52 



BACTERIA 



2000 or 3000 cc. of the suspected water through a Berkefeld 

 filter. When this has been accompHshed, by means of a 

 sterile brush the particulate matter on the 

 candle of the filter is brushed off into 10 or 

 15 cc. of sterilised water. This simple ar- 

 rangement is analogous to the use of gravity 

 or centrifugal methods of securing the solid 

 matter in milk. The smaller quantity of 

 water is then readily examined, and scanty 

 germs more readily detected. A second 

 point elaborating the scheme of water exam- 

 ination is the choice of media for sub-cultur- 

 ing. Mere examination on gelatine is not 

 sufficient. Even in making the primary 

 plate cultivations it is well to vary the media 

 — agar, carbol-gelatine, Eisner, etc. But 

 when colonies have appeared upon these 

 plates it is important to sub-culture with 

 accuracy and good judgment upon all or any 

 media — gelatine, agar, broth, potato, milk, 

 blood serum, glucose agar, glycerine agar, 

 Berkefeld Filter ^^^^^ — \}[i'^t will reveal the real characters of 

 In Position for Fiitra- the bactcrla present. A method proposed 

 be Examined. by Profcssor Sheridan Delepine is to place 

 some of the suspected water in sterilised 

 test-tubes without further treatment, and incubate at 37° 

 C. for twelve or eighteen hours, and then plate out and 

 estimate the number of bacteria as in the ordinary course. 

 '* In polluted water, containing an excess of organic matter," 

 he says, '* an extremely rapid multiplication of bacteria is 

 observed. In unpolluted water, containing only water bacteria 

 and a very small amount of organic matter, very little or no 

 multiplication takes place, and the growth of the water bac- 

 teria liquefying gelatine is checked to a remarkable extent." 

 Thirdly, by none of these methods should we, be able to 



