MacConkey Sorbitol Agar Plate

Principle And Interpretation

MacConkey Sorbitol Agar is recommended by ISO Committee (1) with a slight modification of MacConkey Sorbitol Agar formulated by Rappaport and Henigh (2). This medium is recommended for isolation of enteropathogenic Escherichia coli O157: H7, which ferments lactose but does not ferment sorbitol, hence produces colourless colonies. This organism has been recognized as a cause of hemorrhagic colitis (3). E.coli O157: H7 is a human pathogen associated with hemorrhagic colitis that results from the action of a shiga-like toxin (SLT) (4,5). MacConkey Sorbitol Agar however should not be solely used to detect pathogenic E.coli O157: H7 strains as some non-toxic strains will also not ferment sorbitol (6).On standard MacConkey Agar containing lactose, this strain is indistinguishable from other lactose-fermenting E.coli. In MacConkey Sorbitol Agar Base, lactose is replaced by sorbitol. Unlike most E.coli strains, E.coli O157:H7 ferments sorbitol slowly or not at all (7,8). The growth of E.coli O157:H7 on MacConkey Agar with Sorbitol shows colourless colonies and most of the fecal flora ferment sorbitol and appear pink. MacConkey Agar with Sorbitol therefore permits ready recognition of E.coli O157:H7 (4,5,9). Tryptone and HM peptone supply necessary nutrients like nitrogenous and carbonaceous compounds, minerals, vitamins and trace ingredients for the growth of organisms. Crystal violet and bile salt mixture present in the medium inhibit growth of gram-positive bacteria. The addition of cefixime and tellurite, as FD147 significantly reduces the number of sorbitol non-fermenters that are to be screened during the attempted isolation of E.coli O157:H7. Sodium chloride maintains osmotic equilibrium. Neutral red is an indicator. D-Sorbitol is the fermentable carbohydrate.