Plant and Soil 43, 77-93 (1975) Ms. 2582 METABOLISM OF INORGANIC SULPHUR COMPOUNDS BY THIOBACILL US FERROOXI- DANS AND SOME COMPARATIVE STUDIES ON THIOBACILLUS A2 AND T. NEAPOLITANUS* by D. P. KELLY** and O. H. TUOVINEN*** Microbiology Department, Queen Elizabeth College, Campden Hill, London W8 7AH, England SUMMARY After growth on thiosulphate, Thiobacillus [errooxidans could oxidize poly- thionates and sulphur or ferrous iron, but oxidized thiosulphate only very slowly, with some tetrathionate accumulation. Tetrathionate was metabolized aerobically to sulphate and anaerobically to thiosulphate. Its oxidation was inhibited by thiol-binding reagents. Thiobacillus A2 oxidized thiosulphate, but lacked thiosulphate-oxidizing enzyme and was unable to form or meta- bolize polythionates, although thiol-binding reagents depressed thiosulphate oxidation. T. neapolitanus oxidized thiosulphate and metabolized polythio- nares aerobically and anaerobically. Rhodanese enzymes from all three thio- bacilli were compared. The comparative results are discussed in relation to possible mechanisms of inorganic sulphur compound oxidation. INTRODUCTION Thiobacillus ]errooxidans is characterized by its ability to grow using energy from ferrous iron oxidation at pH I-4. All the strains studied seem able to oxidize sulphur and thiosulphate 13. C o lm e r 3 maintained a strain on thiosulphate for 14 months and was then able to culture it successfully on iron again. Studies have been made of sulphur-grown organisms 19 and some enzymes of inorganic sulphur metabolism have been demonstrated 2o 21 35 35. A number of workers, including ourselves, have experienced considerable diffi- culties in obtaining growth on thiosulphate using iron-grown * Paper read at the Symposium on the Sulphur Cycle, Wageningen, May 1974. ** University of Warwick, Coventry, Warwickshire CV4 7AL, England. *** Waite Agricultural Research Institute, Glen Osmond, South Australia 5064.