(1) Tubercle Bacillus.—The presence of the tubercle bacillus may be taken as positive evidence of the existence of tuberculosis somewhere along the respiratory tract, most likely in the lung. In laryngeal tuberculosis they are not easily found in the sputum, but can nearly always be detected in swabs made directly from the larynx.
Recognition of the tubercle bacillus depends upon the fact that it stains with difficulty; but that when once stained, it retains the stain tenaciously, even when treated with a mineral acid, which quickly removes the stain from other bacteria. The most convenient method for general purposes is here given in detail:
Gabbet's Method.—(1) Spread suspicious particles thinly and evenly upon a slide or a cover-glass held in the grasp of cover-glass forceps. Cover-glasses are easier to handle while staining. Do not grasp a cover too near the edge or the stain will not stay on it well. Tenacious sputum will spread better if gently warmed while spreading.
(2) Dry the film in the air.
(3) Fix in a flame; i.e., pass the cover-glass rather slowly, with film side up, three times (a slide about twelve times) through the flame of a Bunsen burner or alcohol lamp. Take care not to scorch. Should the film be washed off during future manipulations, fixation has been insufficient.
(4) Apply as much carbol-fuchsin as will stay on, and hold over a flame so that it will steam for three minutes or longer, replacing the stain as it evaporates. If the bacilli are well stained in this step, there will be little danger of decolorizing them later.
(5) Wash the film in water.
(6) Apply Gabbet's stain to the under side of the cover-glass to remove excess of carbol-fuchsin, and then to the film side. Allow this to act for one-fourth to one-half minute.
(7) Wash in water.
(8) If, now, the thinner portions of the film are blue, proceed to the next step; if they are still red, repeat steps (6) and (7) until the red has disappeared. Too long application of Gabbet's stain will decolorize the tubercle bacilli.