466 DESCRIPTION OF A BACTERIUM. 



Standstill in neutral media unless the buffer effect is sufficient to 

 absorb the change in H ion concentration accompanying- the 

 transformation of alkaline arsenite into neutral or acid arsenate. 

 The reaction limits are approximately assessed as 



Ph 6.8 to Ph 10. 



Although the organism does not grow under anaerobic 

 conditions in bouillon, it does so readily in presence of nitrate. 

 Arsenate, however, cannot take the place of nitrate, and although 

 arsenite is so vigorously oxidized to arsenate under aerobic con- 

 ditions, the reverse change of reduction is not effected under 

 anaerobic conditions. 



The organism is physiologically very active, and relati\ely 

 small numbers are capable of converting larse amounts of ar- 

 senite into arsenate, even in mineral media containing only very 

 small amounts of organic matter. 



Reducing Organisni. — This organism, which reduces arsen- 

 ate to arsenite with great rajjidity, under suitable conditions, is 

 a vigorously motile bacterium with the grou]) number 

 222.2331033. It appears to belong to the colon-typhoid family, 

 from other members of which it can however be easilv differen- 

 tiated by its high resistance to arsenite and its out- 

 standing capacity to reduce arsenate to arsenite. It has 

 been named Bacterium arsenreduccns. It has peritrichic 

 flagella, usually four to eight in number. It is a poly- 

 morphic, rod-shaped organism, usually about 2 /x in length by 

 about 0.4 /x in breadth, but it may vary from i /i, to 6/x in 

 length and 0.3 to 0.6 fx in l)readth. Disposition to chain forma- 

 tion is marked, and old bouillon cultures may show as many as 

 TO or 20 rods in one long motile chain. It stains well with all 

 ordinary stains and is gram-negative. Occasionally bipolar 



staining may be observed. Litmus milk is rendered slightly 

 alkaline, but no coagulation or {proteolysis occurs. 



Althoufirh o-rowth under anaerobic conditions is moderate, 

 aerobic growth is much better, and in suitable media complete 

 reduction of 0.2 per cent, arsenate can occur in 48 hours even 

 on free exposure to air in li<|uid layers only i cm. deep. The 

 organism can tolerate up to i per cent. As.^Q., as arsenite, and 

 arsenate reduction may proceed almost to this limit. 



In mixed cultures with the oxidising organism reduction of 

 arsenate or oxidation of arsenite can be made to proceed at will 

 by simply varying the composition of the medium. In attenu- 

 ated media bufferized with potassium phosphate, oxidation 

 occurs. On enriching the medium by the addition of glycerine, 

 glucose, or fresh stable manure, the reducing organism out- 

 friultiplies the oxidizing organism and reduction occurs. The 

 significance of this is discussed in relation to dipping tanks in the 

 field and the frequency of their use. 



Other Arsenic-Resistant Organhtns. — A number of other 

 organisms, which neither reduce arsenate nor oxidize arsenite, 



