The
differential media
medium that may eventually give rise to diagnostically useful growth or apparent change in the medium
after the proper incubation.
A few typical examples are as discussed under :
Eosin Methylene Blue Agar [EMB-Agar]
The
EMB-Agar media is employed exclusively to differentiate between the ‘lactose fermenters’
and the
‘non-lactose-fermenters’. In-fact, the EMB-Agar media essentially comprises of : lactose,
salts, and
two dyes viz. eosin and methylene blue. From the observations the following inferences may
be drawn :
(
a) E. coli (a ‘lactose fermenter’) : will produce either a dark colony or one that has a metallic
sheen, and
(
b) S. typhi (a ‘nonlactose fermenter’) : shall appear as an absolute colourless colony.
MacConkey Agar
It has already been discussed under Section 3.
Hektoen Enteric Agar [HE-Agar]
It is invariably used to enhance the overall yield of
Salmonella and Shigella species in comparison
to other
microbiota. It has been observed that the presence of relatively high bile salt concentration
inhibits the general growth of Gram-positive microorganisms specifically. Besides, HE-Agar also retards
(or slows down) the growth of several coliform strains.
Enrichment Media
It has been amply demonstrated and established the critical and judicious incorporation of serum,
blood, or extracts to the particular
‘tryptic soy agar’ or broth shall enormously augment the much
desired growth of a large number of most
‘fastidious microbes’. In actual practice, however, these
media are largely employed to isolate primarily the microorganisms from a host of
‘biological fluids’
such as :
cerebrospinal fluid, pleural fluid, wound abscesses, and sputum. A few typical examples
are as stated under :
Blood Agar*
The critical addition of
‘citrated blood’ to the prevailing ‘tryptic soy agar’ renders it to afford
variable haemolysis
, that in turn allows the precise differentiation of certain species of microorganisms.
It is, however, pertinent to state here that one may observe these distinct
haemolytic patterns on blood
agar. A few such typical variations are as stated under :
(
a) α-Haemolysis. It may be observed due to the formation of greenish to brownish halo**around the colony e.g., streptococcus gardonii and streptococcus pneumoniae.
(
b) β-Haemolysis. It represents the virtual complete haemolysis of blood cells thereby giving
rise to a distinct
clearing effect around growth in the colony e.g., Staphylococcus aureus and Streptococcus
pyogenes
.
(
c) Nonhaemolytic Pattern. In this particular instance practically no change occurs in the mediume.g., Staphylococcus epidermidis and Staphylococcus saprophyticus.
Chocolate Agar
Interestingly, the
‘chocolate agar’ is specifically made from ‘pre-heated blood’ that essentially
caters for the requisite and necessary
growth factors desired urgently to support the bacterial growthe.g., Haemophilus influenzae and Neisseria gonorrhoeae.
Characteristic Media
The very purpose and extensive utility of the so-called
‘characteristic media’ are to test microbes
for ascertaining a few highly specific
metabolic activities, products, or their ensuing requirements.Following are some of the typical examples, namely :