( 402 ) 



nitrogen compound should he added when a small quantity of tap- 

 water is used, which addition is not necessary when cultivatino: in 

 a greater quantity in a flask of the same size. 



3. PURE CULTURE. 



Our bacterium does not grow at all or ojily to a slight extent 

 on the commonly used bacteriological media, these containing too 

 much organic food. But it is easy to produce pure cultures on solid 

 media, when observing the same precautions which I described in 

 the Meeting of the Academy of 27 June 1892 for the pure culture 

 of the ferments of nitrification on agar-plates ^), and to which I 

 referred in the Meetings of 30 March 1901 (Proceedings p. 586) and 

 25 May 1901 (Proceedings p. 5) when discussing the culture condi- 

 tions of the oligonitrophilous Cyanophyceae. 



In all these cases it is necessary as completely as possible to 

 remove all soluble organic substances from the solid medium, which 

 is to be effected by a prolonged washing ^vith distilled ^vater. The 

 agar thus prepared, with the required nutrient salts, for instance in 

 the proportion : 



Distilled water 100 



x\gar 1 .5 



K^HPO, 0.01 



KXOjofNH^Cl) 0.01 



is boiled and plated, and used for strew-or streakcultures originating 

 from a fdni of B. olii/oairbophthi.s. Very soon the common saprophytic 

 l)acteria which never lack in the film, are seen to develop on the 

 plate and when these l)y their growth and respiration hdxe consumed 

 the soluble carbon conqiounds, \\ hicli were not yet removed from the 

 agar by the extraction with water, ./>. o/i(/oon-/joj)/u'hi.s itself begins to 

 grow. This is usually the case after 14 days. Then, however, the 

 colonies become easily recognisable, our bacterium being the only 

 species which in the given circumstances can feed on the atmospheric 

 carbon, and so go on growing, whilst the growth of all othei- 

 species soon comes to a stop. 



Even the colonies of the nitrifying ferments, which, as 1 have 

 demonstrated before (1. c), can grow fairly well on this medium, when 

 instead of nitrate an annnonium salt is used, i-emain very small, 

 never exceeding 1 mM. or less. On the other hand, the colonies 

 of B. oligocarhopldlus attain dimensions of 1 cM. and moi'e ajid 

 may then easily be transferred in a pure condition into test-tubes 



1) Nature, Vol. i6, pag. 264, 1892. 



