CHAPTER XXII. 
THE TUBERCLE BACILLUS GROUP: HUMAN, BOVINE, 
AND AVIAN. 
THE ACID-FAST GROUP. 
There is a well-defined group of bacteria characterized by the 
relatively large amounts of lipoidal substances contained within their 
bodies. These lipoidal or waxy substances confer upon the members 
of the group distinctive staining reactions; ordinary dyes do not 
stain them at all, or at best slowly. The more intense stains con- 
taining a mordant, as carbol-fuchsin, penetrate the waxy envelope, 
especially when heat is applied; once stained the bacteria retain the 
dye tenaciously even after treatment with mineral acids. This resist- 
ance to decolorization with acids has led to the name— the Acid-fast 
group. 
Included within the group of acid-fast bacteria are saprophytic 
types found rather commonly in hay and manure; parasitic organ- 
isms found upon the surface of the body, as the smegma bacillus and 
the nasal secretion l)acillus of Karlinski; and exquisitely pathogenic 
bacteria, B. tuberculosis and B. leprse. The basis of classification 
therefore is chemical rather than morphological, and in this sense the 
definition of the acid-fast organisms does not follow a strictly natural 
system. 
Types of Tubercle Bacilli.— Eour types of tubercle bacilli are 
recognized which are pathogenic respectively for man, cattle, birds, 
and for fishes and reptiles; the human, bovine, avian, and ichthic 
varieties. Considerable discussion has arisen concerning the identity 
of the human, bovine, and avian varieties, some authorities claiming 
that they are identical, although modified somewhat by their con- 
tinuous sojourn in a series of animals of the same kind. The evidence 
for this view is arrayed around the obser\'ation that tubercle bacilli 
of undoubted bovine type occasionally are isolated from tuberculous 
lesions in man (chiefly in children, infrequently in adults). On the 
other hand human bacilli are less commonly found in progressive 
tuberculous lesions of cattle. In spite of many attempts to change 
one type into the other, no experiments have been reported up to the 
present time which are sufficiently conclusive and extensive to war- 
rant the assumption that one variety has been permanently changed 
into the other. Loss or increase of pathogenic properties of one 
strain does not suffice to bridge the gap between it and another strain 
habitually pathogenic for another animal. It is very probable that 
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