MacConkey agar is widely used to examine stools specimens. To shut out the proliferation of gram-positive
bacteria, 1 mg per liter of crystal violet is included in the composition. Similarly, 30 mg per liter of neutral red is used
to act as pH indicator. In the fermentation of lactose by gram-negative bacteria, an acid is produced, in this case, giving
their colonies a red colouration; a colourless colony indicates that the bacteria there do not ferment lactose.
Hektoen Enteric (HE) agar is recognised for a much improvised bacterial colonial differentiation due to its effectiveness
in exposing high-speed lactose-fermenting organism in fecal specimens. Additional capabilities include the detection of hydrogen
sulphide producers. HE agar encompasses 3 carbohydrates – lactose, salicin, and sucrose and in addition, indicator dyes
including Bromthymol blue (65 mg per liter) and acid fuchsin (100 mg per liter). Against the faint pink to red background
of the medium, high-speed lactose-fermenting bacteria shows up as bright yellow to salmon-pink glistening drops. Nonlactose
fermenters are normally blue-green to green in colour, Sodium thiosulfate and ferric ammonium citrate are employed in the
medium to reveal the presence of H2S emitters, which will have black centers in their colonies. It is crucial to
ensure that the HE agar is transparent and green in colour before use.
on the purpose of isolating and differentiating of pathogenic Staphylococcus aureus, Mannitol-salt agar was customized by
concocting a solution of 7.5% Sodium Chloride. The very basis of its function: it inhibits the proliferation of most bacteria.
The medium also include 10g of mannitol per liter for the identification of mannitol fementation. Phenol is added into the
medium in the concentration of 25mg per liter.
A blood agar plate can be made from any animal
blood. However results may alter depending on the different animal blood agars used. Thus, clinical results use
5-10% sheep blood agar plates as the standard for defining hemolytic reaction of bacteria.