EXAMINATION OF SHELL-FISH 725 



treated, the volume of fish + liquor noted, and sterile water is 

 then added to make up to 1 litre ; 100 c.c. liquid therefore corre- 

 sponds to one fish. In. addition, four dilutions of the liquid 

 are prepared 1 in 10, 1 in 100, 1 in 1,000, and 1 in 10,000. With 

 the liquid and dilutions gelatin and agar plate cultivations are 

 prepared for the enumeration of the organisms present. Cul- 

 tures are also made in litmus lactose bile-salt peptone water and 

 in milk for the enumeration and isolation of B. coli and B. Welchii 

 respectively, taking 100 c.c., 10 c.c., and 1 c.c. of the liquid, 

 and 1 c.c. of each of the four dilutions ; in this way the contents 

 of the fish, ranging from one fish to T^^^ of a fish, are examined. 

 The process and principles involved correspond to those described 

 for water. Houston has suggested for oysters as a lenient stan- 

 dard less than 1,000, and as a stringent standard less than 100, B. 

 coli per oyster. Even ten B, coli per fish should be viewed with 

 suspicion, for Hewlett and others have shown that oysters from 

 pure layings contain no B. coli. 



Watercress, etc., may be examined in a similar manner, 100 grm. 

 being weighed out and transferred bit by bit with sterilised 

 forceps and scissors to a flask containing 900 c.c. of sterile water. 

 The flask is shaken vigorously, and the washings examined in a 

 manner similar to that employed for shell-fish. 



FILTERS. Reference has already been made to the 

 removal of organisms in water by sand filtration. With 

 regard to filters for domestic use, few of those in the 

 market are capable of doing more than removing particles 

 of suspended matter, while they allow from 5 to 50 per 

 cent., or even more, of the bacteria present in the water 

 to be filtered, to pass through. Such filters are, of course, 

 useless for the prevention of disease in fact, rather 

 favour it, by engendering a false sense of security ; and 

 when in use for some time without cleaning, the water 

 after filtration may be worse, bacteriologically and 

 chemically, than before filtration. The only efficient 

 filters are those composed of unglazed porcelain or some 

 such material, e.g. the Pasteur-Chamberland, Doulton 

 and Berkefeld. ' 



