BACTERIOLOGY OF SHELLFISH 207 



guinea-pigs, and gives yellowish growths on potato and 

 gelatin streaks. 



Hydrogen- Sulphide- forming Bacteria. Faeces, both 

 human and animal, contain bacteria capable of producing 

 hydrogen sulphide from peptone v/ater. Chamot and 

 Redfield (abst. Analyst, 1915, 351) found this group do 

 not actively ferment carbohydrates. 



The Bacteriology of Shellfish. 



Epidemics of typhoid have arisen through the ingestion 

 of bivalves arid molluscs obtained from beds polluted 

 with sewage containing typhoid bacilli (see p. 100). 

 Oysters are examined for sewage pollution by Houston's 

 method. Ten oysters are taken from a sample, thoroughly 

 cleansed by scrubbing in tap-water and rinsing in sterile 

 water. They are opened aseptically, and the fish is 

 minced and added, together with any fluid contained in 

 the shell, to sterile water in a sterile graduated cylinder, 

 and the bulk is made up to a litre with sterile water. 

 Each 100 c.c. of the emulsion represents one fish. With 

 the liquor thus obtained cultures are made in 



(a) Litmus lactose bile-salt peptone-water for detecting 

 B. coli, amounts being taken varying from 100 c.c. to 

 O'OOOl c.c., representing quantities of the fish from one 

 fish to one-millionth of a fish. 



(b) Sterile milk for detecting B. Welcliii, amounts 

 being taken of from 10 c.c. to 0-001 c.c., representing 

 quantities of fish from one-tenth of a fish to one-hundred- 

 thousandth of a fish. 



Agar and gelatin plates are also made for enumeration. 



Houston's standards are To reject oysters containing 

 1,000 (lenient standard) or 100 (stringent standard) 

 B. coli, or 100 (lenient standard) or 10 (stringent standard) 

 spores of B. Welchii, respectively per oyster. 



On the other hand, Hewlett, finding that oysters from 

 pure layings contain no B. coli, is of opinion that even 

 ten B. coli per fish should be viewed with suspicion. 



Nash is convinced that large numbers of ' indication ' 

 organisms will not always justify the conclusion that the 

 shellfish have been exposed to what is generally under- 

 stood by ' sewage ' pollution that is, human sewage 

 coming down through a sewer outfall. ' Cattle, sheep, 

 and gceso are grazed on low-lying marsh-lands, which 



