Investigating the effect of acid stress on selected mesophilic micro-organisms implicated in bioleaching

During start-up of heap bioleaching, low grade ores are typically treated with acid for agglomeration and to combat the acid neutralising capacity of the gangue minerals. This may stress the bioleaching inocula, particularly upon inoculation during ore agglomeration. Acid addition for agglomeration varies across operations, ore types and their neutralising capacity, with limited information published on recommended concentrations. The initial pH in the agglomeration mix is typically below pH 1.0 and may be as low as pH 0.5. This paper investigates the effect of acid stress in terms of initial acid concentration and exposure duration in submerged culture on mesophilic bacteria typically implicated in mineral sulphide bioleaching and critical for heap colonisation at start-up. Following acid stress, cultures were returned to standard operating conditions in batch stirred slurry reactors and their performance assessed in terms of mineral leach rates, ferrous oxidation and the rate of microbial growth. Increasing acid stress resulted in an increase in the lag period before onset of microbial growth and iron oxidation. Following adaptation, typical growth and ferrous iron oxidation rates were observed under low stress conditions while reduction in the rate and extent of microbial growth and ferrous iron oxidation persisted at extreme conditions. A reduction in yield (microbial cells produced per kg iron oxidised) was observed with increased acid concentration over comparative times. Microbial speciation analysis indicated a substantial decrease in the diversity of surviving bacterial species

MRI and gravimetric studies of hydrology in drip irrigated heaps and its effect on the propagation of bioleaching microorganisms

Heap bioleaching performance is dependent on the contacting of the leach solution with the ore bed, hence on the system hydrodynamics. In this study two experimental setups were used to examine hydrodynamics associated with irrigation from a single drip emitter, one of the most common methods of heap irrigation. A specialist magnetic resonance imaging (MRI) method which is insensitive to the metal content of the ore was used to examine the liquid flow into an ore bed in the immediate vicinity of an irrigation point. The distribution of liquid in, microbial colonisation of and mineral recovery from a bioleach of a large scale 132 kg “ore slice” were subsequently monitored using sample ports positioned along the breadth and height of the reactor. In both systems the lateral movement of the liquid increased with bed depth, though preferential flow was evident. The majority of the liquid flow was in the region directly below the irrigation point and almost no liquid exchange occurred in the areas of lowest liquid content at the upper corners of the bed in which fluid exchange was driven by capillary action. The MRI studies revealed that the liquid distribution was unchanging following an initial settling of the ore bed and that, at steady state, the majority (~60%) of the liquid flowed directly into established large channels. The limited lateral movement of the liquid had a significant impact on the local leaching efficiencies and microbial colonisation of the ore with cell concentrations in the regions of lowest liquid content lying below the detection limit. Hence poor lateral liquid distribution with drip irrigation, and the associated impact on colonisation was identified as a significant disadvantage of this irrigation approach. Further, the need to optimise fluid exchange throughout the ore bed was identified as key for optimisation of leaching performance

The Helicobacter pylori Genome Project : insights into H. pylori population structure from analysis of a worldwide collection of complete genomes

Helicobacter pylori, a dominant member of the gastric microbiota, shares co-evolutionary history with humans. This has led to the development of genetically distinct H. pylori subpopulations associated with the geographic origin of the host and with differential gastric disease risk. Here, we provide insights into H. pylori population structure as a part of the Helicobacter pylori Genome Project (HpGP), a multi-disciplinary initiative aimed at elucidating H. pylori pathogenesis and identifying new therapeutic targets. We collected 1011 well-characterized clinical strains from 50 countries and generated high-quality genome sequences. We analysed core genome diversity and population structure of the HpGP dataset and 255 worldwide reference genomes to outline the ancestral contribution to Eurasian, African, and American populations. We found evidence of substantial contribution of population hpNorthAsia and subpopulation hspUral in Northern European H. pylori. The genomes of H. pylori isolated from northern and southern Indigenous Americans differed in that bacteria isolated in northern Indigenous communities were more similar to North Asian H. pylori while the southern had higher relatedness to hpEastAsia. Notably, we also found a highly clonal yet geographically dispersed North American subpopulation, which is negative for the cag pathogenicity island, and present in 7% of sequenced US genomes. We expect the HpGP dataset and the corresponding strains to become a major asset for H. pylori genomics

Investigating the effect of acid stress on selected mesophilic bioleaching microorganisms

