Despite being a gram positive bacteria, Clostridium tetani can produce a greenish fluorescence color on MacConkey medium containing neutral red indicator, why is that?
It's selective for any microbe that grows in presence of bile salts, NaCl and neutral red. That it is typically used for Gram negative enterics does not exclude the rest of the microbes in the world.
Enterococci also grow on MacConkey and some Mycobacteria can grow on EMB agar.
I guess I am curious as to why or how you found this. Did you purposely put C. tetani on the MacConkey agar to see if it would grow or did you identify the colonies after growth? I assume you grew it anaerobically if it is C. tetani. I have not heard of the green sheen for any organism other than E. coli but that could easily be my ignorance as I have not worked with anaerobes. If you grew the culture in an aerobic environment, the colonies are probably E. coli.
MacConkey agar is both selective and differential medium. Bacteria (either Gram-positive or Gram-negative) that resist bile salts can grow in this medium. Eg. Streptococcus B and D, Staphylococcus spp. and coliforms and other non-fastidious Gram-negatives
But, if MacConkey agar contains crystal violet as an ingredient, it may suppress the growth of Gram-positives, eg. Staphylococcus spp
Also, MacConkey agar is used to differentiate Gram-negatives according to their ability to ferment lactose as this medium contains an indicator(neutral red) and lactose. Bacterial colonies of lactose fermenting organisms give rise to pink colored colonies (E.coli, Klebsiella spp.) whereas non-lactose fermenters appear as colourless/pale colonies (eg. Salmonella, Shigella).
Crystal violet dye and bile salts halt the growth of gram-positive bacteria. but in some cases some gram positive bacteria that have resistance to bile salts can also grow on the medium. And there might be a chance that your culture was contaminated during your work.
have you done any tests to confirm it is a gram positive bacteria.
best regards.
- Badges
- Science method