72 PRINCIPLES OF BACTERIOLOGY 



a special chemical constitution. The latter hypothesis holds good only, 

 if at all, in regard to flagella and spores; while the assumption of the 

 resisting envelope has reference more particularly to the tubercle bacil- 

 lus, and is probably correct. The presence of fatty and waxy bodies 

 in the envelope of these micro-organisms is capable of demonstration. 

 Moreover, after extraction of these bodies by ether the tubercle bacillus 

 loses its power of resisting acids, which peculiar resistance can also be 

 artificially produced in other bacteria having normally no such resist- 

 ing power. In many instances, doubtless, both of these causes, viz., 

 resistant envelope and chemically different constitution, work together 

 to produce the above-mentioned results. 



Individual differences in acid resistance among the difficultly stained 

 bacteria have been observed in tubercle bacilli; according to Ziehl and 

 Ehrlich those having less individual resistance are probably the younger 

 members. Individual differences in staining, in the easily stained bac- 

 teria, have also been noticed; for example, cholera vibrios and allied 

 species are best stained with fuchsin, not so well with methylene blue, etc. 



The relation between staining and degeneration of bacteria is a 

 complicated question. Decrease of staining power takes place during 

 degeneration of the bacterial cell, but it is often difficult to determine 

 the exact moment when this loss of power occurs. Degenerated forms 

 of the cholera bacillus from the abdominal cavity of guinea-pigs thus 

 soon lose their power of staining in mcthylene-blue solution, but stain 

 well in diluted carbol fuchsin. Moreover, bacteria killed by drying and 

 moderate heating, as in the preparation of films, retain their power of 

 staining. Kitasato found dead tubercle bacilli in sputum which took 

 on normal staining. Bacteria killed by chloroform, formalin, etc., 

 still retain their staining properties intact. 



Elective staining properties, whereby certain species of bacteria are 

 exclusively or rapidly and intensely stained by certain dyes, have repeat- 

 edly been observed. Of the greatest practical importance in this respect 

 is the Gram stain used for the differential diagnosis of many species of 

 bacteria; although a distinct classification of bacteria into those 

 which are stained and those which are not stained by Gram's solution 

 has been shown to be impracticable. There are some bacteria, how- 

 ever, which act uniformly toward Gram under all conditions; as, for 

 example, the anthrax bacillus and the pyogenic cocci are always posi- 

 tive, the cholera and plague bacilli and gonococci are always negative 

 to Gram. Other species again are at one time stained and at another 

 decolorized by Gram; thus pyocyaneus is stained only in young indi- 

 viduals. Previous heating or extraction with ether, according to Niki- 

 tine, does not prevent the action of Gram's stain, but treatment with 

 acids or alkalies renders it impossible. Bacteria so treated, however, 

 after one hour's immersion in Loeffler's mordant regain their property 

 of staining with Gram. 



As to the nature of Gram's staining solution it may be mentioned 

 that only the pararosanilines (gentian violet, methyl violet, arid Victoria 

 blue) are suitable for the purpose, whereas the rosanilines (fuchsin 