Thesis submitted in accomplishment of the requirements of the Magister Technologiae in Engineering degree, Department of Chemical Engineering, Cape Peninsula University of Technology 2015Heap bioleaching is a microbially-assisted hydrometallurgical metal extraction process in which metals are solubilised from low grade ore by ferric iron and acid leach agents. Particularly for low grade ores, heap bioleaching provides several advantages over conventional technologies, it is simpler and safer to operate with low capital and operation cost and acceptable recoveries. Key challenges with heap bioleaching systems include the need to minimise leach durations and optimise both rate and extent of metal recovery. There is limited understanding of the sub-processes involved in microbially assisted bioheap leaching (nutrient transport, microorganisms attachment to mineral, effects of metal concentration on microorganisms viability, …). Mineral ore agglomeration is a pre-treatment process typically carried out in the setup of the bioheap operation to dissolve metal oxide residue, neutralise acid consuming gangue, prepare agglomerates of the mixed particle size fraction to optimise heap permeability, prepare the ore surface for microbial attachment and optimise moisture content and mineral exposure to leaching reactions. Most agglomeration processes are carried out with an acidic solution. This may create an acid stress condition for the bioleaching microorganisms when inoculated into the operation. This is particularly relevant when the inoculum is introduced during the agglomeration process and may remain under the highly acidic conditions for a protracted time. However, quantitative data on the recommended acid concentration during agglomeration processes is very limited and is dependent on the ore treated. Similarly little is reported on the response of the acidophilic bioleaching microorganisms to acid stress. This research project contributes to the bioleaching knowledge base by providing an understanding of the effect that acid stress has on the mesophilic species typically implicated in mineral sulphide bioleaching as a function of acid concentration (and resulting acidity) and duration of exposure. The study addresses the following specific key factors:  The effect of acid stress due to acid concentration and exposure time on performance on mesophile microorganisms in terms of the microbial and ferrous iron oxidation.  The interaction of acidity and exposure time with respect to microbial stress on the mesophilic bioleaching system performance.  The nature of the stress response observed i.e. only the lag period or also the rate of ferrous iron and sulphur oxidation on the initiation of the leaching process.  The observed effects on microbial activity mediated through the number of active cells or through the activity of these cells. Quick fit stirred tank reactors (STR) containing 3% pyrite concentrate and 1 litre Norris media (Norris, 1983), aerated with 2 L.min-1 compressed air and stirred at 550 rpm were inoculated with a mixed mesophilic culture mainly Acidithiobacillus ferrooxidans, Acidiplasma cupricumulans, Ferroplasma acidiphilum, and predominantly Leptospirillum ferriphilum following its pre-stress at 0.34M, 0.51M and 0.68M acid (H2SO4), whilst operating as a batch system. A Control, inoculated with an un-stressed culture, was run concurrently. The cultures were subjected to these acid stresses for a period of one hour, three hours and 24 hour and assessed for microbial growth and activity, leaching performance and microbial speciation. Findings showed an increasing period necessary for microbial recovery with increased acid stress and increased time exposure. A similar leaching performance to the Control were recorded soon after the cultures recovered from the stress when the acid concentrations used were low, but the highest acid concentration (0.68M) combined with the longer exposure time (24 h) compromised the overall leaching performance and the required time of recovery was extended to as high as 200 h. Equally the microbial growth rates were similar to that of the Control culture following the recovery period. The yield in terms of microbial cells produced per kg iron oxidised decreased with increased acid stress but not necessarily with increased exposure time. The extent of iron solubilisation, at the time the Control achieved its highest solubilisation, decreased with both increases in acid stress concentrations and in exposure time. Microbial speciation indicated that four of the initial six species in the mixed culture were sensitive to acid stress. Only three species survived the stress in the early stages of the experiment and one specie disappeared during the course of the leaching experiment leaving just two species surviving. Of the two surviving species, Fe. acidiphilum and L. ferriphilum, the latter dominated to a final ratio of 99% to 1%. Some recommendations have been made for future studies, namely:  Acid stress effects should be tested on simulated heap leaching experiments using agglomerated ore.  Acid concentration and exposure time should be increased to assess the extent of microbial recovery and acid tolerance levels.  Similar experiment should be conducted using moderate thermophile and thermophile cultures.  A conglomerate of a more defined mixed culture should be used to assess the acid resistant species.  Physico-chemical conditions resulting from the acid agglomeration, such as shear stress, increase temperature, radiation should be considered to be assessed further