STUDIES ON THERMOPHILIC BACTERIA. I 355 



Indol. Tests for indol were made on nutrient broth cultures 

 that had been incubated for four days at 55°C. Both Ehrhch's 

 test and the Nitroso-indol test were used. It was found that 

 EhrUch's test was much more satisfactorj^ when the tubes were 

 heated sU!ihtl}^ All of the cultures formed indol from Witte's 

 peptone in varjdng amounts. 



Htjdrogen sulfide formation. To determine hydrogen sulfide 

 formation, nutrient broth (made of Witte's peptone) over which a 

 strip of lead acetate paper was suspended by means of the cotton 

 plug, was used. The cultures were incubated for four days at 

 55°C. The blackening of the paper indicated hydrogen sulfide 

 formation. All the cultures studied formed H2S. Streak cul- 

 tures on "Bacto Lead Acetate Agar" plates also showed that all 

 the cultures formed H2S. This latter medium seemed to be well 

 adapted to the determination of tliis characteristic. 



Potato slants. The growth of these thermophiles on potato 

 slants was abundant in most cases even after twenty-four hours 

 at 55°C. The type of growth varied from a filiform to spreading 

 growth. The potato was turned gray, brown, or reddish brown. 

 The cultures could not be kept longer than from two to three 

 days since they dried out so quickly at 55°C., but the growth at 

 this temperature was quite rapid on this and other media. 



Liquefaction of gelatin. The "provisional method" was used 

 to determine this characteristic. The cultures were first accus- 

 tomed to the gelatin medium by preUminary cultivation for eight- 

 een to twenty-four hours in a 1 per cent solution of gelatin at 

 55°C.; then the surface of gelatin in test tubes was inoculated and 

 the tubes incubated for thirty days at 20°C. All the cultures 

 except nos. 6, 15, 20, 32, 51 had partially or completely hquefied 

 the gelatin at the end of thirty days. Gelatin cultures prepared 

 in the same way and incubated at 55°C. for four days were all 

 hquefied with flocculent growth throughout the gelatin and would 

 not harden when placed in the refrigerator. The fact that all 

 the cultures studied hquefied gelatin at 55°C. and some of them 

 at 20°C. can probably be explained by the fact that 20°C. was 

 below the minimum temperature for growth for those cultures 

 which did not hquefy gelatin at that temperature. 



