PERMANENT CULTURES 79 



must be used. This method of examination is exceedingly 

 useful in the examination of bacteria in their fresh state, 

 the progress and changes in growth and pecuharities, such 

 as motility, mode of propagation, and so on, being most 

 distinctly brought under observation. 



Permanent Cnltures. — In order to preserve cultures of the 

 various micro-organisms in a permanent form in such a 

 way as to be available for future reference, evaporation 

 of the water from the nutrient medium must be prevented 

 by hermetically sealing the tube. This is most satisfactorily 

 effected by sealing the tube by the blowpipe just above the 

 culture. Many cultures can be kept in good condition for 

 a long time by simply pushing in the cotton-wool plugs for 

 about half an inch, and then filling up the small space thus 

 left at the top of the tube by means of parafQn or sealing- 

 wax. Praunitz pours a thin layer of strongly-carbolized 

 gelatine over the culture in the tube, which is then allowed 

 to set, after which the tube is corked. 



Cultures can also be killed and rendered permanent by 

 the addition of a drop or two of concentrated formic alde- 

 hyde (formaldehyde) to the tube, after which the tube is 

 corked, the cork being waxed or varnished over. 



The Indol Eeaction. — This test is a useful method for the 

 differentiation of certain organisms ; among those which 

 give the reaction may be mentioned the cholera spirillum, 

 the Bacillus coli communis, the Spirillum Metschnikovi, etc. 

 This test depends upon the interaction of indol (which is 

 one of the products elaborated in some bacterial cultures) 

 with nitrous acid, to form nitroso-indol nitrate, which is of a 

 red colour. The test is applied as follows : To 10 c.c. of 

 the culture in ordinary alkaline peptone broth of the 

 organism which has been growing for twenty-four hours at 

 blood-heat, add 1 c.c. of a solution of potassium or sodium 

 nitrite (containing '02 in 100 c.c), and then a few drops of 



