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Mark Dopson - One of the best experts on this subject based on the ideXlab platform.
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metal resistance or tolerance Acidophiles confront high metal loads via both abiotic and biotic mechanisms
Frontiers in Microbiology, 2014Co-Authors: Mark Dopson, Francisco J Ossandon, Lars Lovgren, David S HolmesAbstract:All metals are toxic at high concentrations and consequently their intracellular concentrations must be regulated. Extremely acidophilic microorganisms have an optimum growth of pH <3 and proliferate in natural and anthropogenic low pH environments. Some Acidophiles are involved in the catalysis of sulfide mineral dissolution, resulting in high concentrations of metals in solution. Acidophiles are often described as highly metal resistant via mechanisms such as multiple and/or more efficient active resistance systems than are present in neutrophiles. However, this is not the case for all Acidophiles and we contend that their growth in high metal concentrations is partially due to an intrinsic tolerance as a consequence of the environment in which they live. In this perspective, we highlight metal tolerance via complexation of free metals by sulfate ions and passive tolerance to metal influx via an internal positive cytoplasmic transmembrane potential. These tolerance mechanisms have been largely ignored in past studies of acidophile growth in the presence of metals and should be taken into account.
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Gene Identification and Substrate Regulation Provide Insights into Sulfur Accumulation during Bioleaching with the Psychrotolerant Acidophile Acidithiobacillus ferrivorans
Applied and Environmental Microbiology, 2012Co-Authors: Maria Liljeqvist, Olena Rzhepishevska, Mark DopsonAbstract:The psychrotolerant acidophile Acidithiobacillus ferrivorans has been identified from cold environments and has been shown to use ferrous iron and inorganic sulfur compounds as its energy sources. A bioinformatic evaluation presented in this study suggested that Acidithiobacillus ferrivorans utilized a ferrous iron oxidation pathway similar to that of the related species Acidithiobacillus ferrooxidans. However, the inorganic sulfur oxidation pathway was less clear, since the Acidithiobacillus ferrivorans genome contained genes from both Acidithiobacillus ferrooxidans and Acidithiobacillus caldus encoding enzymes whose assigned functions are redundant. Transcriptional analysis revealed that the petA1 and petB1 genes (implicated in ferrous iron oxidation) were downregulated upon growth on the inorganic sulfur compound tetrathionate but were on average 10.5-fold upregulated in the presence of ferrous iron. In contrast, expression of cyoB1 (involved in inorganic sulfur compound oxidation) was decreased 6.6-fold upon growth on ferrous iron alone. Competition assays between ferrous iron and tetrathionate with Acidithiobacillus ferrivorans SS3 precultured on chalcopyrite mineral showed a preference for ferrous iron oxidation over tetrathionate oxidation. Also, pure and mixed cultures of psychrotolerant Acidophiles were utilized for the bioleaching of metal sulfide minerals in stirred tank reactors at 5 and 25°C in order to investigate the fate of ferrous iron and inorganic sulfur compounds. Solid sulfur accumulated in bioleaching cultures growing on a chalcopyrite concentrate. Sulfur accumulation halted mineral solubilization, but sulfur was oxidized after metal release had ceased. The data indicated that ferrous iron was preferentially oxidized during growth on chalcopyrite, a finding with important implications for biomining in cold environments.
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Biodiversity, metabolism and applications of acidophilic sulfur-metabolizing microorganisms.
Environmental Microbiology, 2012Co-Authors: Mark Dopson, D. Barrie JohnsonAbstract:Extremely acidic, sulfur-rich environments can be natural, such as solfatara fields in geothermal and volcanic areas, or anthropogenic, such as acid mine drainage waters. Many species of acidophilic bacteria and archaea are known to be involved in redox transformations of sulfur, using elemental sulfur and inorganic sulfur compounds as electron donors or acceptors in reactions involving between one and eight electrons. This minireview describes the nature and origins of acidic, sulfur-rich environments, the biodiversity of sulfur-metabolizing Acidophiles, and how sulfur is metabolized and assimilated by Acidophiles under aerobic and anaerobic conditions. Finally, existing and developing technologies that harness the abilities of sulfur-oxidizing and sulfate-reducing Acidophiles to extract and capture metals, and to remediate sulfur-polluted waste waters are outlined.
