4i8 
THOMPSON YATES LABORATORIES REPORT 
done at 37° C. In this way the formation of any sulphide is indicated by the 
appearance of the black sulphide of iron in the solution ; while sulphide of hydrogen 
or volatile sulphides cause also blackening of the lead paper. 
Most of the bacteria isolated were identified as sulphide formers before their 
richer production in ten per cent, peptone was known. Two per cent, peptone had 
previously been used, standardized at + 10 and + 14 to phenolphthalein (+10 indi- 
cates acidity to the extent of 10 c.cm. normal acid per litre of medium, with 
phenolphthalein as indicator ; — 10 indicates alkalinity to the same degree). Hydro- 
chloric acid and potassium hydrate were used in standardizing. In titration 
10 c.cm. medium was added to 40 c.cm. distilled water, boiling hot, for each test ; 
~ potassium hydrate being used for neutralizing. A faint rose pink colour was 
accepted for the neutral point. 
For purposes of identification two per cent, peptone is probably almost as 
reliable as ten per cent., for even those organisms, which form very little sulphide in ten 
per cent, peptone, yield it also in two per cent. On the other hand, several which 
form very little in two per cent, peptone yield it richly in ten per cent, peptone. 
Various methods have been followed by investigators in the isolation of hydrogen 
sulphide producing bacteria. Lead carbonate is recommended by Beijerinck., 1 added 
to slightly alkaline ordinary 'fleisch' gelatine or agar in sufficient quantity to whiten 
the media. He spreads the material for examination on the surface of the poured 
plates. The hydrogen sulphide producing colonies manifest themselves by their 
dark appearance against the white surface, the darkening being due to the deposit of 
lead sulphide. 
A salt of iron instead of lead has been used and recommended by Stagnitta- 
Balistreri, 2 Fromme/ and others. 
Both of these methods have been tried in this work, and have not been found 
reliable. Sulphide forming colonies grow excellently on the ferro-agar plates, but 
the blackness generally appears slowly, sometimes not at all. The white lead agar 
plates are still less favourable for the formation of sulphide, and unfavourable even 
for the growth of some strong hydrogen sulphide producing bacteria. 
Pakes,' in discussing methods of demonstrating the formation of hydrogen 
sulphide, states that he has found satisfactory a salt of iron or neutral lead acetate in 
three per cent, peptone water. This appears remarkable ; for in experimenting with 
lead acetate it has been found that, with the addition of \ c.cm. saturated solution of 
lead acetate per litre to two per cent, peptone, there is absolutely no blackening 
of the medium after prolonged incubation. This salt, indeed, markedly inhibits 
the growth of the organisms. 
Sodium nitroprusside has been used by Stagnitta-Balistreri, but with 
unfavourable results. 
