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== References ==
== References ==

{{Reflist}}
{{Reflist}}

==Further Reading==
*{{cite journal|last1=Lebedev|first1=Nikolai|last2=Chen|first2=Linxu|last3=Ren|first3=Yilin|last4=Lin|first4=Jianqun|last5=Liu|first5=Xiangmei|last6=Pang|first6=Xin|last7=Lin|first7=Jianqiang|title=Acidithiobacillus caldus Sulfur Oxidation Model Based on Transcriptome Analysis between the Wild Type and Sulfur Oxygenase Reductase Defective Mutant|journal=PLoS ONE|volume=7|issue=9|year=2012|pages=e39470|issn=1932-6203|doi=10.1371/journal.pone.0039470}}

*{{cite journal|last1=Valdes|first1=J.|last2=Quatrini|first2=R.|last3=Hallberg|first3=K.|last4=Dopson|first4=M.|last5=Valenzuela|first5=P. D. T.|last6=Holmes|first6=D. S.|title=Draft Genome Sequence of the Extremely Acidophilic Bacterium Acidithiobacillus caldus ATCC 51756 Reveals Metabolic Versatility in the Genus Acidithiobacillus|journal=Journal of Bacteriology|volume=191|issue=18|year=2009|pages=5877–5878|issn=0021-9193|doi=10.1128/JB.00843-09}}

*{{cite journal|last1=Dopson|first1=Mark|last2=Holmes|first2=David S.|last3=Valdés|first3=Jorge|last4=Mangold|first4=Stefanie|title=Sulfur Metabolism in the Extreme Acidophile Acidithiobacillus Caldus|journal=Frontiers in Microbiology|volume=2|year=2011|issn=1664-302X|doi=10.3389/fmicb.2011.00017}}


==External Links==
*{{EOL|972360|Acidithiobacillus caldus}}
*[http://www.bacterio.net/acidithiobacillus.html LPSN]

{{DEFAULTSORT:Acidithiobacillus caldus}}

{{bacteria-stub}}

Revision as of 15:43, 2 June 2014

Acidithiobacillus caldus
Scientific classification
Domain:
Kingdom:
Phylum:
Class:
Order:
Family:
Genus:
Species:
A. caldus
Binomial name
Acidithiobacillus caldus
(Kelly and Wood 2000)
Type strain
DSM 8584
(Kelly & Wood 2000)
Synonyms

Thiobacillus caldus
(Hallberg & Lindstrom 1994)

Acidithiobacillus caldus formerly belonged to the genus Thiobacillus prior to the year 2000, at which point it was reclassified along with a number of other bacterial species into one of three new genera that better categorize sulfur-oxidizing acidophiles..[1]. As a member of the Gammaproteobacteria class of Proteobacteria, A. caldus may be identified as a Gram-negative bacterium that is frequently found in pairs[2]. Considered to be one of the most common microbes involved in biomining, it is capable of oxidizing reduced inorganic sulfur compounds (RISC's) that form during the breakdown of sulfide minerals[3]. The meaning of the prefix 'acidi-' in the name Acidithiobacillus comes from the Latin word 'acidus', signifying that members of this genus love a sour, acidic environment[1]. 'Thio' is derived from the Greek word 'thios' and describes the utilization of sulfur as an energy source, and 'bacillus' provides a description of the shape of these microorganisms, which are small rods[1]. The species name, caldus, is derived from the Latin word for 'warm' or 'hot', denoting this species' love of a warm environment.

History

Thiobacillus caldus was reclassified into Acidithiobacillus, one of three new genera (also including Halothiobacillus and Thermithiobacillus) created to further classify members of the genus which fall into the alpha-, beta-, and gamma-classes of the Proteobacteria[1]. Thiobacillus species exhibit a tremendous amount of diversity in physiology as well as DNA composition, which was one reason for reclassification of this species into a new genus containing four species of acidophiles (microorganisms which function best in an acidic environment), some of which are also capable of oxidizing iron[II] and sulfide minerals[1].

A. caldus, originally isolated from spoils of unneeded rocks encountered when mining coal, was the first acidophilic species of thermophilic Thiobacilli to be described[2]. The type strain of this species, DSM 8584, also known as strain KU, has been deposited in the Deutsche Sammlung von Mikroorganismen und Zellkulturen , a collection of microorganisms in Germany[1].

Morphology

Acidithiobacillus caldus is a short, rod-shaped, Gram-negative bacterium that possesses motility via a single polar flagellum located on its outer cell wall, which displays characteristics of a typical Gram-negative cell wall[2]. It is approximately 1 μm by 1-2 μm in length and frequently is found in pairs[2].

Different strains of Acidithiobacillus caldus have been shown to vary in size when compared to one another. One of the smaller strains, BC13, has a diameter of approximately 0.7 μm and is about 1.2 μm in length, whereas strain KU is a little lengthier, with a diameter of approximately 0.8 μm and a length of about 1.8 μm[2].

