rods 

 wliich thus acquire a filamentous form, or it may occur 

 as a sheath around true filamentous forms. Some of 

 these iron bacteria are obligate autotrophs, for example 

 Didymohelix ferruginea, and some facultative autotrophs, 

 such as Leptotlirix crassa. The reaction by which they 

 derive their energy is probably : — 



In certain cases the iron may be replaced by manganese. 

 The Hydrogen Bacteria. — There are a number of 

 hydrogen oxidising bacteria occurring in such places as 

 canal mud or swamps and in soils where large amounts 

 of hydrogen are produced by anaerobic processes. The 

 hydrogen bacteria are usually facultative autotrophs 

 which oxidise hydrogen to water in the presence of 

 carbon dioxide, but they can also utilise organic 

 compounds. For example a member of the photo- 

 synthetic "sulphur free" purple bacteria (Aihiorhoda- 

 cece) is known which can oxidise simple alcohols in 

 presence of carbon dioxide with formation of the corres- 

 ponding ketone and reduction of the carbon dioxide to 

 give cell substances. Thus isopropanol is oxidised 

 to acetone : — 



2(Jfr3.C'H0H.CH3 -!- CO2 > 2CH3CO.CH3 + (CH2O) + H2O. 



The oxidation of hydrogen very probably does not pro- 

 ceed directly to water but through the intervention of 

 carbon dioxide with formation of formaldehyde : — 



K.COa 1 2K2 5-H('H() I 2H2O. 



Part of this formaldehyde is used in the synthetic 



