408 J. POSTGATE 



The authenticated species within this group of bacteria are three : Clostri- 

 dium nigrificans (earher known as Desulphovibrio thermodesulphuricans) a 

 spore-forming thermophilic organism; Desulphovibrio orientis, a spore- 

 forming mesophiUc species; Desulphovibrio desulphur leans, a second meso- 

 philic species which has salt-tolerant marine variants sometimes called 

 Desulphovibrio aestuarii. It is the last species, D. desulphuricans and its 

 marine variant, that contain cytochrome Cg. 



D. desulphuricans is the sulphate-reducing bacterium that is most 

 commonly encountered; it is widespread in soils, waters, sewage, industrial 

 effluents and the rumens of sheep ; in fact with proper techniques it can be 

 detected in almost any aqueous environment, though it only multiplies when 

 the oxygen tension is zero and the activities of other microbes have reduced 

 the Ef^ to about —0-1 V. It is of considerable importance in corrosion, 

 pollution, oil technology, formation of sulphur and certain mineral deposits 

 and in a variety of other economic spheres (these matters were also mentioned 

 by Postgate, 1959) and it has been studied in the laboratory to a much greater 

 extent than have the other two species. Its physiology has been discussed in 

 the reviews already cited; for the present purposes, three biochemical 

 properties need to be emphasized. 



1. In addition to conducting reactions of the type given in equation (1), 

 it contains a hydrogenase system enabhng it to reduce sulphate in 

 hydrogen : 



4H2 -h SO4— ^ 4H2O -1- S-- (2) 



2. It can grow with and reduce a variety of partly reduced sulphur anions 

 in place of sulphate. Examples are sulphite, thiosulphate, tetrathionate, 

 dithionite; even colloidal sulphur. Of these only sulphite is a true 

 intermediate in ordinary sulphate reduction, this fact having been 

 demonstrated using labelled substrates. 



3. Just as certain strains of aerobic bacteria can grow without air, so 

 certain strains of D. desulphuricans can grow without sulphate, but such 

 'sulphate free' growth only takes place if pyruvate is present. Ordinarily 

 pyruvate is metabolised by reaction (3) analogous to (1) above. 



4CH3CO • COO- + SO4- - ^ 4CH3 • coo- + 4COo -I- S- - (3) 



but if sulphate is absent the true fermentation (4), yielding gaseous 

 hydrogen, occurs: 



CH3 . CO • COO- + H2O ^ CH3 • coo- + CO2 + Ha (4) 



PROPERTIES OF CYTOCHROME C3 



The properties of cytochrome c^ (Postgate, 1956) and its associated pigment 

 desulphoviridin have been reviewed briefly with an appropriate bibliography 

 (Postgate, 1959). 