Physiological Tolerance

Acidithiobacillus caldus displays tolerance to a broad range of conditions, including acidic pH levels and temperature with the best growth occurring at a pH of 2.0 to 2.5 and a temperature of 45 °C[2]. Optimal growth results in a short generation time of approximately 2-3 hours, depending on the environmental factors present[2]. A. caldus is not considered to be halophilic because it displayed no signs of growth in environments containing NaCl[4].

Temperature

Acidithiobacillus caldus is moderately thermophilic and thrives at an optimum temperature of 45 °C[2]. Certain strains, such as strain KU, have still been shown to exhibit growth on a tetrathionate medium in conditions with a temperature range as low as 32 °C and as high as 52 °C[4]. When grown on a medium containing sulfur, strain BC13 has been found to tolerate temperatures as high as 55 °C[2]. It is thought in the scientific community that there is a genetic basis for the extreme temperature tolerance depictive of A. caldus as compared to other species in its genus, such as A. ferrooxidans and A. thiooxidans[5].

pH

As with all acidophilic microorganisms, A. caldus thrives best in an environment with a low, acidic pH with a preferred pH range of 2.0-2.5[2]. This microorganism is capable of coping with a large pH gradient across the cellular membrane, keeping its intracellular pH around a nearly neutral level of 6.5[6]. Certain strains, including KU and BC13, have been found to display signs of growth in a broad, acidic pH range, with a slow growth rate involving a longer generation time, approximately 45 hours, at a pH of 4.0 and a rate of 6-7 hours at a pH of 1.0[2]. A. caldus has its shortest generation time of 2-3 hours in conditions involving a pH between 2.0 and 2.5[2]. No growth was observed at a pH of 0.5[2], showing that some conditions are simply too acidic to support the growth of even extreme acidophiles.

Metabolism

A. caldus is capable of oxidizing reduced inorganic sulfur compounds along with other substrates including molecular hydrogen, ferrous iron (Fe2+), and formate, in addition to numerous organic compounds and sulfide minerals[2]. It displays chemolithotrophic growth when exposed to substrates containing sulfur, tetrathionate, or thiosulfate, with sulfate being produced as the end-product[2]. Reduced sulfur compounds are utilized by A. caldus in order to support its autotrophic growth in an environment which lacks sunlight[1]. It has been found that the growth of A. caldus is enhanced when the air used for sparging, a process by which bubbles of a chemically inert gas are pumped through a liquid, is supplemented with 2% (w/v) CO2[2]. Neither 0.05% yeast extract (a yeast product formed when a cell's walls are removed and its internal contents are extracted[7]), Casamino acids (an amino acid/peptide mixture common to microbial growth media formed from the acid hydrolysis of casein[8]), nor a 2.5 mM concentration of glucose as the sole substrate have been shown to induce heterotrophic growth of A. caldus[2]. Instead, growth is seen to occur mixotrophically with tetrathionate and yeast extract or glucose[2]. It has been noted that strain BC13 is capable of growth on a glucose medium, but not after being transferred to a glucose medium from one that contained sulfur in addition to glucose[2].

The principle function of bacteria involved in bioleaching is to oxidize ferrous iron to ferric iron (Fe3+), which carries out the oxidization of sulfur and provides energy for essential metabolic functions[3]. Key intermediates in the metabolism of A. caldus are elemental sulfur (S0) and tetrathionate[3]. The hydrolysis of tetrathionate by the key enzyme tetrathionate hydrolase (tetH), composed of 503 amino acids, yields pentathionate, thiosulfate, and sulfur, while elemental sulfur is oxidized by sulfite into sulfate[3].

Genomics

Most of what is known about the genus Acidithiobacillus comes from experimentation and genomic analyses of two of its species: A. ferrooxidans and A. caldus. With a length of 2,932,225 base pairs, the genomic sequence of A. caldus is GC-rich with a GC content (mol%) in the range of 63.1-63.9% for strain KU[4] and 61.7% for strain BC13[2]. DNA hybridization studies have revealed that strains KU and BC13 exhibited 100% homology with each other, yet showed no DNA hybridization of significance (2-20%) with other species in the genus including A. ferrooxidans and A. thiooxidans, or with other similar Proteobacteria, such as Thiomonas cuprina or Thiobacillus thioparus[1].

Strains of Acidithiobacillus caldus have been differentiated from other related Acidithiobacilli, including A. ferrooxidans and A. thiooxidans, by sequence analyses of the PCR-amplified 16S-23S rDNA intergenic spacer (ITS) and restriction fragment length polymorphism (RFLP)[9]. Phylogenetic analysis of ITS sequences was sufficient to differentiate three unique species of Acidithiobacillus that were found to have slightly different physiological tolerances[9]. Bergamo et al. (2004) concluded that the 16S-23S rDNA spacer region is a useful target for developing molecular methods that focus on the detection, rapid differentiation and identification of Acidithiobacilli[9].

