Oxygen Dependency of Neutrophilic Fe(II) Oxidation by Leptothrix Differs from Abiotic Reaction

Neutrophilic Fe(II) oxidizing microorganisms are found in many natural environments. It has been hypothesized that, at low oxygen concentrations, microbial iron oxidation is favored over abiotic oxidation. Here, we compare the kinetics of abiotic Fe(II) oxidation to oxidation in the presence of the bacterium Leptothrix cholodnii Appels isolated from a wetland sediment. Rates of Fe(II) oxidation were determined in batch experiments at 20°C, pH 7 and oxygen concentrations between 3 and 120 μmol/l. The reaction progress in experiments with and without cells exhibited two distinct phases. During the initial phase, the oxygen dependency of microbial Fe(II) oxidation followed a Michaelis-Menten rate expression (K_M = 24.5 ± 10 μmol O₂/l, v_max = 1.8 ± 0.2 μmol Fe(II)/(l min) for 10⁸ cells/ml). In contrast, abiotic rates increased linearly with increasing oxygen concentrations. At similar oxygen concentrations, initial Fe(II) oxidation rates were faster in the experiments with bacteria. During the second phase, the accumulated iron oxides catalyzed further oxidative iron precipitation in both abiotic and microbial reaction systems. That is, abiotic oxidation also dominated the reaction progress in the presence of bacteria. In fact, in some experiments with bacteria, iron oxidation during the second phase proceeded slower than in the absence of bacteria, possibly due to an inhibitory effect of extracellular polymeric substances on the growth of Fe(III) oxides. Thus, our results suggest that the competitive advantage of microbial iron oxidation in low oxygen environments may be limited by the autocatalytic nature of abiotic Fe(III) oxide precipitation, unless the accumulation of Fe(III) oxides is prevented, for example, through a close coupling of Fe(II) oxidation and Fe(III) reduction.     

Enhancement of granulocyte luminescence by murine macrophage secretory products: Suggestive evidence for the release of a new activator

The incubation of macropages (MΦ) in the presence of lipopolysaccharides (LPS) usually results in the release of a variety of immunoregulatory cytokines such as interleukins (IL), tumour necrosis factor (TNF) and colony stimulating factors (CSF). We recently observed that conditioned media (CM) from LPS-treated murine MΦ lines probably contain another protein endowed with granulocyte stimulatory activity. This cytokine, which has an apparent MW of about 55 kDa enhances the PMA-induced luminescence of granulocytes and also stimulates their degranulation as measured by lactoferrin release. In contrast to IL₁ and IL₆ this factor is destroyed by brief treatment at pH 2, but is stable for 60 minutes at 65°C. Unlike CSF, its activity is unchanged by reducing agents such as beta-mercaptoethanol. Furthermore, pretreatment of the MΦ with dexamethasone, in order to reduce the release of IL₁ and TNF, hardly reduces the effect on granulocyte activation. Finally, treatment with a neutralizing polyclonal anti-murine TNF antiserum only partly abolishes its activity. These results show that, in addition to the already well-described cytokines, LPS-treated murine MΦ lines most probably secrete another granulocyte activator.