HETEROTROPHIC BACTERIA REQUIRING COMBINED NITROGEN 149 



mycoides and usually forms round colonies with entire edges; the surface 

 membrane contains granules which tend to be arranged concentrically. 

 Bac. megatherium liquefies gelatin more slowly; its colonies are seldom 

 over 1 cm. in diameter and are characterized by a flocculent center 

 composed of white opaque granules, surrounded by a zone of clear 

 liquefied gelatin; the smaller colonies have no surrounding zone and 

 are recognized only by their granular structure. 11 



These organisms as well as various other spore-forming bacteria are of 

 universal occurrence in the soil. The following forms were demon- 

 strated 12 in the soil of Northern Greenland: Bac. subtilis, Bac. mesen- 

 tericus, Bac. asterosporus, and Bac. malaberensis. According to Flugge 13 

 Bac. mycoides is present in almost every soil examined. Holzmiiller 14 

 also found one form or another of Bac. mycoides in every soil. Gottheil 

 found Bac. asterosporus very abundantly in the soil as well as Bac. 

 ellenbachensis, and to a less extent Bac. cohaerens, Bac. fusiformis, Bac. 

 pctasites, Bac. graveolus, Bac. pumilus, Bac. ruminatus, Bac. subtilis and 

 Bac. tumescens. Bac. simplex was found only once, while the presence 

 of Bac. mycoides and Bac. carotarum could not be established 



According to Ford and associates Bac. cereus is very predominant in 

 Maryland soils, followed by Bac. subtilis; Bac. mesentericus was more 

 abundant than Bac. vulgatus, while Bac. megatherium , Bac. petasites and 

 Bac. mycoides were found only rarely. Five hundred and twenty cul- 

 tures were isolated from eight soils (five near Baltimore and three from 

 Nazareth, Pa.). All soil samples were boiled in water for 20 minutes 

 just before plating, to kill all the vegetative cells of bacteria as well as 

 the other soil organisms and leave only the spores of the spore-forming 

 bacteria. (See tabulation on p. 150.) 



The three most common types of spore-forming bacteria found in 

 the soil by Conn were: Bac. megatherium (averaging about 375,000 per 

 gram), Bac. mycoides (225,000 per gram) and Bac. cereus (180,000 per 

 gram) . 



The relative number of spore-forming bacteria in the soil depend on 

 the length of the incubation period. When a short incubation period 

 is used, nearly half of the colonies on the plate may be found to consist 



11 Conn, 1917 (p. 142). 



12 Barthel, Ch. Recherches bacteriologiques sur le sol et sur les matieres 

 fecales des animaux polaires du Groenland septentrional. Saertryk MeddeL 

 Groenland 64, Kopenhagen. 1922. 



13 Fliigge, 1896 (p. x). 



14 Holzmiiller, 1909 (p. 143). 



