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On agar-agar.—No growth occurs at 37° C. At 20° C. growth is rapid.
Agar-agar streak cultures.—In 24 hours the streak is 2 m.m. broad, of a greyish white colour,
gummy and semi-translucent in appearance, with a thick slightly irregular edge. In two days the
streak is 3 m.m. in width and distinct fluorescence is perceptible, and in a week the agar-agar shows
brilliant greenish-yellow fluorescence. The streak ultimately attains a width of 4 to 5 m.m. In
cultures of three weeks old or more the fluorescence fades to a yellowish brown tint and the growth
becomes more transparent.
In broth.—Growth is very similar to that of B. fluorescens liquefaciens; marked green
fluorescence appears in the surface layer after the third day.
In milk.—The milk does not coagulate, but is gradually digested, the upper layer becoming
yellowish and semi-transparent after two days; this change gradually spreads downwards, and in five
days there is a loose flocculent deposit at the bottom of the tube, the upper layers being semitransparent
and of a slightly yellowish-green tint.
Temperature.—No growth in any medium at 37° C. Grows well at 20° C.
Rapidity of growth.—Grows rapidly, attaining its maximum of growth in a week or 10 days.
Spore formation.—Not observed.
Need of oxygen.—Growth almost absent away from the surface. The fluorescent pigment is
produced only in contact with the air.
Gas production.—Not observed.
Reaction on gelatine.—Liquefies rapidly.
Pigment production.—Produces a greenish yellow fluorescent pigment, apparently identical with
that yielded by the preceding species, B. cloacae fluorescens, but fading more quickly. Liquefied
plates have an unpleasant sour, somewhat faecal smell.
Relations.—Allied closely to the preceding species, but is of somewhat smaller size, and liquefies
gelatine more rapidly. It is also closely allied to B. fluorescens liquefaciens, but differs from it in the
mode and rate of liquefaction of gelatine, and in the greater brilliancy of the fluorescence it produces
in agar-agar cultures, in which it resembles B. pyocyaneus. From the last-named species it differs
in its refusal to grow at 37° C.
conclusions.
If we now proceed to contrast the micro-organisms which we have found present in London
sewage, as above described, with those found in the air of London sewers, and described in a previous
report, certain striking differences become at once apparent. The most striking is the absence of
moulds from sewage. Out of the many thousand colonies which arose in the numerous plates which
were made, moulds occurred only seven times, and of those seven moulds one, and one only, was allied to
the common species existing in sewer air. This result coincides with that obtained by Jordan in the
sewage of Lawrence, Mass. "Moulds," he says, "are found rarely in the sewage and effluents."
Further the same striking result was obtained by Mr. Dibdin in a series of experiments he made on the
number of micro-organisms in Barking sewage. In sewer air on the contrary moulds were found to
be a predominating feature, forming on an average no less than 64.33 per cent. of the total colonies.
The proportion, however, varies according to season; in winter it may be as low as 13.33 per cent.,
while in the summer months it may rise as high as 82.6 per cent. of the total. It is right, however,
to point out that in our sewage experiments, agar-agar, which is a less favourable medium than gelatine
for the growth of moulds, was employed, whereas in the estimation of the organisms present in sewer
air gelatine was used. This, however, does not materially affect our results since most moulds grow
well on agar-agar at a temperature of 22° C. Jordan, and Dibdin, moreover employed gelatine plates
only-
Bacillus coli communis was found, with one exception, in each sample of sewage we examined,
and varied in numbers from 20,000 to 200,000 per cubic centimetre. In addition we isolated in every
case many bacilli which, although indistinguishable in cultural features from bacillus coli communis,
nevertheless gave only one or at most two of the three characteristic chemical reactions. These
bacilli in most instances far outnumbered the true bacillus coli communis. Nevertheless, neither
B. coli communis nor any of its close allies were ever found in sewer air. Furthermore, we have
obtained enormous numbers of sarcina, amounting in one case to over 300,000 per cubic centimetre,
yet we have not found one single colony of Sarcina lutea which is so common in sewer air and fresh air.
The flora of sewage naturally embraces a very large number of different species of bacteria, but
when we consider the enormous number of micro-organisms per cubic centimetre and the extreme
degree of dilution that it is necessary to employ in making cultivations, it will be evident that we have
probably isolated only those species which are present in comparatively large numbers. The following
organisms are those which we have found to be present in numbers varying from 200,000 to 2,500,000
per cubic centimetre—bacillus fluorescens stercoralis, bacillus albus putidus, bacillus fluorescens liquefaciens,
bacillus cloacae fluorescens, bacillus mycoides, proteus cloacinus, proteus Zenkeri, a streptococcus
coagulating milk, staphylococcus pyogenes citreus, sarcina flava and its allies, and diplococcus albicans
tardissimus. Other bacilli, which rapidly liquefy gelatine and produce a green fluorescence, were found
in numbers varying from 10,000 to 200,000 per cubic centimetre.
A further difference between the bacteria of sewer air and those of sewage lies in the relative
proportions of micrococci and bacilli. The bacteria of sewer air were found to consist mainly of
micrococci, bacilli forming but a small proportion of the total species found. The bacilli found were
B. subtilis, B. aureus, B. arborescens, B. helvolus, B. nigrescens, and B. acidi lactici. In sewage itself,
however, there can be no doubt that bacilli preponderate over micrococci, probably in actual numbers,
certainly in the numbers of species present. The large majority of those species which we have been
obliged to pass over were bacilli. This was less noticeable in the fresh sewage taken from
St. Bartholomew's Hospital, where the large number of streptococcus colonies materially altered the