What type of agar is most suitable for isolating enteric gram-negative bacteria?

MacConkey agar is specifically designed to isolate enteric gram-negative bacteria. This medium is both selective and differential. The selective aspect comes from the incorporation of bile salts and crystal violet, which inhibit the growth of gram-positive bacteria, allowing for the isolation of gram-negative organisms typically found in the intestinal tract.

The differential feature of MacConkey agar is its use of lactose and a pH indicator, which helps distinguish between lactose fermenters and non-fermenters. When lactose is fermented by gram-negative bacteria, it produces acid, leading to a color change in the agar; colonies of lactose-fermenting bacteria will appear pink, whereas non-fermenters will retain a pale or colorless appearance. Thus, MacConkey agar not only selects for gram-negative organisms but also allows for the differentiation of species based on their ability to ferment lactose, which is a key trait often analyzed in enteric pathogens.

The other types of agar mentioned serve different purposes. Nutrient agar is a general-purpose medium that supports a wide range of bacteria but is not specific for enteric gram-negative bacteria. Blood agar can grow both gram-negative and gram-positive bacteria and is primarily used for cultivating fastidious organisms and assessing hemolytic activity rather than selectively isolating specific types