18 



STERILIZATION BY FILTRATION 



Berkefeld bougie. The Berkefeld bougie does not lend itself to heating in a 

 flame or incinerator. To clean it, it must be brushed with a stiff brush in a 

 solution of sodium carbonate, washed in running water, and then autoclaved. 1 



2. Filtration of culture media. 



Bougies are also used for rendering sterile media which are to be used 

 for growing cultures, and for freeing a culture medium of the organisms 

 which have been grown in it. 



This may be effected in one of many ways ; but nitration should always 

 be carried out under pressure either by putting pressure upon the liquid 

 to be sterilized, or by aspirating the nitrate at the mouth of the bougie. 



A. Filtration by compression. 



The original method was to pour the unfiltered liquid into a copper reservoir 

 A (fig. 14), and then to force it through the filter K by means of a Gay- 

 Lussac pump P. 



Technique. Close the tap G, and half fill the reservoir A by pouring in the liquid 

 through the opening D. Close D by screwing on the cap, and compress the air in 

 A by working the pump P. The pressure can be read on the manometer F. When 

 the necessary pressure has been attained (2 or 3 atmospheres is generally sufficient), 

 close the tap E and slowly open G. This allows the liquid to pass into the filtering 

 chamber H, which contains a sterile Chamberland filter K (size B) fitted up as 



described above. The liquid is forced through 

 V the filter and issues at the nozzle where it can 



be collected aseptically. 



Collection of the filtrate. 1. [A Cobbett's 

 bulb is a useful piece of apparatus with which 

 to collect and distribute the filtrate. The 

 illustration (fig. 15) shows the bulb, which 

 usually has a capacity of about 200 c.c. To 

 render it available for the present purpose, 

 plug the small bulb V with wool, attach 

 W to the nozzle of the filter K by means of 

 stout red rubber pressure-tubing, and with 

 another piece of rubber tubing connect X with 

 a short length of glass tubing Z, the other end 

 of which has been drawn out to a fairly narrow 

 opening. Select a small india-rubber plug Y with 

 one perforation, slip it over the lower end of Z 

 and push it up until it fits tightly round the 

 tube, then enclose the lower end of Z in a test- 

 tube the mouth of which must be of suitable size 

 to fit the rubber plug Y. After thus fitting up 

 the filter and bulb, autoclave at 120 C. 



[When required for use, fit the filter K in its 

 metal case H (fig. 14), and screw on the cap L 

 firmly. Support the bulb by clamping it to a 

 retort stand. Clip the tubing between X and Z. 



[The filtered liquid will be forced into the 

 bulb, the rate being regulated by the tap G, and when nearly filled turn off the tap 

 G ; take off the test-tube, and by releasing the clip between X and Z the fluid can 



f 1 According to Dr. Andrew Wilson, however, it would appear that Berkefeld bougies 

 must not be autoclaved. "It is a well-known fact that in consequence of the composi- 

 tion and the mounting of the Berkefeld filtering cylinders, they do not stand sterilization 

 in an autoclave at 120 C. The only way effectually to sterilize the cylinder without 

 injuring it is to place it in a vessel with cold or tepid water, and to boil it for about an 

 hour " (Journal of Hygiene, 1909, p. 33). It has already been stated that simple boiling 

 at 100 C., though prolonged, cannot be relied upon to destroy all micro-organisms.] 



FIG. 15 Cobbett's bulb with attach- 

 ments for filtration by compression. 



