232
Comparative Study in the Use of Different Media
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XVII
in faeces, soil, milk and water under Indian conditions and found that MacConkey's bile salt lactose
broth was the most suitable medium for the purpose. A similar conclusion has been reached by
Raghavachari and Sitarama Iyer [1934, 1936 and 1939], who examined the use of several media
for estimating coliform contamination in water. In his studies on the types of colon organisms
present in milk supplies in Pusa, Walton [1931] used MacConkey's broth with litmus as a selective
medium for the detection and growth of coliforms and Endo's agar for confirming them. The use of
solid media for estimating coliform organisms in Indian milks does not seem to have received atten-
tion.
There is thus considerable divergence of opinion as to the best medium for carrying out the
presumptive coliform test in milk. While the English workers favour McConkey's broth, the
American authors recommended several liquid as well as solid media for the purpose. The most
suitable one from among them may be selected and adopted for use in this country. It is well known
that the nature of bacterial flora developing in milk is characteristic of the climatic and environmental
conditions and since India differs so widely from the western countries in these respects, any bacterio-
logical test or medium evolved there must be examined for its applicability as well as reliability in
testing Indian milks before accepting it with confidence. Accordingly the present study was under
taken with a view to compare some of the well-known liquid and solid media available for the
estimation of coliform organisms in milk.
EXPERIMENTAL
Media compared. The following media, which could be obtained locally or prepared in th
laboratory from available materials, were compared for estimating the coliform organisms in milk
by the dilution and plating methods :
Liquid media
MacConkey's broth
Formate ricinoleate broth
Brilliant green bile broth
Solid media
MacConkey's agar
Endo's agar
Eosin methylene blue agar
Violet red bile agar
With the exception of MacConkey's broth, the media were constituted from the Difco dehydrated
products (U.S.A.). The MacConkey's broth was prepared in this laboratory according to the formula
given by Chalmers [1945].
In the course of the investigation, the Difco preparation of MacConkey's agar was exhausted
early and further quantities could not be obtained. Some preliminary trials were, therefore, under-
taken to prepare the medium in this laboratory. A medium prepared from ordinary ingredients
according to the formula given by Chalmers [1945] or using Difco formula gave very unsatisfactory
results. The chief trouble seems to lie with the quality of peptone, bile salt and indicators, as a
result of which characteristic colonies of coliform organisms could not be obtained and other types
of organisms could not be prevented from growing on the medium and giving misleading results.
Ultimately the following composition, employing Difco ingredients and a suitable concentration of
the indicators, was found to give satisfactory results comparable with those yielded by the dehydrated
product and was adopted in subsequent work :
Bacto-peptone
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Proteose-peptone
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Bacto-lactose
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Bacto-bile salts
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Sodium chloride
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Shred agar
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Neutral red (B.D.H.) (0.5 per cent aqueous solution)
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Crystal violet (B.D.H.) (0.1 per cent aqueous solution)
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Tap water . . . . . . . . . . . . . . . .
17 gm.
3 gm.
10 gm.
1.5 gm.
5 gm.
17 gm.
6 ml.
2 ml.
1 litre.
(Final pH 7.1)
All the media were sterilized at 15 lb. pressure for 20 minutes and incubated at 37°C. for 3 to
4 days for detecting and weeding out any contaminated tubes before employing them for the ex-
periments.