Azoarcus tolulyticus
Classification
• Kingdom - Bacteria
• Phylum - Proteobacteria
• Class - Betaproteobacteria
• Order - Rhodocyclales
• Family - Rhodocyclaceae
• Genus - Azoarcus
Species
• Azoarcus tolulyticus
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NCBI: Taxonomy |
Description and Significance
First described in 1995 by researchers at Michigan State University, Azoarcus tolulyticus, is a Gram negative, motile rod-shaped bacteria that can be 1.4-2.8um in length (size dependent on growth medium). It belongs to the nitrogen-fixing genus of Azoarcus. A. tolulyticus is an anaerobic toluene degrader isolated from primarily from petroleum contaminated soils, marine and freshwaters. This microbe would be considered a non-halophilic mesophile as it grows almost exclusively at temperatures ranging from 25-37⁰C, NaCl concentrations below 1% and near neutral to slightly alkaline pH.
In anaerobic environments where there is significant gasoline contamination, BTEX (benzene, toluene, ethylbenzene and xylene) compounds pose serious environmental and human health problems. Health effects from high exposure to BTEX compounds can be as severe as cancer or death. Azoarcus tolulyticus is a facultative anaerobe which allows it to live best in low oxygen environments, like subsurface aquifers. Because of its innate ability to anaerobically degrade toluene which is enhanced through nitrogen stimulation to increase degradation, Azoarcus tolulyticus has the potential for use in the in situ bioremediation of gasoline or petroleum contaminated sites.
Genome Structure
Azoarcus tolulyticus strain Tol-4 was obtained from petroleum contaminated freshwater sediment, 24-25m deep in Northern Michigan. Its genome consists of 1458 base pairs with a G+C content of 56.1-56.6% Base pairs at positions 28 and 1491 correspond with that on the E. coli 16S rRNA gene. Helices located at positions 180 and 220 on the E. coli gene are signature sequences specific to the beta subdivision of the Proteobacteria. Automated fluorescence sequencing of the 16S rRNA revealed that different A. tolulyticus strains had high levels of similarity, of up to 99.9%, but not identical. PCR analysis has shown that multiple isolates have distinct repetitive extragenic palindromic PCR patterns.
Its closest related genus is a selenate respiring species Thauera selenatis which maintains a 93% similarity. A. evansii is the mostly closely related Azoarcus species to A. tolulyticus although it does not it does not degrade toluene anaerobically.
Figure I. - Phylogenetic tree showing the relationships between strains of Azoarcus tolulyticus and related species. Tree above was obtained using DNAPAR and SEQBOOT with A. tolulyticus strains and related species. Percentages indicate estimates on the confidence of the above branches (Zhou 1995).
Cell Structure, Metabolism and Life Cycle
Interesting features of cell structure; how it gains energy; what important molecules it produces.
Eight facultative anaerobic isolates of A. tolulyticus have been identified each of which is capable of performing toluene-degrading denitrification. These bacteria show fatty acid profiles similar to other members of the genus Azoarcus, however distinctions were found in that the newly discovered isolates contained an abundance of 12:0 fatty acid while lacking 3-OH 8:0 fatty acid content. All of the isolates are gram negative, cytochrome c oxidase and catalase positive, and all are capable of fixing nitrogen when grown on N2. Also each of these isolates are capable of producing N2 gas as products of cellular respiration under anaerobic conditions.
A. tolulyticus are short motile rods, although having lengths ranging from 1.4 to 2.1um when grown on toluene, can increase in size to 2.1 to 2.8um (or in long chains) when grown on M-R2A agar. Morphological characteristics when grown on M-R2A agar produces uniform, translucent colonies, yellowish in color and approximately 2-3mm in diameter containing a dark center. The cells lack tryptophanase, B-galactosidsase, urease, and arginine dihydrolase activity. As facultative anaerobes A. tolulyticus is able to use either nitrate or oxygen as terminal electron acceptors during cellular respiration. In aerobic environments oxygen serves as the terminal electron acceptor and is usually the limiting factor due to its low solubility in water along with diffusion constraints in subsurface environments. In anaerobic environments nitrate is used as the terminal electron acceptor. Nitrate is an adequate alternative due to its high solubility in water and high mobility in soil making the rates of degradation comparable to those of aerobic metabolism processes. Evidence has been shown that anaerobic degradation of toluene by A. tolulyticus has also been observed under Fe(III)-reducing, methanogenic, and sulfidogenic conditions.
Ecology and Pathogenesis
Azoarcus tolulyticus which is included in the genus of the free-living nitrogen fixing bacteria are found in a variety of habitats including both contaminated and non-contaminated soils, aquifiers, and sediments. Table I. shows the diversity of habitats in which isolates have been obtained which range from Michigan to California.
Table I. – Sources of strains of Azoarcus tolulyticus and the habitats from which they were isolated from relative to Geographic origin.
The importance of Azoarcus tolulyticus can be attributed to its bioremediation potential in anoxic contaminated sites. Bioremediation of toxic chemicals, such as BTEX compounds (primary contaminants in petroleum) affecting soil and other habitats, are of great importance as they can impact the health and safety of the public and are difficult to remediate effectively. The prospect of in situ bioremediation treatments using A. tolulyticus potentially could be the best solution at achieving cost effective cleanup technology for treating contaminated sites. Although microbial degradation of these compounds is known to occur under aerobic conditions, contamination such as those in aquifiers and soil, housing anaerobic conditions make degradation by aerobic microbes not possible. The ability of A. tolulyticus to utilize nitrate (and other inorganic compounds) as terminal electron acceptors allows for degradation of toxic chemicals in these habitats.
References
Author
Page authored by Eric Tovar and Alyse Waldhorn, student of Prof. Jay Lennon at Michigan State University.
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