FILTRATION OF CULTURES 



77 



to a funnel, the stem of which has been passed through a rubber 

 cork; this cork fits into a conical flask with side arm for 

 connection with exhaust. 



Before any one of the above apparatus is used it ought to 

 be connected up as far as possible and sterilised in the Koch's 

 steriliser. The ends of any important 

 unconnected parts ought to have pieces 

 of cotton wool tied over them. After use 

 the bougie is to be sterilised in the auto- 

 clave. Much of the material kept back on 

 the filter can now be removed by forcing 

 water through in a direction opposite to 

 that of the flow of the fluid during 

 filtration. Alternatively, the candle, after 

 being dried, should be passed carefully 

 through a Bunsen flame to burn off 

 all organic matter. If the latter is 

 allowed to accumulate, the pores become 

 filled up. 



The success of filtration must be tested 

 by inoculating tubes of media from the 

 filtrate, and observing if growth takes 

 place, as there may be minute perforations 

 in a candle sufficiently large to allow 

 bacteria to pass through. Filtered fluids 

 keep for a long time if the openings of 

 the glass vessels in which they are placed 

 are kept thoroughly closed, and if these 

 vessels be stored in a cool place in the dark. 

 A layer of sterile toluol about half an inch 

 thick ought to be run on to the top of the 

 filtered fluid to protect it from the atmospheric oxygen. 



Instead of being filtered off, the bacteria may be killed by 

 various antiseptics, chiefly volatile oils, such as oil of mustard 

 (Roux). These oils are stated to have no injurious effect on the 

 chemical substances in the fluid, and they may be subsequently 

 removed by evaporation. It is hot practicable to kill the 

 bacteria by heat when their soluble products are to be studied, 

 as many of the latter are destroyed by a lower temperature than 

 is required to kill the bacteria themselves. 



Bacteria can be almost entirely removed from fluid cultures 

 by spinning the latter in a centrifuge of very high speed (e.g., 

 C. J. Martin's turbine centrifuge), and this method is sometimes 

 adopted in practice. 



Fro. 81. — Flask for 

 filtering small quanti- 

 ties of fluid. 