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Biodiversity, metabolism and applications of acidophilic sulfur-metabolizing microorganisms: Sulfur metabolizing Acidophiles
Environmental microbiology, 2012Co-Authors: Mark Dopson, D. Barrie JohnsonAbstract:Extremely acidic, sulfur-rich environments can be natural, such as solfatara fields in geothermal and volcanic areas, or anthropogenic, such as acid mine drainage waters. Many species of acidophilic bacteria and archaea are known to be involved in redox transformations of sulfur, using elemental sulfur and inorganic sulfur compounds as electron donors or acceptors in reactions involving between one and eight electrons. This minireview describes the nature and origins of acidic, sulfur-rich environments, the biodiversity of sulfur-metabolizing Acidophiles, and how sulfur is metabolized and assimilated by Acidophiles under aerobic and anaerobic conditions. Finally, existing and developing technologies that harness the abilities of sulfur-oxidizing and sulfate-reducing Acidophiles to extract and capture metals, and to remediate sulfur-polluted waste waters are outlined.
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Bioleaching in brackish waters--effect of chloride ions on the acidophile population and proteomes of model species.
Applied Microbiology and Biotechnology, 2011Co-Authors: Carla M. Zammit, Mark Dopson, Stefanie Mangold, Venkateswara Rao Jonna, Lesley Mutch, Helen R. Watling, Elizabeth L.j. WatkinAbstract:High concentrations of chloride ions inhibit the growth of acidophilic microorganisms used in biomining, a problem particularly relevant to Western Australian and Chilean biomining operations. Despite this, little is known about the mechanisms Acidophiles adopt in order to tolerate high chloride ion concentrations. This study aimed to investigate the impact of increasing concentrations of chloride ions on the population dynamics of a mixed culture during pyrite bioleaching and apply proteomics to elucidate how two species from this mixed culture alter their proteomes under chloride stress. A mixture consisting of well-known biomining microorganisms and an enrichment culture obtained from an acidic saline drain were tested for their ability to bioleach pyrite in the presence of 0, 3.5, 7, and 20 g L−1 NaCl. Microorganisms from the enrichment culture were found to out-compete the known biomining microorganisms, independent of the chloride ion concentration. The proteomes of the Gram-positive acidophile Acidimicrobium ferrooxidans and the Gram-negative acidophile Acidithiobacillus caldus grown in the presence or absence of chloride ions were investigated. Analysis of differential expression showed that acidophilic microorganisms adopted several changes in their proteomes in the presence of chloride ions, suggesting the following strategies to combat the NaCl stress: adaptation of the cell membrane, the accumulation of amino acids possibly as a form of osmoprotectant, and the expression of a YceI family protein involved in acid and osmotic-related stress.
D. Barrie Johnson - One of the best experts on this subject based on the ideXlab platform.
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Biodiversity, metabolism and applications of acidophilic sulfur-metabolizing microorganisms.
Environmental Microbiology, 2012Co-Authors: Mark Dopson, D. Barrie JohnsonAbstract:Extremely acidic, sulfur-rich environments can be natural, such as solfatara fields in geothermal and volcanic areas, or anthropogenic, such as acid mine drainage waters. Many species of acidophilic bacteria and archaea are known to be involved in redox transformations of sulfur, using elemental sulfur and inorganic sulfur compounds as electron donors or acceptors in reactions involving between one and eight electrons. This minireview describes the nature and origins of acidic, sulfur-rich environments, the biodiversity of sulfur-metabolizing Acidophiles, and how sulfur is metabolized and assimilated by Acidophiles under aerobic and anaerobic conditions. Finally, existing and developing technologies that harness the abilities of sulfur-oxidizing and sulfate-reducing Acidophiles to extract and capture metals, and to remediate sulfur-polluted waste waters are outlined.
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Biodiversity, metabolism and applications of acidophilic sulfur-metabolizing microorganisms: Sulfur metabolizing Acidophiles
Environmental microbiology, 2012Co-Authors: Mark Dopson, D. Barrie JohnsonAbstract:Extremely acidic, sulfur-rich environments can be natural, such as solfatara fields in geothermal and volcanic areas, or anthropogenic, such as acid mine drainage waters. Many species of acidophilic bacteria and archaea are known to be involved in redox transformations of sulfur, using elemental sulfur and inorganic sulfur compounds as electron donors or acceptors in reactions involving between one and eight electrons. This minireview describes the nature and origins of acidic, sulfur-rich environments, the biodiversity of sulfur-metabolizing Acidophiles, and how sulfur is metabolized and assimilated by Acidophiles under aerobic and anaerobic conditions. Finally, existing and developing technologies that harness the abilities of sulfur-oxidizing and sulfate-reducing Acidophiles to extract and capture metals, and to remediate sulfur-polluted waste waters are outlined.
