TUBERCULOSIS. 253 



sidered by Ehrlich to depend upon the fact that the tubercle- 

 bacilli possess an especially resistant cell-membrane, which 

 permits the stain to enter the body of the cell only with the 

 aid of mordants (aniline water, carbolic acid, etc.). This 

 membrane, later in the process of decolorization, prevents the 

 entrance of acids into the interior of the bacillus, so that the 

 microorganisms do not give up their stain. The tubercle-bacil- 

 lus can be stained by Gram's method. 



According to recent investigations by Robert Koch, the 

 tubercle-bacilli contain two unsaturated fatty acids, one of which 

 is soluble in dilute alcohol, and is saponified by sodium hy- 

 droxid ; whereas the other is not saponifiable, and is soluble only 

 in boiling absolute alcohol and ether. Both fatty acids take 

 the specific stain of the tubercle-bacilli ; but as one of them is 

 soluble in alcohol, only the other remains after decolorization, 

 and this fixes the stain and must be looked upon as the medium 

 of the color-reaction. By means of hot sodium hydroxid the 

 fatty acids may be slowly driven out of the bodies of the bacilli 

 and their escape in the form of tingible drops uniting into 

 larger drops can be observed under the microscope. According 

 to Koch, these fatty acids form a coherent layer within the 

 bodies of the bacilli, to which they afford protection against 

 external influences. 



Culture of Tubercle-bacilli. Pure culture of tubercle- 

 bacilli is difficult, and mainly because the microorganisms 

 develop very slowly and require a temperature of 37 C. 

 (98.6 F.) for their growth. Their temperature-minimum 

 is 29 C. (84.2 F.), their temperature-maximum 41 C. 

 ( 1 05. 8 F.), and their temperature-optimum 37 C.(98.6 F.) or 

 38 C. (100.4 F. ). Tubercle-bacilli thrive well upon blood- 

 serum, upon from four to six per cent, glycerin-agar, and upon 

 glycerin-bouillon. The preparation of glycerin-agar plates for 

 the purpose of isolating tubercle-bacilli from the mixture of 

 bacteria in tuberculous sputum is scarcely possible, as the tubercle- 

 bacilli develop so slowly that they are overgrown and suppressed 

 by the colonies of the other bacteria. Their cultivation will, 

 therefore, be successful only if the material examined is uncon- 

 taminated. To this end the following plan of procedure is 

 pursued : Several guinea-pigs animals that are extremely sus- 

 ceptible to tuberculosis are inoculated with material containing 

 tubercle-bacilli. After the lapse of about four weeks the first of 

 the inoculated animals will die, the autopsy revealing marked 

 tuberculosis of the abdominal viscera. One of the remaining 

 guinea-pigs is now killed, its skin cleansed most carefully by 

 means of hot water and i : 1000 solution of mercuric chlorid, 

 the skin turned back with a knife sterilized in the flame, the 

 peritoneum opened with another instrument similarly treated, 



