72 BACTERIOLOGICAL CHEMISTRY 



the esters break down with liberation of inorganic phos- 

 phate. As sulphur can be oxidised for long periods in 

 the absence of carbon dioxide and with only a limited 

 amount of inorganic phosphate it is probable that the 

 latter is involved in a cycle of reactions, as in alcoholic 

 fermentation by yeasts (see Chapter XV). It is con- 

 sidered that the energy provided by the oxidation of 

 sulphur is used in the synthesis of a storage carbohydrate 

 which can subsequently be broken down again, via 

 phosphorylation, during carbon dioxide utilisation. The 

 phosphate esters which have been isolated from Th. 

 thio-oxidans include adenosine-3-triphosphate (not 

 adenosine-5-triphosphate which occurs in muscle, yeasts 

 Esch. coli, B. suhtilis, Stajoli. aureus and Ps. fhiorescens), 

 fructose- l:6-diphosphate, glucose-6-phosphate, glucose-1- 

 phosphate and co -enzyme I (see Chapter XV). This 

 suggests that the internal carbon metabolism of the 

 autotrophs is similar to that of the heterotrophs. The 

 reactions may be expressed as : — 



( 1 ) 2S + 3O2 + 2H2O + inorg. phosphate >2H2S04 + phosphate ester. 



(2) CO2 -I- 2H2 -f phosphate ester ^(CHgO) +H2O 4-inorg. phosphate. 



The symbol (CHgO) does not necessarily represent 

 formaldehyde but may be a carbohydrate. 



The anaerobic organism, Th. denitrificans , is an 

 obligate autotroph which oxidises sulphur, hydrogen 

 sulphide, thiosulphate or tetrathionate to sulphuric acid 

 at the expense of the oxygen of nitrates. For example 

 thiosulphate may be oxidised as follows : — 



5Na28203 + 8KNO3 + 2NaHC03 > 6Na2804 + 4K2SO4 + iNg + 2CO2 + HgO 



It occurs widely in soils, water and mud, from which it 

 was isolated by Beijerinck. 



The morphologically more complex sulphur bacteria, 

 including the purple pigmented, photosynthetic Thiocystis 

 and the colourless, chemosynthetic Thiothrix and 

 Beggiatoa, are characterised by the presence of globides 



