MICROSCOPICAL TECHNIC 435 



with a number of chemically very different substances (ovokeratin, 

 neurokeratin, elastin). [VAN GIBSON'S stain contains picric acid and 

 stains elastin specifically. Tr.] 



The Staining of Bacteria. 



Most cocci and bacteria have a definite acid character evidenced 

 by the fact that they migrate to the anode in the electrical current 

 (see p. 205). 



Though they usually stain intensely with basic dyes (f uchsin, meth- 

 ylene blue, thionin, etc.), nevertheless bacteria exhibit considerable 

 differences in staining capacity. Though all cocci with which I am 

 acquainted stain very intensely, some bacteria, e.g., paratyphoid and 

 bacilli of hog erysipelas, are stained more faintly. Spores stain with 

 especial difficulty, the more poorly the older they are; it is obvious 

 that the solid capsule offers great resistance to the penetration of 

 the stain. The tubercle bacillus is most difficult to stain, which may 

 be attributed chiefly to its high keratin content, inasmuch as other 

 keratin-containing substances (bristles, hair, epidermis, etc.) stain 

 just as poorly. The difficulty in staining the tubercle bacillus was 

 formerly attributed to the wax contained. HELBIG, however, 

 showed that complete removal of the wax did not increase the stain- 

 ing capacity. 



GRAM'S stain is quite unique; it is extensively employed for the 

 classification of bacteria (we distinguish Gram-positive and Gram- 

 negative). It is performed as follows: we first stain with methyl 

 violet or some related basic dye and then subject the specimen to the 

 action of iodin (dissolved in KI). After this treatment, some bac- 

 teria readily give up the dye to alcohol and are decolorized, whereas 

 others firmly retain it. In the latter case, a firm combination has 

 been formed. A thorough study from modern points of view would 

 be of great value, since it would explain the difference in the nature 

 of the two groups of bacteria. It is important to mention that, by 

 GRAM'S method, a differentiation of the structure of individual bac- 

 teria may be revealed. The so-called BABE'S corpuscles are not de- 

 colorized by a brief action of alcohol. Upon this fact depends M. 

 NEISSER'S method for identifying diphtheria bacilli. 



We have as yet little insight into the actual basis of differentiation 

 by GRAM'S stain. It has actually only been established that GRAM 

 positive bacteria show a greater permeability for dies, stain more 

 quickly and intensely and retain the dye more strongly upon de- 

 colorization with alcohol. Probably the only purpose of the treat- 

 ment with iodin is to increase the size of the dye molecule or increase 

 its fixation by the bacillus (EISENBERG). 



