sometimes put in their own category. These include Serratia and Citrobacter. Mucoid colonies Some organisms, especially Klebsiella and Enterobacter, produce mucoid colonies which appear very moist and sticky and slimy This phenomenon happens because the organism is producing a capsule, which is predominantly made from the lactose sugar in the agar. Variant A variant, sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enterohemorrhagic E. coli serotype E. coli O157:H7, by the presence of colorless circular colonies that are non-sorbitol fermenting. See also R2a agar MRS agar (culture medium designed to grow Gram-positive bacteria and differentiate them for lactose fermentation). References Biochemistry detection reactions Microbiological media Cell culture media