230 BACTERIA 



are some of the chief contributors to the elucidation of this 

 problem. 



The mode of infection of oysters by pathogenic bacteria 

 is briefly as follows : The sewage of certain coast towns is 

 passed untreated out to sea. At or near the outfall, oyster- 

 beds are laid down for the purpose of fattening oysters. 

 Thus they become contaminated with saprophytic and path- 

 ogenic germs contained in the sewage. It will be at once 

 apparent that several preliminary questions require attention 

 before any deductions can be drawn as to whether or not 

 oysters convey virulent disease to consumers. To the solu- 

 tion of these Dr. Cartwright Wood was one of the first to 

 address himself. 



The precise conditions which render one locality more 

 favourable than another in respect to oyster culture are not 

 fully known. But it has been observed that they do not 

 flourish in water containing less than three per cent, of salt. 

 Hence they are absent from the Baltic Sea, which, owing to 

 the fresh water flowing into it in rivers, contains a smaller 

 percentage of salt than three. Oysters appear in addition, 

 to favour a locality where they find their chosen food of 

 small animalculae and particles of organic matter. Such a 

 favourable locality is the mouth of a river, where tides and 

 currents also assist in bringing food to the oyster. Unfor- 

 tunately, however, in a crowded country like England such 

 localities round her coasts are frequently contaminated by 

 sewage from outfalls. Thus the oysters and the sewage 

 come into intimate relation with each other. 



Professor Giaxa carried out some experiments in 1889 at 

 Naples which appeared to show that the bacilli of cholera 

 and typhoid rapidly disappeared in ordinary sea-water. 

 Other observers at about the same time, notably Foster and 

 Freitag, arrived at an opposite conclusion. In 1894 Profes- 

 sor Percy Frankland, in a report to the Royal Society, de- 

 clared " that common salt, whilst enormously stimulating 



