2. INTRODUCTION
Mc Leod’s media, also known as Tellurite blood agar is a selective
medium used for isolation and cultivation of Corynebacterium species.
Selective media are used for the growth of only selected
microorganisms to ensure the survival or proliferation of cells with
certain properties, such as antibiotic resistance or the ability to
synthesize a certain metabolite.
The media contains indicator which changes colour when bacterium
grows in them.
3. Corynebacterium is a genus of gram-positive, facultatively
anaerobic, non-motile bacteria that exhibits a fermentative
metabolism (carbohydrates to lactic acid) under certain
conditions.
Corynebacteria constitute a diverse group of bacteria that
includes saprophytic associations as well as plant and animal
pathogens.
Most species are normal flora of humans present virtually at
all anatomic sites.
Many species of Corynebacteria can be isolated from various
places such as soil, water, blood, and human skin.
Pathogenic strains of Corynebacteria can infect plants,
animals, or humans.
4. COMPOSITION
Mc Leod’s media is mainly composed of heated rabbit’s blood and
potassium tellurite.
The other ingredients are-
Biopeptone- provides nitrogenous compounds
Sodium chloride- maintains the osmotic equilibrium of the medium
Dipotassium hydrogen phosphate- buffer the medium
Corn starch- neutralizes the toxic metabolites
Monopotassium phosphate-buffer the medium
Agar- for solidification
Haemoglobin and Vitamino Growth Supplement-stimulate good
growth
5. PRINCIPLE
Potassium tellurite acts as a selective
agent and has inhibitory activity
(formation superoxide radicles) against
most gram-positive and gram-negative
bacteria except Corynebacterium species.
Corynebacterium reduces the potassium
tellurite present in the Mc Leod’s media
to metallic tellurium which is
accumulated as black deposits inside the
cell.
As a result of which, grey-black colonies
are produced in the media.
Fig: Mc Leod’s agar plate
6. USES
Used for isolation and cultivation of Corynbacterium
species. Throat or nasal swab is directly inoculated and
streaked on this agar medium.
Depending upon the colonial morphology of
Corynbacterium in Mc Leod’s media, it can be classified
into C. gravis, C. intermedius and C. mitis. These cultural
biotypes are more flat with the latter two having larger
colony size.
7. LIMITATIONS
Slow growth.
Colony seen after 48 hours.
Organisms like staphylococci and streptococci can also
produce such colonies, has to be distinguished later by
gram stain morphology.
