In the zone inhibition antibacterial test, chitosan kills the bacteria by direct contact with them. But how does chitosan kill the surrounding bacteria while not having direct contact with them?
(Assuming chitosan film at a neutral pH in culture medium)
Assuming that this really happens, I would speculate that the effect might be due to chelation of anions that the bacteria need to grow from the medium by the polycationic chitosan.
Another possibility is that impurities in the chitosan diffuse into the medium and kill the bacteria.
By aggregation and immobilization of nutrient elements and their ions resulting in cellular starvation, inactivation and death.
You might consider running this experiment across a pH range and running it in both rich and minimal media. I agree with the answers above, it is less likely a direct effect of chitosan if it is real.
