102 MEDICAL BACTERIOLOGY 



of these saprophytes are pathogenic to man. Some are pathogenic to laboratory 

 animals. They are differentiated from the tubercle bacillus as follows: 



1. When stained with carbol fuchsin and treated with Pappenheim's solution 

 for 20 minutes they appear blue. 



2. They do not produce tuberculin. 



3. Many of them grow on plain bouillon, agar, and gelatin and at room tem- 

 perature. (Their growth, in many respects, resembles that of the tubercle 

 bacillus.) 



There has been considerable discussion and disagreement as to the possibility 

 of differentiating the tubercle from non-pathogenic acid-fast bacilli by methods 

 of staining, which should stimulate consideration of the following facts : 



First, the vast majority of tubercle bacilli, found in tissue, tuberculous exu- 

 dates and pus, or directly cultured from them, when stained with carbol fuchsin 

 cannot be decolorized with acids and alcohol within an hour. 



Second, occasionally, tubercle bacilli found in tissue, tuberculous exudates 

 pus, or in cultures, are decolorized in a few seconds or minutes with acid and 

 alcohol or with acid alone, after staining with carbol fuchsin. 



Third, a very large majority of the non-pathogenic acid-fast bacteria, no 

 matter where found, are decolorized in a few minutes with alcohol, after staining 

 with carbol fuchsin. 



Fourth, occasionally non-pathogenic acid-fast bacteria, from various sources, 

 are found which resist decolorization by alcohol as long or longer than the 

 majority of tubercle bacilli for hours. 



