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3. The chief mode of cultivation employed was that of agar-agar plates, mainly because a large
number of the bacteria of sewage rapidly liquefy gelatine, and also because in certain cases the plates
were incubated at 37° C. By the ordinary device of keeping the plates upside down all trouble from
the condensation of water was avoided. It is true that gelatine is in general a better medium than
agar-agar for the growth of micro-organisms, and subsequent experiments indicated that we should
possibly have found larger numbers had we used it, nevertheless the presence of liquefying organisms
made its employment almost impossible. Before the more slowly growing colonies have time for
development the plates become useless from the rapid liquefaction of the gelatine. Our attention was
also specially directed to the possibility of the occurrence of certain disease germs in the sewage,
notably those of typhoid, cholera, and diphtheria, and these three organisms all grow well on agaragar.
From each sample of sewage taken, 6 to 10 plates were made, and the number of colonies which
appeared showed an agreement which convinced us that the method of dilution adopted was a good one.
4. Methods of examination.—Each plate was examined from day to day, and the number of
colonies counted accurately with a hand lens. When the number became constant, i.e., when new
colonies had ceased to appear, an average was struck between the different plates. Calculation of the
total number of micro-organisms was made from the plates which had been incubated at 22° C., since
very many bacteria do not grow at a temperature of 37° C. In our latter experiments greater accuracy
was obtained by making careful drawings of each plate and numbering the different colonies.
From every kind of colony which appeared, so far as we were able to judge, sub-cultures were
made, and these were identified and named as completely as the means at our command allowed. This
involved not only microscopic observations, but sub-cultures in all the ordinary media employed, agaragar,
gelatine, beef-broth, milk, potatoes, &c., and also in many cases the use of chemical tests. But
in a very large number of cases, owing to imperfect descriptions and to the fact that many of the
bacteria with which we met seem to be hitherto undescribed, identification has been impossible. In a
few cases we give descriptions of such new species as appear to us of importance, but in most instances
we are obliged to pass over new or undescribed species, as to work out all their characters would be the
labour of some years.
Inasmuch as we desired to keep a very careful watch for the occurrence of the typhoid fever
bacillus in sewage, and since this organism is very similar to Bacillus coli communis (a normal
inhabitant of the human intestine and one of the commonest of sewage bacteria), and indeed
only to be distinguished from it with certainty by various chemical reactions, it was necessary to
employ special chemical tests in the case of all colonies which seemed likely to be the typhoid bacillus.
Particulars of the methods employed will be given later, when we describe in detail our search for this
organism in sewage.
It was clearly probable that the bacteria present in fresh sewage might prove very different
from those in older and more decomposed sewage. Sewage may be a favourable medium for the
growth of one species, an unfavourable one for another. The latter will therefore tend to disappear
from older sewage, which will contain larger and larger numbers of those particular organisms for which
it is a suitable soil. We therefore determined to examine, in the first instance, the most recent and
unaltered sewage to which we could gain access, but which nevertheless should be a fairly representative
sample. We selected the Smithfield outfall from St. Bartholomew's Hospital, as the area drained
by this sewer is sufficiently large to give a fair sample of average fresh sewage, including as it does,
not merely that from hospital wards but from several dwelling houses, and laboratories of various
kinds. The hospital has, moreover, during the past few years been newly drained, and the sewage is
delivered at the outfall within a few minutes of its discharge into the drains, having no prolonged
sojourn in old and foul brick drains to alter its composition. We hoped thus to gain some idea of the
bacterial flora of perfectly recent and unaltered sewage.
We next examined a sample of somewhat older sewage from a much larger area, but including
that from St. Bartholomew's Hospital—that, namely, from the Fleet sewer in Farringdon-street under
Holborn Viaduct, which takes the sewage from the districts of Holborn and Smithfield. Later we
examined sewage from the Barking outfall, which represents the sum total of all sewage north of the
Thames, and from the Crossness outfall, which is a similar summation of all sewages south of the
Thames. Ultimately, with the special object of searching for the bacillus of typhoid fever, we
examined sewage from the drain which leaves the scarlet and typhoid fever blocks at the Eastern Fever
Hospital at Homerton, and again that from a sewer in Bridge-street, Homerton, about a quarter of a
mile from the hospital, which conveyed the sewage of the Homerton district, including the hospital,
into the main sewer.
the total number of bacteria present in sewage.
Various published determinations show the number of organisms present in sewage to be
enormous. The results of Mr. Jordan's examination of the sewage of Lawrence, Mass., gave an
average of only 708,000 living bacteria per cubic centimetre, his highest result being 3,963,000 per c.c.
He obtained far higher results during the summer months than at other times. We have found, and
our results agree with those of other observers in England, that the number of bacteria in London
sewage is considerably higher than this. Determinations of the number of micro-organisms in the
sewage of the King's Scholars' Pond sewer in November and December, 1891, mentioned in a
previous report, showed 2,618,000 and 3,179,000 respectively. The figures we have obtained are as
follows:—
In perfectly fresh sewage from St. Bartholomew's Hospital on January 26th, 1894, at 10.30 a.m.
from which cultivations were made immediately, without allowing any time for the multiplication of
the micro-organisms, we found on an average 2,781,650 bacteria per cubic centimetre. Two agar-agar
plates, inoculated each with 1 c.c. of the sewage diluted 10,000 times, and incubated at 22° C. yielded