CULTIVATION 451 



turbidity, while the surface shows a thin pellicle composed of 

 spirilla in a very actively motile condition. Growth takes place 

 under the same conditions equally rapidly in peptone solution 

 (1 per cent, with *5 per cent, sodium chloride added). 



In milk also the organism grows well, and produces no 

 coagulation nor any change in its apj>earance, at least for 

 several days. 



On all the media the growth of the cholera spirillum is a 

 relatively rapid one, and especially is this the case in peptone 

 solution and in bouillon, a circumstance of importance in relation 

 tn its separation in cases of cholera (vide p. 459). 



The cholera organism is one which grows much more rapidly 

 in the presence of oxygen than in anaerobic conditions ; in the 

 complete exclusion of oxygen very little growth occurs. 



A 



Kic. !#>. Colonies of the cholera spirillum in a gelatin plate 

 three days' growth. A shows the granular surface, liquefaction just 

 i ommencing ; in B liquefaction is well narked. 



I l!"l H'-'i-t !<,n. This is one of the most important tests 

 in the diagnosis of the cholera organism. It is always given by 

 a true cholera spirillum, and though the reaction is not peculiar 

 to it, the number of organisms which give the reaction under 

 the conditions mentioned are comparatively few. The test is 

 made by adding a few drops of pure sulphuric acid to a culture 

 in bouillon or in peptone solution (1 per cent.) which has been 

 incubated for twenty-four hours at 37 C. ; in the case of the 

 cholera spirillum a reddish-pink colour is produced. This is due 

 to the tact that both indol and a nitrite are formed by the 

 spirillum in the medium, and hence in applying the test for 

 indol tin- addition of a nitrite is not necessarily the red colour. 

 Here, as in testing for the production of indol by other bacteria. 

 it is found that not every specimen of peptone is suitable, and 

 it is advisable to select a peptone which gives the characteristic 



