Rappaport and Henig described a medium for isolation of enteropathogenic E coli serotype. Sorbitol MacConkey agar is also a type of medium based on formula Rappaport and Henig. It is selective and differential media. This media is for detection enterohaemorrhagic Escherichia coli serotype O 157: H7.
E. coli serotype O 157: H7 is causative agent haemorrhagic colitis and diarrhoea in infants. E. coli serotype O 157: H7 produces a type of toxin. This toxin is Shiga like Toxin (SLT). Shiga like toxin also known as Verocytotoxin. Shiga like toxin causes diarrhoea, severe enterohaemorrhagic Colitis and Haemolytic Uraemia Syndrome (HUS).
Generally, the standard MacConkey agar contains lactose. Most of the E. Coli ferment and appear pink coloured colonies. Sorbitol is also a type o sugar. In sorbitol MacConkey agar, sorbitol is added in place of lactose. Most Escherichia Coli species ferment the sorbitol. Thus, sorbitol fermenter e coli species appear in pink colony. But E. coli serotype O 157: H7 doesn’t ferment the sorbitol and form colourless colonies. Since E. coli serotype O 157: H7 doesn’t use sorbitol, it grows using peptone in the medium only.
Composition of Sorbitol MacConkey Agar:
The main component which make MacConkey sorbitol agar (SMAC) as selective and differential are Bile salts, crystal violet, D-Sorbitol and Neutral Red.
Peptone and Proteose peptone present in the SMAC provide nutrients such as organic carbon nitrogen and vitamins.
D Sorbitol: D sorbitol is the fermentable sugar. Since enteropathogenic bacteria doesn’t ferment the sorbitol . They appear as colorless colonies.
Bile salts and crystal violet are added for preventing the growth of gram positive bacteria. Therefore Bile salts and crystal violet acts as the selective agent and inhibits the growth of gram negative bacteria.
Sodium chloride– The main role of sodium chloride is to prevent bacterial cell lysis. NaCl helps in maintaining osmotic environment .
Neutral Red: The pH indicator in SMAC agar is neutral red .
Agar– Agar acts as solidifying agent
For preparation of 1000 ml of sorbitol MacConkey Agar add amounts of following ingredients shown in table.
Principle of Sorbitol MacConkey Agar
MacConkey sorbitol Agar is a selective and differential medium. It is used for isolation and differentiation of enteropathogenic serotypes of E. Coli. The peptone and proteose peptone present in the medium are source of nitrogen, carbon, long chain amino acids, minerals , vitamins and trace elements for bacteria. Bile salt and crystal violet in the medium inhibit the growth of gram positive bacteria such as enterococci and staphylococci. It is selective media which selects gram negative bacteria. The sodium chloride present in the medium maintains osmotic balance and prevents cell lysis. Combination of sorbitol and the neutral red differentiates gram negative enteric bacteria. Most of the E. coli species ferments sorbitol therefore appear in pink color. Enterohaemorrhagic E. coli serotype O 157: H7 doesn’t ferment sorbitol, therefore grow and produces colorless colonies.