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Attachment of acidophilic bacteria to solid surfaces: The significance of species and strain variations
Hydrometallurgy, 2006Co-Authors: M. Afzal Ghauri, Naoko Okibe, D. Barrie JohnsonAbstract:Abstract Sixteen strains of acidophilic bacteria were screened for their abilities to adhere to pyrite ore, glass beads and ferric hydroxysulfates. These were three culture collection and two isolated strains of the iron- and sulfur-oxidizer, Acidithiobacillus ferrooxidans, two each of the sulfur-oxidizer Acidithiobacillus thiooxidans and the iron-oxidizer Leptospirillum ferrooxidans (the type strain and a mine isolate in either case), five heterotrophic Acidophiles (four Acidiphilium and one Acidocella sp.) and two moderately thermophilic iron/sulfur-oxidizers (Sulfobacillus thermosulfidooxidans and Sulfobacillus acidophilus). Considerable variations were found between different species of Acidophiles, and also between different strains of the same species, in how they attached to the three solid materials tested. Attachment to the solid substrata generally increased with time (over 100 min) though > 99% of one At. ferrooxidans isolate (strain OP14) were attached to pyrite after just 10 min exposure. Most Acidophiles attached more readily to pyrite than to glass beads, and attachment to ferric hydroxysulfates was highly variable, though one At. ferrooxidans isolate (strain SJ2) and one heterotrophic acidophile (Acidocella sp. het-4) both attached strongly to ferric iron precipitates (jarosites and schwertmannite) that formed in cultures of At. ferrooxidans grown at pH > 2. The results of these experiments showed that even closely related strains of acidophilic bacteria can display very different propensities to attach to solid materials, an observation that may explain the somewhat disparate results reported on occasions by research groups that have examined single, or limited numbers of strains, of Acidophiles (mostly At. ferrooxidans). The significance of differential attachment of mineral-oxidizing and other Acidophiles to pyrite and other solids is discussed in the context of biohydrometallurgy.
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Microbiology of a wetland ecosystem constructed to remediate mine drainage from a heavy metal mine
Science of The Total Environment, 2004Co-Authors: Kevin B. Hallberg, D. Barrie JohnsonAbstract:A pilot passive treatment plant (PPTP) was constructed to evaluate the potential of a composite wetland system to remediate acidic, metal-rich water draining the former Wheal Jane tin, in Cornwall, England. The treatment plant consists of three separate and controllable composite systems, each of which comprises a series of aerobic wetlands for iron oxidation and precipitation, a compost bioreactor for removing chalcophilic metals and to generate alkalinity, and rock filter ponds for removing soluble manganese and organic carbon. To understand the roles of microorganisms in remediating acid mine drainage (AMD) in constructed wetland ecosystems, populations of different groups of cultivatable acidophilic microbes in the various components of the Wheal Jane PPTP were enumerated over a 30-month period. Initially, moderately acidophilic iron-oxidising bacteria (related to Halothiobacillus neapolitanus) were found to be the major cultivatable microorganisms present in the untreated AMD, though later heterotrophic Acidophiles emerged as the dominant group, on a numerical basis. Culturable microbes in the surface waters and sediments of the aerobic wetlands were similarly dominated by heterotrophic Acidophiles, though both moderately and extremely acidophilic iron-oxidising bacteria were also present in significant numbers. The dominant microbial isolate in waters draining the anaerobic compost bioreactors was an iron- and sulfur-oxidising moderate acidophile that was closely related to Thiomonas intermedia. The Acidophiles enumerated at the Wheal Jane PPTP accounted for 1% to 25% of the total microbial population. Phylogenetic analysis of 14 isolates from various components of the Wheal Jane PPTP showed that, whilst many of these bacteria were commonly encountered Acidophiles, some of these had not been previously encountered in AMD and AMD-impacted environments.
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Biooxidation of pyrite by defined mixed cultures of moderately thermophilic Acidophiles in pH-controlled bioreactors: significance of microbial interactions.
