Activated charcoal has been previously shown to induce in vitro expression of virulence factors by Listeria monocytogenes. In trying to elucidate the nature of the charcoal action, we found that the treatment of brain heart infusion medium with activated charcoal followed by charcoal removal does not result in an increase of virulence factor expression. At the same time, the addition of fresh charcoal to the charcoal-treated medium induces expression, suggesting that the effect of activated charcoal cannot be explained only by changes in medium composition. In addition, we observed that activated charcoal induced expression of virulence factors even when L. monocytogenes was physically separated from charcoal particles by either a nitrocellulose membrane or a thin layer of agar. We propose that the interaction of charcoal with some listerial product(s) might be responsible for the effect observed.
A mutant strain characterized by hyperproduction of a number of cell wall and supernatant proteins was isolated after N'methyl-N'-nitro-N-nitrosoguanidine treatment of Listeria monocytogenes strain NCTC10527. Several of these proteins were identified as virulence factors. When a wild-type strain was grown in the presence of activated charcoal it was shown to exhibit the same protein pattern as the isolated mutant.
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