gram+ and gram-

Brian_D Lanoil lanoilb at
Wed Sep 24 17:33:08 EST 1997

>I have a sixth form student investigating the antiseptic/antibiotic
>properties of garlic extracts. There is evidence to suggest that the
>effect is different on gram + and gram- bacteria. Can someone tell me if
>there is a simple way to stain a mixed plate of bacteria differentially
>to show up one of the types. I know that we can utilise gram staining on
>smears but this is inappropriate for colonies on an agar plate.
>Thanks in anticipation
>Paul Cotton

Although I dont know about staining colonies on a plate, there are 
differential media which will allow only Gram + or only Gram - bacteria 
to grow.  Generally, the Gram + media contain high salts, which kills 
Gram - bacteria but not Gram + due to the difference in thickness of the 
cell walls.  Gram - media contain dyes, such as eosin blue, which are 
toxic to cells.  Because Gram - bacteria have the outer lipid envelope on 
the cell wall, the dyes cannot enter the cell and so they are able to 
grow, whereas Gram + die due to dye entering through pores in the cell 
wall.  Recipes for these media can be found in most media books.

I hope this helps.

Good luck

Brian D. Lanoil   Department of Microbiology
Oregon State University   Nash 222
Corvallis, OR 97331   (541)737-3334
lanoilb at
"Quippy quote here..."

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