Biotechnology and Bioengineering, 2004Co-Authors: Naoko Okibe, D. Barrie JohnsonAbstract:The oxidative dissolution of pyrite (FeS2) by pure and mixed cultures of moderately thermophilic Acidophiles was studied in shake flask cultures and in pH-controlled bioreactors, incubated at 45°C. Various combinations of seven eubacteria (a Leptospirillum sp. (MT6), Acidimicrobium ferrooxidans, Acidithiobacillus caldus, an Alicyclobacillus sp. (Y004), and three Sulfobacillus spp.) and one archaeon (Ferroplasma sp. MT17) were examined. Pyrite dissolution was determined by measuring changes in soluble iron and generation of acidity, and microbial populations were monitored using a combined culture-dependent (plate counts) and culture-independent (fluorescent in situ hybridization) approach. In pure cultures, the most efficient pyrite-oxidizing acidophile was Leptospirillum MT6, which was unique among the prokaryotes used in being obligately autotrophic. Mixed cultures of Leptospirillum MT6 and the sulfur-oxidizer At. caldus generated more acidity than pure cultures of the iron-oxidizer, though this did not necessarily enhance pyrite dissolution. In contrast, a mixed culture of Leptospirillum MT6 and the obligate heterotroph Alicyclobacillus Y004 oxidized pyrite more rapidly and more completely than a pure culture of Leptospirillum MT6, in synchronized bioreactors. Although the autotroph, At. caldus, and the “heterotrophically inclined” iron-oxidizer, Am. ferrooxidans, were both ineffective at leaching pyrite in pure culture, a mixed culture of the two bacteria was able to accelerate dissolution of the mineral. Concentrations of dissolved organic carbon accumulated to >100 mg/L in some mixed cultures, and the most effective bioleaching systems were found to be consortia containing both autotrophic and heterotrophic moderate thermophiles. A mixed culture comprising the autotrophs Leptospirillum MT6 and At. caldus, and the heterotroph Ferroplasma MT17, was the most efficient of all of those examined. Mutualistic interactions between physiologically distinct moderately thermophilic Acidophiles, involving transfer of organic and inorganic carbon and transformations of iron and sulfur, were considered to have critical roles in optimizing pyrite dissolution. © 2004 Wiley Periodicals, Inc.
Elizabeth L.j. Watkin - One of the best experts on this subject based on the ideXlab platform.
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Draft Genome Sequence of Acidihalobacter ferrooxidans DSM 14175 (Strain V8), a New Iron- and Sulfur-Oxidizing, Halotolerant, Acidophilic Species.
Genome Announcements, 2017Co-Authors: Himel N. Khaleque, Joshua P. Ramsay, Riley J. T. Murphy, Anna H. Kaksonen, Naomi J. Boxall, Elizabeth L.j. WatkinAbstract:ABSTRACT The use of halotolerant Acidophiles for bioleaching provides a biotechnical approach for the extraction of metals from regions where high salinity exists in the ores and source water. Here, we describe the first draft genome of a new species of a halotolerant and iron- and sulfur-oxidizing acidophile, Acidihalobacter ferrooxidans DSM-14175 (strain V8).
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Bioleaching in brackish waters--effect of chloride ions on the acidophile population and proteomes of model species.
Applied Microbiology and Biotechnology, 2011Co-Authors: Carla M. Zammit, Mark Dopson, Stefanie Mangold, Venkateswara Rao Jonna, Lesley Mutch, Helen R. Watling, Elizabeth L.j. WatkinAbstract:High concentrations of chloride ions inhibit the growth of acidophilic microorganisms used in biomining, a problem particularly relevant to Western Australian and Chilean biomining operations. Despite this, little is known about the mechanisms Acidophiles adopt in order to tolerate high chloride ion concentrations. This study aimed to investigate the impact of increasing concentrations of chloride ions on the population dynamics of a mixed culture during pyrite bioleaching and apply proteomics to elucidate how two species from this mixed culture alter their proteomes under chloride stress. A mixture consisting of well-known biomining microorganisms and an enrichment culture obtained from an acidic saline drain were tested for their ability to bioleach pyrite in the presence of 0, 3.5, 7, and 20 g L−1 NaCl. Microorganisms from the enrichment culture were found to out-compete the known biomining microorganisms, independent of the chloride ion concentration. The proteomes of the Gram-positive acidophile Acidimicrobium ferrooxidans and the Gram-negative acidophile Acidithiobacillus caldus grown in the presence or absence of chloride ions were investigated. Analysis of differential expression showed that acidophilic microorganisms adopted several changes in their proteomes in the presence of chloride ions, suggesting the following strategies to combat the NaCl stress: adaptation of the cell membrane, the accumulation of amino acids possibly as a form of osmoprotectant, and the expression of a YceI family protein involved in acid and osmotic-related stress.
