82 BACTERIA IN THE AIR 



A still more remarkable illustration of the effect of a moist 

 perimeter upon the contained bacteria is to be found in sewer air. 

 For long it has been known that air polluted by sewage emanations 

 is capable of giving rise to various degrees of ill-health. These 

 chiefly affect two parts of the body ; one is the throat and the other 

 the intestine. Irritation and inflammation may be set up in both or 

 either by sewer air. Such conditions are in all probability produced 

 by a lowering of the resistance and vitality of the tissues, and not by 

 a conveyance of bacteria in sewer air or by any stimulating effect 

 upon bacteria exercised by sewer air. What evidence we have is 

 against such factors. Several series of investigations have been 

 made into the bacteriology of sewer air, amongst others by Uffel- 

 mann, Oarnelly and Haldane, and Laws and Andrewes. From their 

 labours we may formulate four simple conclusions : — 



1. The air of sewers contains very few micro-organisms indeed, 

 sometimes not more than two organisms per litre (Haldane), and 

 generally fewer than the outside air (Laws and Andrewes). 



2. There is not, as a rule, intimate relationship between the 

 microbes contained in sewer air and those contained in sewage. 

 Indeed, there is a marked difference which forms a contrast as 

 striking as it is at first sight unexpected. The organisms isolated from 

 sewer air are those commonly present in the open air. Micrococci 

 and moulds predominate, whereas in sewage moulds and micrococci 

 are rare, and bacilli are most numerous. Liquefying bacteria, too, 

 which are common in sewage, are extremely rare in sewer air. 

 Bacillus coli communis, which occurs in sewage from 20,000 to 

 200,000 per c.c, is altogether absent from sewer air. 



3. As a rule it may be said that only when there is splashing in 

 the sewage, or when bubbles are bursting (Oarnelly and Haldane), is 

 it possible for sewage to part with its contained bacteria to the air 

 of the sewer. But under these conditions it may part with a 

 considerable number. 



4. Pathogenic organisms and those nearly allied to them are 

 found in sewage, but are absent in sewer air. Uffelmann isolated 

 the Staphylococcus pyogenes aureus (one of the organisms of sup- 

 puration), but such a species is exceptional in sewer air. Hence, 

 though sewer air is popularly held responsible for directly conveying 

 virulent micro-organisms of various diseases, there is up to the 

 present no evidence of a substantial nature in support of such views. 

 In 1894, Laws and Andrewes found an average of 2,781,650 bacteria 

 per c.c. in fresh sewage, and in older sewage from 3,400,000 per c.c, 

 to 11,216,000, and they pointed out that temperature and dilution 

 of sewage were determining factors in the number of bacteria present. 

 They consider that sewage may become a medium for the dissemina- 

 tion of the typhoid bacillus, and that sewage-polluted soil may possibly 



