Posted by Caitlin on 12/12/08

Hello,

I'm developing an informal Gram staining demonstration for
use with small groups of museum visitors. It's going quite
well, but I'm struggling to find a metaphor that will
explain to younger kids why Gram positive bacteria retain
the crystal violet stain and why the Gram negative bacteria
do not, and doesn't contradict itself when you tell them
that Gram negative bacteria are often more harmful.

In case it's been a while since you did a Gram stain (as it
was for me), it is a technique for distinguishing between
types of bacteria. Gram positive bacteria have a thick cell
wall rich in peptidoglycan that retains the violet stain
even when washed with ethanol. In Gram negative bacteria,
this dye washes out because their peptidoglycan is small
and coated in an outer membrane. This outer membrane also
helps to hide the bacteria from the immune system and can
have toxins embedded in it.

OK, hope that's clear. Thanks for any help! -Caitlin

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