Acidiphilium organovorum sp. nov., an Acidophilic Heterotroph Isolated from a Thiobacillus ferrooxidans Culture

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Abstract
Acidiphilium organovorum sp. nov. is a gram-negative, heterotrophic bacterium that was isolated from a culture of Thiobacillus ferrooxidans which had been grown autotrophically on FeSO4-basal salts medium for several years. Purification of A. organovorum was carried out on a 1.0% glucose-basal salts medium (pH 3.0) solidified with agarose. Growth was enhanced by adding high concentrations of glucose (0.5 to 2.0%) and by supplementing the medium with yeast extract and trace amounts of FeSO4. However, these supplements were not necessary for growth. A wide variety of organic compounds were suitable substrates for growth, but inorganic forms of reduced sulfur or ferrous iron were not. Doubling times of 2.5 h and cell densities of >2 × 1010cells per ml were obtained at the optimal temperature of 37°C and pH 3.0. The guanine-plus-cytosine content of the deoxyribonucleic acid was 64 mol%. A. organovorum contains at least three distinct plasmids; one of these plasmids is larger than 30 kilobase pairs, and two are smaller than 4.0 kilobase pairs. Homology studies in which we compared the total deoxyribonucleic acid of A. organovorum with the total deoxyribonucleic acids of Acidiphilium cryptum and several Thiobacillus species indicated that A. organovorum is most closely related to A. cryptum. A. organovorum can be differentiated from Thiobacillus acidophilus by its higher temperature optimum, its faster growth rate, and its inability to utilize reduced forms of sulfur or iron as energy sources. The abundant cell growth that occurs in a medium which either is rich in organic compounds or completely lacks nutritional supplements distinguishes A. organovorum from A. cryptum. The other physiological and genetic characteristics which we examined are in close agreement with the characteristics of members of the genus Acidiphilium. The type strain of A. organovorum is strain ATCC 43141.