Hydrogen Peroxide Production and Oxidation of Ferrous Iron by Lactobacillus delbrueckii ssp. bulgaricus

Hydrogen peroxide was produced by Lactobacillus delbrueckii ssp. bulgaricus ATCC 11842 only in the presence of glucose at pH 6.5 and 5.0 in the presence of air. Production of H₂O₂ was minimal following the heating of cells, in the presence of catalase, and at low partial pressure of O₂. Production of H₂O₂ was lower at pH 5.0 than at pH 6.5; H₂O₂ was apparently produced by a cytosolic NADH oxidase. Lactobacillus delbrueckii ssp. bulgaricus was able to bind nanomolar quantities of iron when incubated in the presence of micromolar amounts of Fe²⁺. This binding was the result of oxidation of the Fe²⁺ by H₂O₂ produced and the subsequent surface binding of the resulting Fe(III), most likely in the form of Fe(OH)₃. Ferric iron that was complexed with citrate or lactate was not bound. Little, if any, Fe²⁺ was internalized by the cells, and the particulate fraction of the microorganism lacked ferroxidase activity. The Fe(OH)₃ associated with the cells was bound to specific binding regions of the cell surface that were sensitive to trypsin digestion and that could be protected against such digestion by binding endogenously or exogenously produced Fe(OH)₃. The particulate fraction of L. delbrueckii ssp. bulgaricus was able to bind more Fe(OH)₃ than comparable quantities of intact cells, the additional binding sites apparently originating from the cytosol side of the membranes. We concluded that L. delbrueckii ssp. bulgaricus was able to remove Fe²⁺ from its environment in the presence of air but not under anaerobic conditions. In this respect, this organism is less effective than bifidobacteria as a host defense factor in vivo.