Mannitol  eggyolk polymyxin agar Blood agar Immunological detection of Bacillus cereus toxin
A.  A soil suspension (10g/100ml) was pasteurized, diluted (1:100) and a 0.1 ml sample was spread on MEP (mannitol; egg yolk; polymyxin) agar. MEP agar selects for the growth of polymyxin resistant organisms; Bacillus cereus is polymyxin resistant. Bacillus cereus colonies can be differentiated from colonies of the other polymyxin resistant organims growing on the plate by their inability to ferment mannitol and the presence of lecithinase. B.cereus colonies appear pink (cannot ferment mannitol, pH increases - phenol red turns reddish) and have a halo (insoluable lipids are released by the action of lecithinase, an enzyme found in B. cereus). The colony labeled "1" is a good example of how we would expect Bacillus cereus to appear on MEP agar. B. The colonies labelled 1-9 were inoculated onto blood agar medium through a nitrocellulose membrane. The clearing zones visible are due to Beta-hemolysis of the red blood cells in the agar. The ability to lyse red blood cells closely parallels toxin production in B. cereus strains. The nitrocellulose membrane has been removed from this blood agar plate for subsequent immunological detection of Bacillus cereus toxin. C. The nitrocellulose membrane is shown after the immunodetection procedure for Bacillus cerus toxin. The negative controls, colonies inconsistent with the appearance of Bacillus cereus on MEP were inoculated at numbers 7 & 9 and correlate with the absence of color development on the membrane. Although there was hemolysis at #6, there is little if any toxin.