Gram stain test



A gram stain is performed to help identify whether unknown bacterial contaminants are gram-positive or negative to help narrow down the number of possible beer spoilers.  The gram stain method uses crystal violet, iodine, 95% ethyl alcohol, and safranin to stain the bacterial cell wall.  Gram-positive cells retain the crystal violet stain and turn violet, whereas the gram-negative cells retain the safranin stain and turn pink.  Often used in conjunction with a catalase and oxidase test (see below).


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