The isolation of
‘Bacteria’
methods, such as :
(
a) Selective and diagnostic media,
(
b) Bismuth sulphite agar, and
(
c) Selective media for staphylococci.
The aforesaid
three methodologies invariably used for the isolation of bacteria shall be treated
individually in the sections that follows :
Selective and Diagnostic Media
McConkey’s medium was first and foremost introduced in 1995 so as to isolate
Enterobacteriaceae
from faeces, urine, foods, water etc. The medium essentially comprises of several
nutrients
viz., bile salts, lactose, and an appropriate indicator.
Bile salts
categorically serve as an important natural surface-active agent which, fails to
inhibit the growth of the
Enterobacteriaceae, but distinctly inhibits the growth of the specific Grampositive
microorganisms that are probably present in the material to be examined.
Lactose
aids in the production of ‘acid’ from E. coli and A. aerogenes upon this culture medium
thereby changing the colour of the suitable indicator added ; besides, the said two microorganisms may
also
adsorb a certain amount of the indicator that may eventually get duly precipitated around the
growing cells. Importantly, the
bacteria responsible for causing typhoid and paratyphoid fever, and
bacillary dysentery fail to
ferment lactose ; and, therefore, the resulting colonies produced duly by
these organisms appear to be absolutely transparent.
Modifications of McConkey’s Medium
— are as stated under :
(1) Synthetic surface-active agent may replace the
‘Bile Salts’,
(2) Selectivity of
McConkey’s medium may be enhanced significantly by the incorporation of
inhibitory dyes
e.g. crystal violet, neutral red. In fact, these dyes particularly suppress the growth of
Gram-positive microorganisms
viz., Staphylococci.
Bismuth Sulphite Agar
The discovery of the
bismuth sulphite agar medium dates back to 1920s for the identification
of
Salmonella typhi in pharmaceutical preparations, foods, faeces, urine, and water. It essentially
comprises of a
‘buffered nutrient agar’ consisting of bismuth sulphate, ferrous sulphate, and anindicator brilliant green.
E. coli
gets inhibited at a concentration 0.0025% of brilliant green employed, whereas another
organism
Salmonella typhi shall grow predominantly. It has been observed that bismuth sulphite does
exert certain degree of inhibitory effect upon
E. coli.
S. typhi
, in the presence of glucose, causes reduction of bismuth sulphite to the corresponding
bismuth sulphide
(i.e., a black compound), thereby ascertaining the fact that the investigative organism
may generate H
2S from the S-containing amino acids in the medium, which in turn shall interact with
FeSO
4 to produce a distinct black precipitate of FeS (ferrous sulphide).
Selective Media for Staphylococci
The presence of the
Staphylococci organisms in various specimens viz., pharmaceutical products,
food items
, and pathological specimens, may ultimately cause food poisoning as well as serious
systemic infections.
A few typical examples of selective media for various organisms are as follows :
(
a) Enterobacteria — a surface active agent serves as the main-selector.
(
b) Staphylococci — NaCl and LiCl serve as the main selectors. Besides, Staphylococci ingeneral are found to be sufficiently tolerant of NaCl concentrations upto an extent of 7.5%.