60 AIDS TO BACTERIOLOGY 



Gram-GilntTier Method. The section is treated with 

 a 3 per cent, solution of hydrochloric acid for a few 

 seconds after the first alcoholic washing. By this treat- 

 ment cleaner and brighter preparations are obtained. 



CHAPTER V 

 THE ACID-FAST ORGANISMS 



MOST organisms when stained with carbol-fuchsin are 

 readily decolorised by weak solutions of mineral acids. 

 The tubercle, leprosy, and smegma bacilli are, however, 

 notable exceptions, and retain the stain even after treat- 

 ment with 25 per cent, sulphuric acid or with 30 per cent, 

 nitric acid. This persistent retention of the stain is also 

 shown by some saprophytes (vide p. 74), and by some 

 members of the Streptotrichece. It is probable that the 

 degree of resistance of streptothrices to decolorisation is 

 directly proportionate to the vitality of the organism. 

 A pathogenic streptothrix from an old lesion or from 

 one that has been vigorously treated with antiseptics will 

 probably be decolorised, and will be Gram-negative, 

 whereas acid-fast bacteria are Gram-positive. It is 

 believed that some strains of the tubercle bacillus arc 

 naturally not acid-fast. In young cultures of tubercle 

 bacilli some, presumably immature, rods are not acid- 

 fast while older ones are. 



The Tubercle Bacillus. 



Bacillus tuberculosis is a non-motile, slender rod often 

 slightly curved, with rounded ends. It is from 2-5 to 

 5 /LI long and 0-2^ thick (see also p. 159). When stained 

 it often shows unstained areas (' beading ') which a 

 minority of observers consider to be spores. In the moist 

 state the thermal death-point is 60 to 65 C. Milk which 

 has been kept at 70 C. for thirty minutes (Pasteurisation) 

 is regarded as safe, but there is some question as to whether 

 tubercle bacilli are not protected by the pellicle that forms 

 on milk heated in an open vessel. The Board of Agri- 

 culture recommend an exposure to 85 C. for fifteen 

 minutes for rendering creamery products harmless. 



