Agar Plate

MacConkey agar

 Macconkey agar with lactose (left) and non-lactose (right)

A MacConkey agar plate with an active bacterial culture.

MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.

 Contents

It contains bile salts (to inhibit most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus i.e. Staphylococcus aureus), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.
Composition:

• Peptone - 17 g
• Proteose peptone - 3 g
• Lactose - 10 g
• Bile salts - 1.5 g
• Sodium chloride - 5 g
• Neutral red - 0.03 g
• Agar - 13.5 g
• Water - add to make 1 litre; adjust pH to 7.1 +/- 0.2

There are many variations of MacConkey agar depending on the need. If the spreading or swarming of Proteus species is required, sodium chloride is omitted. Crystal violet at a concentration of 0.0001% (0.001 g per litre) is included when we need to check if Gram-positive bacteria are inhibited.

 History

The medium was developed by Alfred Theodore MacConkey while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Uses

Acting as a visual pH indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).
This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.

 Lac+

By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies. The bile salts precipitate in the immediate neighborhood of the colony, causing the medium surrounding the colony to become hazy.

Lac-

Non-Lactose fermenting bacteria such as Salmonella, Proteus specis, Pseudomonas aeruginosa and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies formed in the plate. But they can also look golden to brown with dark centers. They are circular colonies and arranged randomly.

 Slow

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter

 Variant

A variant, Sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enteropathogenic E. coli serotypes such as E. coli O157:H7, by the presence of white circular colonies that are non-sorbitol fermenting.

Endo agar

 
Endo agar (also called Endo's medium) is a microbiological growth medium with a faint pink colour. Originally developed for the isolation of Salmonella typhi, it is now used mostly as a coliform medium. Most gram-negative organisms grow well in this medium, while growth of gram-positive organisms is inhibited. Coliform organisms ferment the lactose in this medium, producing a red colour (i.e. Eschericha coli), whereas non-lactose-fermenting organisms produce clear, colourless colonies, i.e. Salmonella sp..

 Typical composition

Endo agar typically contains :

• 1.0 % peptone
• 0.25 % dipotassium hydrogen phosphate (K₂HPO₄)
• 1.0 % lactose
• 0.33 % anhydrous sodium sulfite (Na₂SO₃)
• 0.03 % fuchsine
• 1.25 % agar

Blood agar types

 

• Blood agar plate (BAP)

Contains mammalian blood (usually sheep or horse), typically at a concentration of 5–10%. BAP are enriched, differential media used to isolate fastidious organisms and detect hemolytic activity. β-hemolytic activity will show lysis and complete digestion of red blood cell contents surrounding colony. Examples include Streptococcus haemolyticus. α-hemolysis will only partially lyse(the cells are either lysed or not- it is the digestion that may be incomplete) the hemoglobin and will appear green. An example of this would be Streptococcus viridans. γ-hemolysis (or non-hemolytic) is the term referring to a lack of hemolytic activity.

Contains meat extract, tryptone, sodium chloride, and agar.

Mannitol salt agar

An MSA plate with Micrococcus sp. (1), Staphylococcus epidermis (2) and S. aureus (3) colonies.

Mannitol salt agar or MSA is a commonly used growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time.  It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria. It is also a differential medium, containing mannitol and the indicator phenol red. Coagulase-positive Staphylococci produce yellow colonies with yellow zones, whereas coagulase-negative Staphylococci produce small pink or red colonies with no color change to the medium. If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow.  It is used for the selective isolation of presumptive pathogen (pp) Staphylococci.