14 GENERAL CONSIDERATIONS 



natural conditions, apart from the animal body. As Andrewes 1 

 points out, " They are certainly abundant in the air of London, 

 but these common air-streptococci are, as I have elsewhere 

 shown, identical with the forms most abundant in the horse- 

 dung which is amongst the chief constituents of London street 

 dust. They occur, too, in water, but in proportion to its con- 

 tamination with sewage, or at least, with animal excreta. Much 

 the same is true of earth and soil. In all these situations strep- 

 tococci may, indeed, be found, but always, it would seem, as the 

 result of pollution with organic matter." 



Their presence in contaminated and absence from pure soil 

 is shown in Chapter IV. 



Outside the animal body they may survive for considerable 

 periods but do not thrive. The evidence as to the duration 

 of viability and vitality outside the animal body is somewhat 

 conflicting, but in general it would appear that the majority are 

 delicate organisms and rapidly die out, but that a small number 

 of hardy strains may persist for very long periods. 



In relation to their viability in water one of the most recent 

 investigations is that of Houston 2 , who studied the viability of 

 100 streptococcus strains, isolated from lumps of excreta in 

 sewage, in Thames river water sterilized by passing through 

 a Pasteur filter. About 100,000 of each of the 100 varieties 

 were added separately to 100 tubes, each containing 10 c.c. of 

 the filtered water. Cultures (i c.c. of water) were made from 

 them each week. The tubes were kept in a dark cupboard at 

 a temperature varying from 8 to i6C. The following is a 

 summary of the results: 



3 per cent, died in I week. 

 II ,,2 weeks. 

 32 3 

 38 4 



8 ,,5 



4 6 

 2 9 



I still alive in 9 weeks, 



(one tube became contaminated.) 



1 Lancet, 1906, n, Nov. 24 (Horace Dobell Lecture). 



2 Metropolitan Water Board, Fifth Research Report. 



