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Exp. 10.—Nine colonies typical of B. Coli were counted in a phenol-gelatine plate
cultivation containing 0.000l c.c. of crude sewage; several of these were tested in gelatine
" shake " cultures and showed gas formation in 24 hours.
Exp. 14.—At least 100,000 colonies of B. Coli were counted per c.c. of crude sewage.
Crossness crude sewage.
Exp. 1. B. Coli was detected in 0.000025 c.c. of crude sewage.
Exp. 3. A large number of colonies of B. Coli were counted in a phenol-gelatine plate
cultivation containing 0"01 c.c. of crude sewage.
Exp. 4. Same remarks as in experiment 3.
Exp. 8. Thirteen colonies typical of B. Coli were counted in a phenol-gelatine plate
cultivation containing 0.0001 c.c of crude sewage.
Exp. 11. 200,000 germs of B. Coli were found per c.c. of crude sewage.
Exp. 15. Same remarks as in experiment 11.
The Bacillus Coli Communis is perhaps the most characteristic and is one of the most abundant
of the different species of sewage micro-organisms, and sewage is the chief and most dangerous source
of pollution of potable waters. The B. Coli Communis is not present, at all events in any numbers,
in pure waters, and it is evident from the above figures that sewage may contain at least 100,000
germs of this micro-organism per c.c. It is obvious, then, that there exists a bacteriological method
of detecting the pollution of water with minimal quantities of sewage, which is of very great
delicacy.
In future work this micro-organism will be searched for in the effluent from the filter beds at
Barking and Crossness. Its presence in such an effluent may be considered to show the derivation of
the liquid from sewage, and hence to indicate the possible presence of other and perhaps dangerous
bacteria which have survived the biological processes at work in the filter beds, but does not signify
that the liquid is necessarily of a degraded, offensive, and putrescible character.
* Protean forms.
Although the proteus vulgaris is usually considered as the type of the various protean species
to be found in putrefying liquids, it would seem that in the case of London crude sewage the protean
form which is most abundant differs from the typical proteus vulgaris in that the cells are shorter and
more frequently united in pairs, and also inasmuch as the colonies show less tendency to produce the
so-called " swarming islands " on the surface of the gelatine.
The term "protean forms" is meant to include all phase forms of protean species and to
comprise those protean-like microbes which liquefy the gelatine rapidly, produce gas in gelatine shakecultures,
cause the decomposition of albuminous matter, and which microscopically show the presence
of actively motile cells, varying in size from very short rods to long threads, and show also involution
forms.
In all the samples of sewage the protean forms were very abundant, and in experiments 14 and
15 (Barking and Crossness sewage) the number of gas-forming protean bacteria was found to be at
least 100,000 per c.c. of sewage.
Pasteur considered that putrefaction was a process mainly dependent on the activity of anaerobic
bacteria; and that the removal of oxygen, either artificially or by the simultaneous growth of aerobic
species, was essential for the production of decomposition of albuminous matter. This view is not
generally accepted at the present time, since many aerobic bacteria are known which undoubtedly cause
putrefaction; and as our knowledge extends it will probably be found that a very large number of
different bacteria, besides those with which we are already familiar, are concerned either directly
or indirectly in the process of decomposition of organic matter into its simplest component parts.
Many bacteria are facultative anaerobes, and it is probable that these under one set of conditions
may produce an aerobic putrefaction and under another set of conditions may give rise to an anaerobic
decomposition of organic matter.
The subject is a complex one, but it seems certain that putrefaction may be produced by aerobic
bacteria under aerobic conditions, by facultative anaerobes under semi-anaerobic or even under anaerobic
conditions, and by strictly anaerobic species under strictly anaerobic conditions.
No doubt in the case of London sewage, as it flows along the sewers towards the outfalls, there
is aerobic putrefaction at work, which is accompanied and followed by a decomposition produced by
facultative anaerobes, and later, perhaps, by a putrefactive fermentation brought about by strictly
anaerobic species.
The volatile products which are evolved during the process of aerobic putrefaction are carbonic
acid, ammonia, &c., while the gases which are given off during the anaerobic decomposition of organic
matter are hydrogen, marsh gas, sulphuretted hydrogen, &c. Speaking generally, it may be said that
there is less nuisance and perhaps less danger attending aerobic than anaerobic putrefaction, and if the
two processes are equally efficacious in splitting up the various and complex kinds of organic matter
present in the sewage into their elementary and innocuous constituents, then preference must be given
to the aerobic process.
In the bacteriological "treatment" of sewage, a preliminary increase in the number of bacteria
causing liquefaction and decomposition of solid organic matter might well be considered advantageous ;
afterwards, however, and associated with a decline in the amount of albuminoid ammonia, one might hope
to find a decline in the number of putrefactive bacteria and an increase in the number of nitrifying
organisms and in the amount of oxidised nitrogen.
* See No. 12 (Plate I.).