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14
Certain points calling for comment may be added to the above table. After the tenth day
several of the flasks became contaminated with other organisms, so that exact estimation of the number
of typhoid bacilli was impossible. We had reason, however, for believing that the total extinction of
the typhoid bacilli did not take place quite as soon as the above figures indicate, since on the twelfth
day cultivations from most of the flasks, made by taking 0.5 c.c. of the undiluted mixtures, showed a
large number of colonies, many of which were certainly typhoid. The flask containing typhoid with
B. fluoresceins stercoralis proved perfectly sterile on the twelfth day, even though 0.5 c.c. of the
mixture was used. The negative results on the tenth day from the other five flasks were obtained
from plates made with the diluted mixtures, and it is possible that the distilled water used for dilution
exercised an unfavourable influence on bacilli previously weakened by their sojourn in an unsuitable
medium. In one flask, that containing typhoid with the mixture of all four organisms, some typhoid
bacilli were found living even alter the lapse of 30 days, when a sufficiently large amount of the
undiluted mixture was used for cultivation.
In spite of possible errors thus arising during the later stages of this experiment, we believe
that the figures in the table are reliable up to the end of the tenth day, at least no labour was spared
to make them so. So far as deductions can be safely drawn from a single series of experiments, the
following conclusions seem to be warranted. Bacillus typhosus seems capable of slight multiplication
in sterilised sewage during the first 24 hours only, thenceforward it becomes gradually extinct. The
presence of certain non-pathogenic organisms, commonly present in sewage, appears to affect its
extinction. Of the four organisms which we tested B. fluorescens stercoralis alone seems to have any
marked effect upon the vitality of B. typhosus, and this effect is practically absent when other
organisms are present at the same time. The mixture of the four non-pathogenic bacteria had no
effect in hastening the extinction of B. typhosus, indeed the reverse appears to have been the case.
These preliminary experiments are necessarily very incomplete, and afford only an indication of
the probable fate of typhoid bacilli which gain access in a living condition to sewage. It seems however
clear that sewage does not form a medium in which much, if any, growth is possible for them
under natural conditions, and their death is probably only a matter of a few days or at most one or two
weeks. But this degree of resistance may, nevertheless, be sufficient to allow of their being carried in
sewage to remote distances and of their being able to produce disastrous results should they gain access
to any water supply. As our knowledge accumulates, it becomes more and more evident that water
supply and, as an incidental result, our milk supply constitute the chief channels of infection by
which typhoid fever is communicated, and this is true also of cholera and possibly of other infectious
diseases. It is, therefore, of the first importance to determine in an exhaustive manner how far
sewage is a possible soil for the growth of these and other disease germs which admittedly gain access
to it, and also to determine what precise influence their non-pathogenic companions may exert on them.
In the conclusions to Part I. of this report we endeavoured to show that sewer air has no power
of taking up bacteria from the sewage with which it is in contact. A strong argument in favour of
this view is the fact that the very organisms which are most abundant in sewage are precisely those
which are absent from sewer air. In the course of previous experiments on sewer air, the nature of
the organisms in some 1,200 litres of sewer air was carefully determined. Not once was Bacillus coli
communis, or any of the predominant organisms of sewage found, though we have shown above that the
former is present in sewage in numbers varying from 20,000 to 200,000 per cubic centimetre. If this
be so, how infinitely improbable becomes the existence of the typhoid bacillus in the air of our sewers.
That sewage is a common medium for the dissemination of typhoid is certain; that sewage-polluted
soil may give up germs to subsoil air is possible; but that the air of sewers themselves should play
any part in the conveyance of typhoid fever appears to us, as the results of our investigations, in the
highest degree unlikely.
J. Parry Laws.
Frederick W. Andrewes.
13th December, 1894.