290 BACTERIA PATHOGENIC TO MAN 



greatest percentage of the bacteria some of them are exposed to less 

 heat than those in the rest of the fluid receive. 



The resisting power of this bacillus to chemical disinfectants, drying, 

 and light is considerable, but not as great as it is apt to appear, for, 

 as in sputum, the bacillus is usually protected by mucus or cell pro- 

 toplasm from penetration by the germicidal agent. It is not always 

 destroyed by the gastric juice in the stomach, as is shown by successful 

 infection experiments in susceptible animals by feeding them with 

 tubercle bacilli. They are destroyed in sputum in six hours or less 

 by the addition of an equal quantity of a 5 per cent, solution of carbolic 

 acid. Bichloride of mercury is less suitable for the disinfection of 

 sputum. lodoform has no effect upon cultures until 5 per cent, is 

 added. The fumes from four pounds of burning sulphur to each 

 1000 cubic feet of air space will kill tubercle bacilli in eight hours when 

 fully exposed to the action of the gas, providing that they are moist, 

 or that abundant moisture is present in the air. 



Formaldehyde gas is quicker in its action, but not much more efficient. 

 Ten ounces of formalin should be employed for each 1000 cubic feet 

 of air space. 



The tubercle bacillus in sputum when exposed to direct sunlight is 

 killed in from a few minutes to several hours, according to the thickness 

 of the layer and the season of the year; it is also usually destroyed by 

 diffuse daylight in from five to ten days when placed near a window. 

 Protected in cloth the bacilli survive exposure to light for longer periods. 

 This fact is worthy of note, as it has an important hygienic bearing. 

 Thus, tuberculous sputum expectorated upon sidewalks, etc., being 

 often exposed to the action of direct sunlight, will in many cases, especially 

 in summer, be disinfected by the time it is in a condition to be carried 

 into the air as dust. For this and other more important hygienic rea- 

 sons, consumptive patients should occupy light, sunny rooms and live 

 as much as possible in the open air. 



Dried sputum in places protected from abundant light has occasionally 

 been found to contain virulent tubercle bacilli for as long as ten months. 

 For a year at least it should be considered dangerous. The Roentgen 

 rays have a deleterious effect on tubercle bacilli in cultures, but 

 practically none upon those in tissues. 



The tubercle bacillus is a strict parasite that is to say, its biological 

 characters are such that it could scarcely find natural conditions outside 

 of the bodies of living animals favorable for its multiplication. Under 

 exceptional conditions, such as in freshly expectorated sputum, tubercle 

 bacilli may increase for a limited time. 



Cultivation of the Tubercle Bacillus. On account of their slow 

 growth and the special conditions which they require, tubercle bacilli 

 cannot be grown in pure culture by the usual plate method on the 

 ordinary culture media. Koch first succeeded in cultivating and 

 isolating this bacillus on coagulated blood serum, which he inoculated 

 by carefully rubbing the surface with sections of tuberculous tissue and 

 then leav ng the culture, protected from evaporation, for several weeks 



