The chemical and biological properties of the constituents of tetrathionate media have been investigated.
(1) The selective properties of thiosulphate and tetrathionate are expressed in graphic form, a method which clearly illustrates their differential effect on various intestinal organisms and is of value in preparing media to suit particular needs.
(2) Thiosulphate, tetrathionate, iodide and, indirectly, chalk all have some selective action, but the most important single constituent is tetrathionate.
(3) The discovery that certain organisms, mainly of the Salmonella group, are capable of reducing tetrathionate rapidly to thiosulphate suggests that in tetrathionate media part at least of the selective action is due to the ability of the tetrathionate to act as an alternative hydrogen acceptor to oxygen.
(4) In the light of these facts many of the defects and puzzling features of tetrathionate media can be explained, especially the frequent heavy growth of Proteus.
(5) A medium containing balanced tetrathionate shows a different kind of selectivity from media of Muller's type and has given consistent results as a routine solid medium for plating of faecal specimens.
(6) A diminution in the amount of thiosulphate in media of Muller's type is desirable, since Bact. typhosum is relatively sensitive to thiosulphate.
(7) The possible value of a biochemical approach to the study of selective media is emphasized.
It is a pleasure to thank Prof. M. E. Delafield and Prof. G. S. Wilson for constructive criticism of this paper, and we are much indebted to Dr C. Hamilton Willkie for taking the photograph.