Four mesophilic acidophilic bacteria isolated from the Caribbean island of Montserrat, have been studied to establish their taxonomic relationship to other metal-mobilizing bacteria and to analyze their potential role in mineral processing. Two of the isolates (coded S-10 and M- 12) have some physiological and morphological traits in common with Thiobacillus ferrooxidans (Gram-negative, iron-oxidizing mesophilic rods) but differend from T.ferrooxidans in displaying chemolitho-heterotrophic growth in ferrous iron/yeast extract medium, and greater sensitivities to some metals. Isolates RIV-14 and L-15 were, in contrast, Gram-positive, spore-forming rods that displayed considerable metabolic flexibility, and resembled moderately thermophilic Sulfobacillus spp. These isolates were also more sensitive to some cationic metals (e.g. copper) than T.ferrooxidans but less sensitive to molybdate than isolates S-10 and M-12. Bioreactor experiments with isolates RIV-14 and L-15 showed that they were capable of very rapid ferrous iron oxidation (equivalent culture doubling times as low as 2.5 h at optimum temperatures 33 deg C and 37 deg C respectively). These two Gram-positive isolates were also found to be more tolerant to extreme acidity than T.ferrooxidans and 'Leptospirillum ferrooxidans': in media set at pH 1.0 they displayed culture doubling times of 6.8 h (RIV-14) and 6.5 h(L-15). All of the Montserrat isolates were able to oxidize pyrite in pure culture.
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