A COMPARATIVE STUDY IN THE USE OF DIFFERENT MEDIA FOR THE
ESTIMATION OF COLIFORM ORGANISMS IN MILK
BY M. A. KRISHNASWAMY AND H. LAXMINARAYANA, Indian Dairy Research Institute, Bangalore
(Received for publication on 26 August 1946)
I N VIEW of the great importance attached to the presence of members of the coli-aerogenes group
in milk and milk products, the attention of workers in the field has been focussed on the develop-
ment of suitable methods for their detection and estimation. The dilution extinction method
(originally introduced by Lister) using various liquid media has been widely employed for the purpose
and recently plating on solid media for the enumeration of coliform organisms and the direct differentia-
tion of coli and aerogenes types have also come into practice. Most of the media were originally-
introduced for water analysis and subsequently suggested for the examination of milk.
MacConkey's lactose peptone bile broth [1905 and 1908] has been almost exclusively used for
the presumptive coliform test in England [Ministry of Agriculture, 1934] while in the United States
a standard lactose broth, differing from the former mainly in the omission of bile salt, was in general
use for a long time. The serious drawbacks of the latter medium in permitting the growth of other
types of organisms and giving many false positives were realized early and this led to the introduction
by various American workers of several selective liquid media, which, while permitting the growth
of coliform bacteria, would inhibit other organisms. Two of these namely, two per cent brilliant
green bile broth [Jordan, 1927] and formate ricinoleate broth [Stark and England, 1935] have been
generally regarded as most satisfactory [Farrell, 1937 ; McCrady and Langevin, 1932 ; McCrady
and Archambault, 1934; Noble and White, 1935; Babel and Parfitt, 1936] and they are now re-
commended for the presumptive coliform test in the bacteriological examination of milk in the
United States [American Public Health Association, 1941].
Solid media were originally employed for differentiating between the various members of the
coli-aerogenes group and confirmation of positive broth tubes in the presumptive test. MacConkey's
agar [1905] has been most commonly employed in England for the purpose, although several other
selective media namely, Eijkman's medium, Simmon's citrate agar and Koser's citrate medium, have
also been used for distinguishing between the two groups [Wilson and colleagues, 1935]. In other
countries, Endo's [1904] medium, litmus lactose agar, Levine's [1918] eosin methylene blue agar
and recently violet red bile agar (Difco laboratories, U.S.A.) and desoxycholate agar [Liefson, 1935]
have been widely used.
The employment of solid media for the enumeration of coliform organisms and direct differentia-
tion of coli and aerogenes types have received attention only recently. Its main advocates have
been Tonney and Noble [1932] who recommended a complex ferrocyanide citrate agar medium.
Kon [1932-33] could not get clear-cut results on any of the solid media suggested by previous workers.
Wilson et al [loc. cit.] conclude that MacConkey's medium (whether liquid or solid) is of great value
in the estimation of coliform bacilli. According to McCrady and Archambault [loc. cit.], Bartram
and Black [1937], Babel and Parfitt [loc. cit.] and Miller and Pricket [1938] violet red bile agar gives
most satisfactory results in 24 hours, while Chilson and Eglinton [1936] and Yale [1937] advocate
the use of desoxycholate agar. The last two media namely, violet red bile agar and desoxycholate
agar, have now been recommended for the presumptive coliform test in milk and eosin methylene
blue or Endo's agar for subsequent isolation of colonies for confirmation and identification
[American Public Health Association, 1941].
Although no legal standards or official methods for the bacteriological examination of milk
have been prescribed in India, the presumptive coliform test using MacConkey's broth [Ministry
of Agriculture, 1934] is employed in certain public health and dairy laboratories for assessing the
hygienic quality of water and milk. Cunningham and Raghavachari [1924] and Raghavachari
[1926] studied the application of some recent methods of differentiating lactose fermenting organisms
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