Influence of oxidation state on iron binding by Bacillus licheniformis capsule

We examined ferric (Fe3+) and ferrous (Fe2+) iron binding by the anionic gamma-glutamyl capsule polymer of Bacillus licheniformis ATCC 9945. The addition of FeCl3 to B. licheniformis capsule under aerobic conditions resulted in flocculation due to the capsule-induced formation of amorphous, rust-colored ferrihydrite. Significant binding of iron, which could be attributed to binding by both the anionic capsule and the ferrihydrite precipitate, occurred. In contrast, the addition of FeCl2 to B. licheniformis capsule under anaerobic conditions resulted in significantly less iron being bound and no color change or flocculation occurring. Capsule-bound ferric iron could be partially released upon addition of several reducing agents. From these observations, it can be concluded that the oxidation state of iron significantly influences its tendency to be bound by anionic bacterial polymers such as capsules.