IV]
M. A. KRISHNASWAMY AND H.
LAXIMINARAYANA
235
TABLE II
Comparative efficiency of different media for grading milks by the presumptive coliform test
Type of milk
Pasteurised milk . . .
Farm produced milk . .
Village milk . . .
Bulk milk (Private dairies) .
Coliform
bacilli
Absent
(ml.)
10°
10– 1
10– 2
10– 3
10– 2
10– 3
10– 4
10– 3
10– 4
10– 5
..
Number of samples indicated in different media
present
(ml.)
10
10°
10– 1
10– 2
10– 1
10– 2
10– 3
10– 2
10– 3
10– 4
10– 5
Mc.
Broth
2
4
4
12
2
2
9
2
4
4
8
F.R.
Broth
3
3
5
11
..
4
9
2
4
4
8
B.G.S.
Broth
2
4
6
10
2
4
7
2
5
3
8
Mc.
Agar
5
1
7
9
2
8
3
2
6
2
8
Endo's
Agar
4
2
7
9
4
3
6
3
5
2
8
E.M.B.
Agar
5
1
7
9
2
4
7
2
6
2
8
V.R.B.
Agar
5
1
7
9
3
4
6
4
5
2
7
Since the minimal quantity of milk showing the presence of coliform organisms is taken as the
criterion for assessing the extent of contamination for grading purposes, the comparative efficiency
of each medium from this point of view has been examined in Table II. The samples have been
grouped according to their source and general level of bacteriological quality, and the relative propor-
tions of positives in the highest dilutions of each series indicated by the different media are shown.
Even when judged from this angle, the liquid media compare well with each other and about 85
per cent of all the samples tested are placed in the same grades by each of the media as indicated
by the results of the presumptive test in the four groups. All the three broths may, therefore, be
said to be equally efficient for the presumptive coliform test in milk, but as MacConkey's broths is more
familiar and can be easily prepared in any laboratory from ingredients available locally, it offers some
advantages over the other two media for use in this country.
Similar agreement is found among the solid media, although in one case (village milk) Mac-
Conkey's agar has given a smaller number of positives than the others in the highest dilution of the
series, which may be due to sampling errors or to a higher inhibitory character of the former medium.
Though the four media have yielded fairly comparable results from the point of view of the presump-
tive coliform test, MacConkey's and violet red bile agars have been found more satisfactory for the
purpose than either Endo's or Rosin methylene blue agar due to the drawbacks pointed out earlier
in respect of the latter two media. The latter have, however, proved useful for confirmatory studies.
In the opinion of the authors, MacConkey' s agar is most suitable as it has been found to give highly
typical and clear colonies of coliform organisms and permit very few non-coliforms to develop on
the plates.
From a comparison of the solid with liquid media, it is observed that the results do not agree
so closely and only about 72 per cent of the samples are placed in the same grades by all the media
The broths have generally indicated a greater number of positives than the solid media in the highest
dilutions of each series and a good number of samples (20 per cent) found positive in the former are
recorded as negative and thus up-graded by the latter media. It cannot be definitely concluded
from this that the solid media give an erroneous estimate of the coliform content, although they
may be considered to exert a slightly greater inhibitory effect on the coliform organisms and present
a more conservative picture than the liquid media. It is well known that the dilution method of
counting bacteria is subject to very large sampling errors even when several tubes are used in each
dilution and that the results must be regarded as only approximate [Halvorson and Ziegler, 1933 ;
Wilson et al., loc. cit.]. Accordingly a positive presumptive evidence based on the appearance of
five or more typical colonies of coliform organisms on a single or duplicate set of plates may be
considered to be as reliable as that obtained from the results of fermentation observed in a number of
broth tubes, The plating method is much simpler and gives clear results in 24 hours, whereas the
R