sorbitolMacConkey

Sorbitol MacConkey agar (SMAC) is a selective differential culture medium derived from MacConkey agar in which sorbitol is substituted for lactose as the fermentable carbohydrate. It is used primarily to isolate and differentiate enteric bacteria, especially Escherichia coli O157:H7, a major foodborne pathogen.

The medium contains bile salts and crystal violet to inhibit Gram-positive bacteria, a pH indicator such as neutral red, and sorbitol. Fermentation of sorbitol by a lactose-nonfermenting organism lowers pH, producing a color change (usually pink to red) in susceptible colonies; sorbitol-nonfermenting colonies remain colorless or pale. On SMAC, most nonpathogenic E. coli and many other Enterobacteriaceae ferment sorbitol and appear pink or red, whereas E. coli O157:H7 typically does not ferment sorbitol within 24 hours and forms colorless or pale colonies. Some sorbitol-fermenting non-O157 strains may appear pink, so identification requires confirmation.

Limitations include occasional false negatives or positives; some O157:H7 isolates may ferment sorbitol slowly, and a fraction of non-O157 pathogens may produce similar colonies. Therefore SMAC results are interpreted alongside other tests and confirmatory assays such as serotyping, Shiga toxin detection, PCR, or latex agglutination.

SMAC is widely used in food safety testing, clinical microbiology, and environmental monitoring as an initial screen for E. coli O157:H7, often as part of outbreak investigations.