36 LABORATORY EXERCISES IN BACTERIOLOGY. 



(B) Sterilization by Filtration. The principle of separation of solid admixtures 

 from fluid media by means of nitration through porous substances has several applica- 

 tions in bacteriology in the matter of sterilization. 



(a) While bacteria are, of course, of much smaller size than the spaces in ordinary 

 filter paper or the meshes of a cotton plug or fabric, without the force of currents of 

 liquid to carry them they are unable to pass, save by continuous growth, through the 

 intricate spaces of a moderately thick layer of the paper or cotton ; and plugs of cotton 

 or several thicknesses of paper or fabric are therefore commonly utilized, after thorough 

 sterilization, as a means of protection to the interior of flasks, tubes, etc., from the en- 

 trance of bacteria from the surrounding air, although permitting fairly free ingress and 

 egress to the atmosphere itself. Moreover, if such material be kept dry, and free from 

 soiling with any of the nutrient substances contained in the flasks or tubes, there is no 

 opportunity for the penetration by growth of any organisms settling upon their exposed 

 portions, and the protection from external contamination thus afforded is practically 

 complete. 



Exercise 13. From a small glass tube blow a bulb as exhibited in figure 

 8. Plug each end with a bit of cotton and sterilize in the usual manner. 



Introduce into the bulb a small amount 

 of melted agar or gelatine and again 

 sterilize the whole. Now attach the 

 apparatus to a suction pump and for 

 several minutes draw through the tube 

 and over the medium contained in the 

 bulb air which has been filtered through 

 FIG. 8. the cotton plug in the exposed orifice of 



the tube. Repeat the same with a 



second tube similarly arranged, but not having a cotton plug in the end 

 of the tube, the air drawn through the tube being unfiltered. Place both 

 tubes in the incubator and at the end of the first, second, and third day 

 note the presence or absence of growth in each. 



(b) Filtration through various porous substances has commonly been used in the 

 crude purification of water and other liquids ; and the same principle is employed in the 

 laboratory for sterilization of liquids from bacterial contamination when the ordinary 

 methods of heating or chemical purification are not to be permitted because of the im- 

 portant physical changes liable to be produced in the liquids subjected to such processes. 

 Filtration through paper, cotton, or other coarsely porous substances is, however, of 

 little or no value, the bacteria in the fluids being readily conveyed with the currents of 

 the liquid through the spaces of the filter; and specially prepared filters made of un- 

 glazed porcelain are commonly employed for the purpose. The Pasteur-Chamberland 

 filter, widely used for the filtration of water for drinking, is an excellent type. Among 

 various models, one of the most simple and efficient is that of Kitasato, a view of 

 which is shown in the accompanying diagram (Fig. 9). Such porcelain filters may be 

 utilized in the sterilization of various liquid culture media, of water, or in the separation 

 of bacteria from infected media when it is desired to obtain the various chemical pro- 

 ducts of bacterial activity free from the bacteria themselves. When used, inasmuch as 



