262 Value of Antiseptics 



Gaseous Disinfection. — If the germicide to be studied be a gas, 

 as in the case of sulphurous acid or formaldehyd, a different method 

 must, of course, be adopted. 



It may be sufficient to place a few test-tube cultures of various 

 bacteria, some with plugs in, some with plugs out, in a closed 

 chamber in which the gas is evolved. The germicidal action is 

 shown by the failure of the cultures to grow upon transplantation 

 to fresh culture-media. This crude miethod may be supplemented 

 by an examination of the dust of the room. Pledgets of sterile 

 cotton are rubbed upon the floor, washboard, or any dust-collecting 

 surface present, and subsequently dropped into culture media. 

 Failure of growth under such circumstances is very certain evidence 

 of good disinfection. These tests are, however, very severe, for in 

 the cultures there are immense numbers of bacteria in the deeper 

 portions of the bacterial mass upon which the gas 'has no oppor- 

 tunity to act, and in the dust there are many sporogenous organisms 

 of extreme resisting power. Failure to kill all the germs exposed in 

 such manner is no indication that the vapor cannot destroy all 

 ordinary pathogenic organisms. 



A more refined method of making the tests consists in saturating 

 strips of blotting-paper, absorbent cotton, various fabrics, etc., 

 with cultures and exposing them, moist or dry, to the action of the 

 gas. Such materials are best made ready in Petri dishes, which are 

 opened immediately before and closed immediately after the ex- 

 periment. If, when transferred to fresh culture media, the ex- 

 posed objects fail to give any growth, the disinfection has been 

 thorough so far as the particular test organism is concerned. If the 

 penetrating power of a gas, such as formaldehyd, is to be tested, it 

 can be done by inclosing the infected paper or fabrics in envelopes, 

 boxes perforated with small holes, tightly closed pasteboard boxes, 

 and by wrapping them in towels, blankets, mattresses, etc. 



Easier of execution, but rather more severe, is a method in 

 which cover-glasses are employed. A number of them are sterilized, 

 spread with cultures of various bacteria, allowed to dry, and then 

 exposed to the gas as long as required. They are subsequently 

 dropped into culture media to permit the growth of the organisms 

 not destroyed. 



Animal experiments may also be employed to determine whether 

 or not a germ that has survived exposure to the action of reagents 

 has its pathogenic power destroyed. An excellent example of this 

 is seen in the case of the anthrax bacillus, a virulent form of which 

 will kill rabbits, but after being grown in media containing an in- 

 sufficient amount of a germicide to kill it, will often lose its rabbit- 

 killing power, though still able to fatally infect guinea-pigs, or may 

 lose its virulence for both rabbits and guinea-pigs, though still 

 able to kill white mice. 



