Acidithiobacillus thiooxidans

Acidithiobacillus thiooxidans, formerly known as Thiobacillus thiooxidans until its reclassification into the newly designated genus Acidithiobacillus of the gamma subclass of Proteobacteria, is a Gram-negative, rod-shaped bacterium that uses sulfur as its primary energy source. It is mesophilic, with a temperature optimum of 28 °C. This bacterium is commonly found in soil, sewer pipes, and cave biofilms called snottites. A. thiooxidans is used in the mining technique known as bioleaching, where metals are extracted from their ores through the action of microbes. A. thiooxidans is a Gram-negative, rod-shaped bacterium with rounded ends that occurs in nature either as singlecells, as is the most common case, or sometimes in pairs, but rarely in triplets. Its motility is due to a polar flagellum. It is an obligate acidophile with an optimal pH less than 4.0, but it also qualifies as an obligate aerobe and chemolithotroph. Described as a colorless, sulfur-oxidizing bacterium, A. thiooxidans does not accumulate sulfur either within or outside of its very small cells, which have an average size around 0.5 µm in diameter and 1 µm or less in length. A. thiooxidans has so far not grown on agar or other solid media, instead it prefers liquid media with a strong, evenly dispersed clouding throughout, and it produces no sediment formation or surface growth. Although it does not grow on traditional organic media, it will not be harmed by a medium containing peptone or glucose. Media best suited for its growth are those that are inorganic and allow A. thiooxidans to use sulfur as a source of energy. The following characteristic reactions accompany the growth of A. thiooxidans in the presence of tricalcium phosphate: the layer on the surface of the medium formed by sulfur tends to drop to the bottom, tricalcium phosphate is dissolved by the product of sulfur oxidation, sulfuric acid, giving soluble phosphate and CaSO4 + 2 H2O, and radiating monoclinic crystals that hang from the sulfur particles floating on the medium surface or protruding upward from the bottom are formed by the precipitation of calcium sulfate. The medium becomes acidic with a pH around 2.8 and remains stationary until all the calcium phosphate has been dissolved. Anything with the tendency to change the medium to an alkaline state would be considered harmful to the uniform growth of A. thiooxidans, but if it is left unharmed by an excess of acid or alkali, numerous consecutive generations may be kept alive on the liquid media. A.s thiooxidans thrives at an optimum temperature of 28-30 °C. At lower temperatures (18 °C and below) and at 37 °C or higher, sulfur oxidation and growth are significantly slower, while temperatures between 55 and 60 °C are sufficient to kill the organism. A. thiobacillus, a strictly aerobic species, fixes CO2 from the atmosphere to meet its carbon requirements. In addition, other essential nutrients are required in varying amounts.A general lack of knowledge exists for acidophilic microorganisms in terms of the oxidation systems of reduced inorganic sulfur compounds (RISCs). Fazzini et al. (2013) presented the first experimentally validated stoichiometric model that was able to quantitatively assess the RISCs oxidation in A. thiooxidans (strain DSM 17318), the sulfur-oxidizing acidophilic chemolithotrophic archetype. By analyzing literature and by genomic analyses, a mix of formerly proposed models of RISCs oxidation were combined and evaluated experimentally, placing thiosulfate partial oxidation by the Sox system (SoxABXYZ), along with abiotic reactions, as the central steps of the sulfur oxidation model. This model, paired with a detailed stoichiometry of the production of biomass, provides accurate predictions of bacterial growth. This model, which has the potential to be used in biohydrometallurgical and environmental applications, constitutes an advanced instrument for optimizing the biomass production of A. thiooxidans. A. thiooxidans derives all of the energy needed to satisfy its carbon requirement from the fixation of CO2. An important distinction can be made between sulfur-oxidizing and nitrifying bacteria by their response to the introduction of carbon to the culture in the form of carbonates and bicarbonates. Carbonates keep the medium alkaline, thus preventing growth of A. thiooxidans which grows best under acidic conditions, while bicarbonates have been shown to allow a healthy growth if kept in small concentrations. Bicarbonate, however, is unnecessary because the CO2 from the atmosphere appears to be sufficient to support growth of A. thiooxidans, and would actually have an injurious effect in that it would tend to make the medium less acidic. A. thiooxidans requires only small amounts of nitrogen due to its small amount of growth, but the best sources are ammonimum salts of inorganic acids, especially sulfate, followed by the ammonium salts of organic acids, nitrates, asparagine, and amino acids. If no nitrogen source is introduced into the medium, some growth is observed, with A. thiooxidans deriving the necessary nitrogen from either traces of atmospheric ammonia, distilled water, or the contamination of other salts. A. thiooxidans is obligately aerobic because it uses atmospheric oxygen for the oxidation of sulfur to sulfuric acid.