240 OUTLINES OF BACTEEIOLOGY 



the bacteria which feed on manure. Ammonium compounds are 

 largely produced, these in their turn being successively changed into 

 nitrites and nitrates. The latter again may be partially still further 

 reduced by denitrifying bacteria and free nitrogen liberated. As there 

 are so many different kinds of bacteria in sewage, a very large number 

 of diverse substances are produced as a result of their activity. Many 

 of these are gases, so we find carbon-dioxide, hydrogen, sulphuretted 

 hydrogen, and marsh-gas in sewage. If sulphur-bacteria are present, 

 the sulphuretted hydrogen will be converted into sulphates. We know 

 as yet very little of the actual processes that take place during the 

 decomposition of sewage, or of the precise role played by the various 

 bacteria, we only know the general trend of the decompositions. The 

 work of the majority of these bacteria is of a highly beneficent nature 

 in sewage as it is in manure, and in all places where organic matter is 

 being broken down \ the saprophytic and nitrifying bacteria, by using 

 up the organic matter, changing this into other substances, prevent 

 the multiplication of the pathogenic bacteria which are also always 

 present in sewage. If unchecked, the latter would also consume the 

 organic matter in sewage, but would change it into, amongst others, 

 a number of highly poisonous bodies. 



A list of the principal sewage bacteria has been furnished by Dr. 

 Rideal, which is as follows : 



(L. = liquefying gelatine. S.L. = slightly liquefying. N.L. =not liquefying.) 



Obligatory anaerobes. 



Spirillum rugula (very active : gives rise to faecal odour). 



Sp. amyliferum. 



Bac. enteritidis sporogenes. 



Bac. amylobacter, L. 



B. butyricus, L. (gives much gas). 

 Facultative anaerobes or aerobes. 



B. putrificus coli, N.L. (decomposes albuminous substances, with 

 liberation of ammonia, whether air is present or not). 



Spirillum plicatile, serpens, undula, tenue, and volutans. 



Vibrio saprophilus, aureus, flavus, flavescens, N.L. 



B. mycoides, L. ^ produce ammonia from nitrogenous organic 



Proteus vulgaris, L. I matter and denitrify. 



B. fluorescens liquefaciens, L. ; and non-liquefaciens, N.L. 



Micrococcus ureae, N.L. Bac. ureae, N.L. (convert urea into am- 

 monium carbonate). 



B. mesentericus, L. 



Proteus mirabilis and P. Zenkeri, L. 