David S Holmes - One of the best experts on this subject based on the ideXlab platform.
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metal resistance or tolerance Acidophiles confront high metal loads via both abiotic and biotic mechanisms
Frontiers in Microbiology, 2014Co-Authors: Mark Dopson, Francisco J Ossandon, Lars Lovgren, David S HolmesAbstract:All metals are toxic at high concentrations and consequently their intracellular concentrations must be regulated. Extremely acidophilic microorganisms have an optimum growth of pH <3 and proliferate in natural and anthropogenic low pH environments. Some Acidophiles are involved in the catalysis of sulfide mineral dissolution, resulting in high concentrations of metals in solution. Acidophiles are often described as highly metal resistant via mechanisms such as multiple and/or more efficient active resistance systems than are present in neutrophiles. However, this is not the case for all Acidophiles and we contend that their growth in high metal concentrations is partially due to an intrinsic tolerance as a consequence of the environment in which they live. In this perspective, we highlight metal tolerance via complexation of free metals by sulfate ions and passive tolerance to metal influx via an internal positive cytoplasmic transmembrane potential. These tolerance mechanisms have been largely ignored in past studies of acidophile growth in the presence of metals and should be taken into account.
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α-fur, an antisense RNA gene to fur in the extreme acidophile Acidithiobacillus ferrooxidans
Microbiology, 2014Co-Authors: Claudia Lefimil, Eugenia Jedlicki, David S HolmesAbstract:A large non-coding RNA, termed α-Fur, of ~1000 nt has been detected in the extreme acidophile Acidithiobacillus ferrooxidans encoded on the antisense strand to the iron-responsive master regulator fur (ferric uptake regulator) gene. A promoter for α-fur was predicted bioinformatically and validated using gene fusion experiments. The promoter is situated within the coding region and in the same sense as proB, potentially encoding a glutamate 5-kinase. The 3′ termination site of the α-fur transcript was determined by 3′ rapid amplification of cDNA ends to lie 7 nt downstream of the start of transcription of fur. Thus, α-fur is antisense to the complete coding region of fur, including its predicted ribosome-binding site. The genetic context of α-fur is conserved in several members of the genus Acidithiobacillus but not in all Acidophiles, indicating that it is monophyletic but not niche specific. It is hypothesized that α-Fur regulates the cellular level of Fur. This is the fourth example of an antisense RNA to fur, although it is the first in an extreme acidophile, and underscores the growing importance of cis-encoded non-coding RNAs as potential regulators involved in the microbial iron-responsive stimulon.
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Metal resistance or tolerance? Acidophiles confront high metal loads via both abiotic and biotic mechanisms
Frontiers Media S.A., 2014Co-Authors: Mark Edopson, Francisco Eossandon, Lars Elövgren, David S HolmesAbstract:All metals are toxic at high concentrations and consequently their intracellular concentrations must be regulated. Acidophilic microorganisms have an optimum growth pH < 3 and proliferate in natural and anthropogenic low pH environments. Some Acidophiles are involved in the catalysis of sulfide mineral dissolution, resulting in high concentrations of metals in solution. Acidophiles are often described as highly metal resistant via mechanisms such as multiple and/or more efficient active resistance systems than are present in neutrophiles. However, this is not the case for all Acidophiles and we contend that their growth in high metal concentrations is partially due to an intrinsic tolerance as a consequence of the environment in which they live. In this perspective, we highlight metal tolerance via complexation of free metals by sulfate ions and passive tolerance to metal influx via an internal positive cytoplasmic transmembrane potential. These tolerance mechanisms have been largely ignored in past studies of acidophile growth in the presence of metals and should be taken into account
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Comparative genomics of the oxidative stress response in bioleaching microorganisms
Hydrometallurgy, 2012Co-Authors: Juan Pablo Cárdenas, David S Holmes, Francisco Moya, Paulo C. Covarrubias, Amir Shmaryahu, Gloria Levicán, Raquel QuatriniAbstract:Abstract Bioleaching Acidophiles inhabit environments with unusually high concentrations of iron that can potentially cause oxidative stress via the Fenton reaction in which dangerous reactive oxygen species (ROS) are generated. ROS can cause damage to proteins, nucleic acids, lipids and other macromolecules and thus have deleterious effects on cell growth and survival. Many of these microorganisms are chemolithotrophs with unusually high oxygen consumption rates that may exacerbate the problem of oxidative stress. Although some knowledge has been gained in recent years regarding the oxidative stress response in a few Acidophiles, the general strategies used by them to face ROS challenges are still inadequately understood. Comparative genomics and multiple bioinformatic tools were used to explore 44 sequenced genomes of acidophilic bacteria and archaea in order to reconstruct their individual oxidative stress responses and to identify conserved strategies. The analyses revealed that Acidophiles lack genes encoding typical oxidative stress response regulators and have an underrepresentation of classical ROS consumption enzymes (e.g. catalases) although they have a complete repertoire of repair systems for macromolecules (DNA, proteins and lipids). This suggests that stress mitigation is an active strategy in Acidophiles confronting unavoidable ROS formation in their environment. Insights into the oxidative stress response in bioleaching Acidophiles may contribute to a better understanding of the aspects that influence fitness of the microbial consortium driving bioleaching.