Applications

Since its discovery in 1994, A. caldus has been found to have a significant practical application in the industrial field of biomining and mineral biotechnology, contributing to the enhanced recovery of desired minerals from rocks known as ores[3]. Metals such as gold have been recovered from ores which contain pyrite (also known as Fool's Gold) and arsenopyrite, two sulfide minerals that are often associated with considerable amounts of this precious metal[2]

Biomining refers to both biooxidation, where the sulfide mineral surrounding the desired metal is oxidized to expose the metal of interest, and bioleaching, where the sulfide mineral is solubilized to obtain the metal of interest[3]. Due to the exothermic nature of bioleaching, the thermophilic nature of A. caldus allows for less cooling and quicker rates of bioleaching overall[2]. Bacteria belonging to the genus Acidithiobacillus possess the ability to oxidize sulfidic ores and thereby solubilize metals[2]. This ability has contributed to a general public interest in this microorganism because of its application in the industrial bioleaching of metals from ores and because of its effective means by which to recover precious metals[2]. Bacteria involved in bioleaching function primarily to produce Fe3+ from the oxidation of ferrous iron, which is then used to carry out sulfur oxidization, which provides an essential energy source for important cellular metabolic functions[3]

References

  1. ^ a b c d e f g h Kelly, Donovan P. and Ann P. Wood (2000). "Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermithiobacillus gen. nov."International Journal of Systematic and Evolutionary Microbiology50: 511-516. Cite error: The named reference "Kelly" was defined multiple times with different content (see the help page).
  2. ^ a b c d e f g h i j k l m n o p q r s t u v w x Hallberg, Kevin B. and E. Börje Lindström. (1994). "Characterization of Thiobacillus caldus sp. nov., a moderately thermophilic acidophile." Microbiology 140 (12): 3451-3456. Cite error: The named reference "Hallberg" was defined multiple times with different content (see the help page).
  3. ^ a b c d e f g Rzhepishevska, Olena I., et al. (2007). "Regulation of a novel Acidithiobacillus caldus gene cluster involved in metabolism of reduced inorganic sulfur compounds."Applied Environmental Microbiology73 (22): 7367-7372. Cite error: The named reference "Rzhepishevska" was defined multiple times with different content (see the help page).
  4. ^ a b c You, Xiao-Yan, et al. (2011). "Unraveling the Acidithiobacillus caldus complete genome and its central metabolisms for carbon assimilation." Journal of Genetics and Genomics38 (6): 243-252. Cite error: The named reference "You" was defined multiple times with different content (see the help page).
  5. ^ Valdes, Jorge, Inti Pedroso, Raquel Quatrini, and David S. Holmes. (2008). "Comparative genome analysis of Acidithiobacillus ferrooxidans, A. thiooxidans and A. caldus: insights into their metabolism and ecophysiology." Hydrometallurgy 94: 180-184.
  6. ^ Mangold, Stephanie, et al. (2013). "Response of Acidithiobacillus caldus toward suboptimal pH conditions." Extremophiles17 (4): 689-696.
  7. ^ Herbst, Sharon (2001). Food Lover's Companion. Hauppauge, New York: Barron's Educational Series, Inc.
  8. ^ Mueller, J. Howard and Everett R. Johnson. (1941). "Acid hydrolysates of casein to replace peptone in the preparation of bacteriological media". The Journal of Immunology 40 (1): 33-38.
  9. ^ a b c Bergamo, Rogério F., Maria Teresa M. Novo, Ricardo V. Verı́ssimo, Luciana C. Paulino, Nancy C. Stoppe, Maria Inês Z. Sato, Gilson P. Manfio, Paulo Inácio Prado, Oswaldo Garcia Jr., Laura M.M. Ottoboni (2004). "Differentiation of Acidithiobacillus ferrooxidans and A. thiooxidans strains based on 16S-23S rDNA spacer polymorphism analysis." Research in Microbiology 155 (7): 559-567. http://dx.doi.org/10.1016/j.resmic.2004.03.009

Further Reading

  • Lebedev, Nikolai; Chen, Linxu; Ren, Yilin; Lin, Jianqun; Liu, Xiangmei; Pang, Xin; Lin, Jianqiang (2012). "Acidithiobacillus caldus Sulfur Oxidation Model Based on Transcriptome Analysis between the Wild Type and Sulfur Oxygenase Reductase Defective Mutant". PLoS ONE. 7 (9): e39470. doi:10.1371/journal.pone.0039470. ISSN 1932-6203.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  • Valdes, J.; Quatrini, R.; Hallberg, K.; Dopson, M.; Valenzuela, P. D. T.; Holmes, D. S. (2009). "Draft Genome Sequence of the Extremely Acidophilic Bacterium Acidithiobacillus caldus ATCC 51756 Reveals Metabolic Versatility in the Genus Acidithiobacillus". Journal of Bacteriology. 191 (18): 5877–5878. doi:10.1128/JB.00843-09. ISSN 0021-9193.


External Links