SULPHIDE PRODUCING ORGANISMS 
419 
Lead paper suspended in the necks of the tubes, and preferably kept moist, has 
found greatest favour as an indicator. Rubner 4 mentions, as an objection to it, the 
fact that mercaptans form with lead acetate a yellow colour, gradually becoming 
dark, which might lead to mistaking this evidence for that of weak and slow 
hydrogen sulphide production. He prefers an organic salt of iron in fluid 
medium. 
Factors influencing sulphide formation. These may be considered as follows : — 
{a) Composition of media. 
(F) Reaction. 
(r) Temperature. 
(c/) Oxygen. 
A. Composition of Media 
This is the most important consideration for the successful demonstration of 
sulphide formation. Peptone was observed by Petri and Maassen 6 to be a very 
favourable ingredient, and to be more efficient in more concentrated solutions. 
Appel s and Pares* speak of it favourably. Beijerinck and some others do not 
mention it. Stagnitta-Balistreri 1 claim equally good results in peptone bouillon 
and peptone-free bouillon. They were unable, however, to obtain positive results 
with a number of organisms which Petri and Maassen found to yield hydrogen 
sulphide freely in peptone solution. 
Experiments performed with the organisms presented in this article confirm the 
observations of Petri and Maassen. Bacteria, which in peptone-free bouillon yield 
sulphide slowly and in small amount, may produce it freely in two per cent, peptone, 
more richly in five per cent., and yet much more in ten per cent, peptone. 
Petri and Maassen suggest that the efficiency of the peptone depends upon 
the loose combination of at least a part of its sulphur. 
This unstable combination is evidenced by the fact that the ferro-peptone 
medium, heated in the autoclave at 120 0 to 130° for twenty minutes, yields some of 
its sulphur as sulphide of iron. With longer heating more sulphide is formed. 
Sulphide is formed likewise from peptone solution not containing iron, by similar 
heating, as is indicated by the giving-off of hydrogen sulphide after the addition of 
dilute acid. Further, treatment of peptone with dilute solution of potash (y), in the 
cold, liberates sulphur as sulphide. 
Beef extract, on the other hand, does not give up its sulphur nearly so readily 
through such agencies. 
The excessive formation of sulphide in ten per cent, peptone is probably not 
accounted for merely by the increase of the sulphur supply ; for the increase, in some 
instances, of hydrogen sulphide and many other noxious compounds formed is too 
