BACTERIOLOGY OF WATER 225 



of man, with the consequence that it is often impossible 

 to identify the source of pollution by bacteriological 

 results. 



Typhoid, cholera, paratyphoid, dysentery, and perhaps 

 anthrax, are the principal water-borne diseases. Polio- 

 myelitis, gore throat, conjunctivitis, suppurative otitis 

 media, and frontal sinus suppuration, are all supposed 

 sometimes to follow visits to swimming-baths used by 

 infected persons. 



Several factors contribute to the increase or decrease of 

 the number of bacteria in water. Sunlight (p. 9) plays 

 an unimportant part in reducing the number, unless the 

 depth of water is very shallow and exposure is prolonged. 

 Cold inhibits growth of bacteria, and, ceteris paribus, a 

 higher bacterial content should be found in warm weather. 

 Other influences may, however, cause the winter content 

 of bacteria to be greater than that of summer. When 

 food material is ample in quantity more organisms are 

 met with. Most bacteria tend to settle to the bottom of 

 a bulk of water, and this sedimentation constitutes an 

 important factor in the self-purification of waters. Storage 

 is an important process in the purification of water on 

 a large scale. By storage time is given for any typhoid 

 or cholera bacilli to die out, owing to lack of nourish- 

 ment, to a temperature unfavourable to development (in 

 temperate climates), and to antagonism of normal water 

 bacteria. 



In the M.W.B. reservoirs, Thames and Lea waters can 

 be stored for fifty-two days. The reduction in the number 

 of organisms is sometimes over 99 per cent. It has been 

 suggested that, owing to bacteria clumping during storage, 

 and each clump producing a single colony, and therefore 

 being counted as one organism, the diminution is more 

 apparent than real. While this may be the case, clumps 

 of bacteria settle quicker than do individuals, and the 

 clumping may accelerate sedimentation (see p. 99). 



Houston found that when typhoid bacilli or cholera 

 vibrios were added to raw Thames, Lea, or New River 

 waters, a reduction of 99-9 per cent, of these organisms 

 in each case was attained in a week. Cholera vibrios 

 could not be found in three weeks, but eight weeks' 

 storage was found to be necessary for the disappearance 

 of B. typJiosus (cf. p. 99). This bacterial purification is 



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