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9
REPORT ON TIIE BACTERIOLOGICAL EXAMINATION OF SAMPLES OF
WATER AND OF WATERCRESS.
By. Dr. A. C. Houston.
SUMMARY OF CONTENTS.
Methods—Page 9.
Section I.—Results of the quantitative and qualitative bacteriological examination of the water "feeding" watcrcress
beds in localities (1) free, from objection on topographical grounds, and (2) open to condemnation as the result of local
inspection.—(Table I.). Page 11.
Section II.—The comparative bacteriological examination of the water representing the first and final "washings" of
cress obtained from pure and impure, beds.—(Table II.). Page 13.
Section III.—The results as regards subsequent isolation of B. coli or coli-like microbes of the addition of cress in
known quantities directly to liquid culture media both before and also after prolonged ivashing.—(Table III.). Page 13.
Section IV.—Comparative bacteriological examinations of the leaf and the stalk of watercress.—(Table IV.). Page 10.
Section V.—The question of standards. Page 16.
Section VI.—Summary and conclusions. Page 17.
Appendix A.—Analysis of the biological attributes of the B. coli or coli-like microbes isolated, from the various waters
(Section I.). Page 18.
Appendix B.—Analysis of the biological attributes of the B. coli or coli-like microbes isolated from the "washings "
of the cress.—(Section II.). Page 18.
Appendix c—Analysis of the biological attributes of the B. coli or coli-like microbes isolated from the primary
medium into which the cress had been previously directly introduced.—(Sections III. and IV). Page 18.
The circumstances which have brought watercress into some disrepute during recent years need
not be described in this report. Bu the trade is undoubtedly at present suffering from the effects
of a "watercress scare" somewhat analogous to the "oyster scare" now on the wane. This report
is not concerned with the epidemiological evidence implicating watercress as a possible means of
spreading disease nor with the topographical conditions of the watercress beds in the vicinity of London
or elsewhere. No attempt will be made, therefore, otherwise than incidentally, to correlate the
bacteriological facts with epidemiological experience and the results of topographical observations.
The matter is important because, although the facts of the competent bacteriologist may be accepted
without hesitation, the proper interpretation to be placed on the facts is matter for conjecture. The
administrator is faced with the dilemma of how best to safeguard the interests of public health without
causing injury to trade.
In carrying out the investigation I have ventured to exceed to some extent my original
instructions, which had reference only to the bacterioscopic analysis of water. The reason was that
certain preliminary experiments showed that the bacteriological examination of the cress was likely
to be as important as the examination of the water " feeding " the beds.
METHODS.
(1.)—Water.
The various waters were examined by the B. coli, B. enteritidis sporogenes and streptococcus tests. It is unnecessary
to enter into any detail as the methods employed were those described in my reports on the subject to the
Local Government Board and Royal Commission on Sewage Disposal
(2.)—Watercress.
(A) The bacteriological examination of the first and final " washings " of the cress.
One hundred grammes of watercress1 are weighed out and transferred bit by bit by means of a sterilised forceps
and scissors into a flask containing 900 c.c. of sterile water. The flask is vigorously shaken for some time and then
cultures are made of the " washings " of the cress as follows:—
(а).—One hundred c.c. are transferred into a flask containing 100 c.c. double strength bile-salt glucoso
peptone medium. (This culture is to be thought of as representing the first " washings " of 10 grammes of
the cress.)
(b).—Ten c.c. are transferred into a tube containing 10 c.c. of double strength bile-salt glucose peptone
medium. (This culture is to be thought of as representing the first " washings " of 1 gramme of the cress.)
(c).—One c.c. is added to a tube containing 10 c.c. single strength bile-salt glucose peptone medium. (This
culture is to be thought of as representing the first "washing" of 0.1 gramme of cress.)
Dilutions are made as follows :—
Ten c.c. of the washings are added to 90 c.c. of sterile water in a flask called (2).
Ten c.c. of (2) are added to a second flask containing 90 c.c. of sterile water called (3).
Ten c.c. of (3) are similarly added to a third flask called (4), 10 c.c. of (4) to (5), and, lastly, 10 0.0. of (5)i
to a fifth flask called (6).
One c.c. from flasks (2), (3), (4), (5) and(6)2 are added to tubes containing bile-salt glucose peptone,
and they represent the first washings "of .01, ".001. "0001, .00001, and .000001 gramme respectively of
cress.
1 One hundred grammes by weight of watercress were found by volume to measure in each case about 120 o.o. A similar
result was obtained with a sample of cress boughtfromashop. Cresstends to float, otherwise it would be more accurate
to add the 100 grammes to an empty sterile 1,000 c.c. measuring cylinder and fill up with sterile water to the
1,000 c.c. mark. For all practical purposes the gramme by weight of cress and the cubic centimetre
by measure of water may be considered identical.
2 Called (2), (3), (4), (5), (6), because 1 c.c. represents respectively the " washings" of .01, .001, .0001 .00001, and
.000001 gramme of cress.
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