| Summary: | The aims of this work were to study the gold leaching by the isolated bacteria from silicate ore. Three strains were isolated and identified as <i>Macrococcus caseolyticus</i>, <i>Acinetobacter calcoaceticus,</i> and <i>Bacillus sp</i>. MBEA40. However, only <i>M. caseolyticus</i> and <i>A. calcoaceticus</i> were capable of gold bioleaching. In order to examine only the effect of microorganisms involved in the gold bioleaching process, minimal medium and ethanol mineral salt medium without amino acids were used for culturing <i>M. caseolyticus</i> and <i>A. calcoaceticus</i><i>,</i> respectively. The result showed that the growth supernatant (in the absence of microorganisms) of both strains might be more suitable to leaching gold from ore than leaching by microorganisms (in the presence of microorganisms) directly. This might be due to the fact that there is no interference of gold absorption and metal toxicity in microorganisms in the long-term operation. The result also confirmed that amino acids/peptides/proteins produced by microorganisms might be involved in gold bioleaching, as shown in the high-performance liquid chromatography (HPLC) results. The Fourier transform infrared spectroscopy (FTIR) study also found that amine groups and carboxylic groups played important roles in gold bioleaching by <i>M. caseolyticus</i> and <i>A. calcoaceticus</i>. In addition, the bioleaching process had significantly higher gold leaching than mixed pure amino acids due to the growth supernatant containing mixed amino acids/peptides/proteins and other compounds. Therefore, the growth supernatant of <i>M. caseolyticus</i> and <i>A. calcoaceticus</i> can be applied in gold bioleaching under neutral pH conditions, which is considered to be a safe, not corrosive, and environmentally friendly leaching process. This study is also needed further study in order to increase the percentage of gold bioleaching and decrease times.
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