Manganese and Defenses against Oxygen Toxicity in Lactobacillus plantarum

Lactobacillus plantarum is aerotolerant during log-phase growth on glucose, but is an obligate aerobe on polyols. Respiration was cyanide resistant and under certain conditions was associated with the accumulation of millimolar concentrations of H₂O₂. On glucose, optimal growth was observed in the absence of O₂. Extracts of L. plantarum did not catalyze the reduction of paraquat by reduced nicotinamide adenine dinucleotide, but plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) was readily reduced. Such extracts produced O₂⁻ in the presence of NADH plus plumbagin. Plumbagin caused a 10-fold increase in the rate of respiration of intact cells in the presence of glucose and also imposed a loss of viability which was dependent upon both glucose and O₂. Although extracts of L. plantarum were devoid of true superoxide dismutase activity, this organism was comparable to superoxide dismutase-containing species in its resistance toward hyperbaric O₂ and toward the oxygen-dependent lethality of plumbagin. L. plantarum required Mn-rich media and actively accumulated Mn(II). Soluble extracts were found to contain approximately 9 μg of Mn per mg of protein and 75 to 90% of this Mn was dialyzable. Such extracts exhibited a dialyzable and ethylenediaminetetraacetic acid-inhibitable ability to scavenge O₂⁻. This O₂⁻-scavenging activity was due to the dialyzable Mn(II) present in these extracts and could be mimicked by MnCl₂. Cells grown in Mn-rich media were enriched in dialyzable Mn and were more resistant toward oxygen toxicity and toward the oxygen-dependent plumbagin toxicity than were cells grown in Mn-deficient media. L. plantarum exhibited no nutritional requirement for iron and little or no iron was present in these cells, even when they were grown in iron-rich media. L. plantarum thus appears to use millimolar levels of Mn(II) to scavenge O₂⁻, much as most other organisms use micromolar levels of superoxide dismutases.