Microbes Must Eat 77 



Berkefeld Filters. These filters of varying porosities are com- 

 posed of diatomaceous earth which is obtained from the siUca-hke 

 skeletons of marine or fresh-water algae. These are coarse Berke- 

 feld V (German viel), medium N (German normal), and fine 

 Berkefeld W (German wenig) filters, but the effectiveness of any 

 of these devices may be materially impaired unless the filters are 

 kept scrupulously clean. The V filter may allow some of the 

 smaller bacteria to pass through, but it serves a useful purpose as 

 the first step in filtering masses of organisms from fluids when 

 centrifugation is not convenient. The N filters usually remove 

 100% of the suspended bacteria, and the W filter, having a fine 

 porosity, can usually be relied upon to free fluids of all cells. It is 

 a wise precaution, however, to run sterility checks on all filtrates 

 whenever freedom from microbes is essential. 



If the pores become coated with grease, the adsorptive power of 

 the pore walls may be neutralized, and organisms may slip through 

 the filter. Stuart Mudd has calculated that Chamberland and 

 Berkefeld filters that are "tight" to bacteria have mean pore space 

 sizes in the range of 3 to 4 microns. 



Cleaning of filters can sometimes be accomplished by merely 

 passing a continuous stream of clear water through them for several 

 hours. Heating in ovens is usually discouraged because the ex- 

 treme temperature changes may cause the filter to crack, often 

 inside where the eye cannot readily detect the fissure. A suggested 

 cleaning technic involves passing a five-tenths per cent solution of 

 potassium permanganate through the filter, followed by a 5% sodium 

 bisulfite solution before allowing streams of clear water to wash out 



o 



all traces of these cleansing agents. 



Seitz Pads. Discussion of these filters can be dismissed by 

 saying that they are made by compressing shredded asbestos into 

 pads which can be inserted into special chambers, sterilized in the 

 autoclave, attached to side-arm flasks, and used to filter liquids with 

 the aid of negative pressure to speed up the process. 



Sintered Glass Filters. Germany led in the production of these 

 devices made from Jena glass, but American pyrex glass is now 

 being manufactured into excellent sintered glass filters. This spe- 