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Genomic insights into microbial iron oxidation and iron uptake strategies in extremely acidic environments.
Environmental Microbiology, 2011Co-Authors: Violaine Bonnefoy, David S HolmesAbstract:Summary This minireview presents recent advances in our understanding of iron oxidation and homeostasis in acidophilic Bacteria and Archaea. These processes influence the flux of metals and nutrients in pristine and man-made acidic environments such as acid mine drainage and industrial bioleaching operations. Acidophiles are also being studied to understand life in extreme conditions and their role in the generation of biomarkers used in the search for evidence of existing or past extra-terrestrial life. Iron oxidation in Acidophiles is best understood in the model organism Acidithiobacillus ferrooxidans. However, recent functional genomic analysis of Acidophiles is leading to a deeper appreciation of the diversity of acidophilic iron-oxidizing pathways. Although it is too early to paint a detailed picture of the role played by lateral gene transfer in the evolution of iron oxidation, emerging evidence tends to support the view that iron oxidation arose independently more than once in evolution. Acidic environments are generally rich in soluble iron and extreme Acidophiles (e.g. the Leptospirillum genus) have considerably fewer iron uptake systems compared with neutrophiles. However, some Acidophiles have been shown to grow as high as pH 6 and, in the case of the Acidithiobacillus genus, to have multiple iron uptake systems. This could be an adaption allowing them to respond to different iron concentrations via the use of a multiplicity of different siderophores. Both Leptospirillum spp. and Acidithiobacillus spp. are predicted to synthesize the acid stable citrate siderophore for Fe(III) uptake. In addition, both groups have predicted receptors for siderophores produced by other microorganisms, suggesting that competition for iron occurs influencing the ecophysiology of acidic environments. Little is known about the genetic regulation of iron oxidation and iron uptake in Acidophiles, especially how the use of iron as an energy source is balanced with its need to take up iron for metabolism. It is anticipated that integrated and complex regulatory networks sensing different environmental signals, such as the energy source and/or the redox state of the cell as well as the oxygen availability, are involved.
Dennis A Savaiano - One of the best experts on this subject based on the ideXlab platform.
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Improvement of lactose digestion by humans following ingestion of unfermented acidophilus milk: influence of bile sensitivity, lactose transport, and acid tolerance of Lactobacillus acidophilus.
Journal of Dairy Science, 1997Co-Authors: Azlin Mustapha, Tianan Jiang, Dennis A SavaianoAbstract:The influence of bile sensitivity, lactose transport, and acid tolerance of Lactobacillus acidophilus on in vivo digestion of lactose was investigated. Four strains of L. acidophilus exhibiting varied degrees of lactose transport, beta-galactosidase activity, and bile sensitivity were used to prepare unfermented acidophilus milks. Lactose malabsorption was evaluated by measuring breath H2 excretion off 11 lactose maldigesting subjects following ingestion of four acidophilus test milks. Test meals were fed in a randomized double-blind protocol. Consumption of acidophilus milk (2% fat) containing strains B, N1, and E significantly reduced mean total H2 production compared with that of the control reduced-fat (2% fat) milk, but milk containing strain ATCC 4356 did not differ from the control. Acidophilus milk containing L. acidophilus N1 was the most effective of the four acidophilus milks in improving lactose digestion and tolerance. Strain N1 exhibited the lowest beta-galactosidase activity and lactose transport but the greatest bile and acid tolerance of the four strains. The results indicated that bile and acid tolerance may be important factors to consider when L. acidophilus strains are selected for improving lactose digestion and tolerance.