S
very seldom that there is any doubt about the result, but when this does occur an extra
24: hours in the incubator usually settles the matter definitely one way or the other.
This may be taken as the working details of the method used. in the following
report, the various media will be found in the table given in the appendix. The
method, it will be observed, does not differ materially from all others in daily use,
the important improvement being that the organisms, that grow in the bile salt broth,
are, by the use of MacConkey's system of passing each organism through certain
fermentation tests, separated out into their individual species.
It may be well to discuss the value of the several tests in a little greater detail :-
(1) The total colonies on agar—This should always form part of the examination.
t affords very valuable information regarding the bacilli,. other than the lactose
fermenters, that are present in a given sample. By comparing the total colonies with the
number of lactose fermenters present :in the bile salt broth the relation between fecal and
ordina\ry water organisms is roughly ascertained. The particular value of the total counts
is not so much in Comparing samples from different parts of the Presidency, though this
is possible, but in comparing the various samples derived from the same source. Here
it may . be pointed out that as a routine method we usually take 1 or 5 Samples from
each system one from the natural water, whether it be well, spring or river ; one
taken immediately after filtration (whether it be filtered through a gallery or through
constructed filter beds) ; and 2 or 3 samples from the distributary System, and. the
service reservoir if there he one. - We not infrequently find that the number of
lactose fermenting organisms in a system varies very little wherever the samples are
taken, but . the total colonies in I cc. gives us much valuable information as to the
efficiency or otherwise of the filtration, the action of the pipes on the water, and the
presence or otherwise of silt in the terminal branches of distributary system. One is
Perfectly prepared to admit that gelatine as a culture medium is superior to agar for.
tins work, because besides giving all the information agar does, it differentiates
between the liquifiers and. the non-liquifiers of gelatine ; further the colonies of the
various bacilli present certain characteristic appearances on this medium. Neither
of these advantages can be claimed for agar-aga• ; but the difficulties in the wa y of
working with gelatine are so great on the plains, that they seriously interfere with its
usefulness.
(2)
Sporogenes lest--There is a great difference of opinion regarding.
the utility or otherwise of seeking for the presence of sporogenes spores in a sample
of drinking water. Many eminent Bacteriologists regard this test as useless as an
indication of pollution, for the simple reason that spores will live for months in
water. Houston however makes the following remark on the subject :—" The test
as ordinarily
fæcal
carried out is a better measure of the degree of impurity of impure
water than an indication of the purity of pure water."
Further, as we are only at the beginning of the work on the bacteriology of Indian
waters, we consider it better to err on the side of doing too many investigations rather
than too few. Until it is definitely settled. that the value of this spore-bearer, as an
indication of pollution is practically nil, it is recommended. that the test be done. It is
simple to carry out and requires only sterile milk as a medium.
(3) Bile salt broth.—The use of MacConkey's bile salt broth, as an indicator
of fæcal contamination, is too well recognised to require any further comment from
us. Of course, if lactose instead of glucose be used in its preparation, only lactose
fermenters will grow in the medium, and this narrows the scope of the work somewhat,
but it possesses one advantage, viz., that if a rough indication of the amount of
contamination be required, a fairly satisfactory opinion can be given from the results
obtained in the bile salt broth alone, without any sub-culturing and separating of
fæcal
the
organisms present. In the laboratory of the King Institute this is occasionally done
for samples which we do not consider it necessary to examine fully. This is what we
designate as the ' short method. " It differs in no way from the long method, except
that no further sub-culturing of organisms from the bile salt medium is carried out.
The results are reported "Evidence of contamination is present in . ...cc."
It has been pointed out by MacConkey in his paper, a Contribution to the
Bacteriology of Milk (
fæcal
Journal of Hygiene, July 1906), that in dealing with faecal
contamination generally, greater separation of individual bacilli is necessary, and in
order to do this he recommends his method. of identifying the separate organisms