Clostridium thermocellum

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A Microbial Biorealm page on the genus Clostridium thermocellum

Classification

Higher order taxa

Bacteria; Firmicutes; Clostridia; Clostridiales; Clostridiaceae; Clostridium

Species

Clostridium thermocellum

NCBI: Taxonomy

Description and significance

Like most species from the Clostridium genus, C. thermocellum is a bacteria that has a rod-like shape for its cell body. It is classified as a gram-positive bacteria which means that the cell body is only surrounded by a single bilayer lipid membrane. Because it is a gram-positive bacterium, the outside of the cell membrane also contains a thick cell wall known as murein, which is made of peptidoglycans. C. thermocellum is an anaerobic and thermophilic organism that produces spores. In addition, it has cellular structures and mechanism that give it motility in the environment it resides in. The organism was completely isolated and sequenced at the DOE Joint Genome Institute. The bacterium had its genome completely sequenced because it contains a unique extracellular enzyme system capable of breaking down insoluble cellulose into ethanol which is vital for biomass energy. Because of its ability to break down cellulose, it is highly likely that C. thermocellum can be isolated from many plant organisms. In addition, it can be isolated from cow and horse manure because it is required in the animals' digestive system to break down the cellulose of grass.

Genome structure

The number of nucleotides present in the genome of C. thermocellum has been discovered and reported to be at 3,843,301 base pairs which makes up 3307 genes [1]. The nucleotides make up one double-stranded circular DNA. The circularity of the chromosome is advantageous for C. thermocellum because exonucleases cannot digest the ends of its DNA[4]. Another advantageous feature of the circularity of the chromosome is that the chromosome can be supercoiled which drastically lowers the energy barrier making it easier for DNA to be activated and separated into single strands for transcription into RNA. In regards to genomic organization, C. thermocellum like all other prokaryotic organisms lack a true nucleus for DNA storage and transcription. Instead, the entire genome is tightly packaged in a small region called the nucleoid and transcription takes place between the cytoplasm and nucleoid. In addition to the circular chromosome, C. thermocellum also has a plasmid which is an extrachromosomal genetic element that is not essential in affecting its lifestyle. Unlike the circular chromosome which is necessary for viability, the loss of the plasmid will not affect the bacteria's ability to reproduce. Because C. thermocellum is known as a degrader of cellulose, its DNA contains specific nucleotide sequences that make up the genes that encode for the system of enzymes that are necessary for cellulose degradation.

Cell structure and metabolism

The general interior cellular structure of C. thermocellum is similar to most rod-shaped bacteria. It contains a nucleoid region for packaging and transcription of the DNA into RNA. A nuclear envelope emcompasses the nucleoid. In the cytoplasm, a chain or cluster of ribosomes form polyribosomes which function as a continuous protein factory that translates mRNA into polypeptide chains.

Aside from the general internal framework of the cell, C. thermocellum is a gram-positive bacteria. As a result, the exterior cellular structure of the bacteria consists of a thick cell wall that is composed of peptidoglycan, a complex polymer of sugars and amino acids. The type of peptidoglycan that forms the cell wall of C. thermocellum is known as murein which protects its single bilayer membrane from high turgor pressure and also provides the cell its shape and rigidity.

The most interesting and special external cellular feature for this bacteria however stems from its unique and trademark ability to degrade insoluble cellulose. The cellulosome, a complex system of cellulolytic enzymes, is located at the exterior of the C. thermocellum cell. The cellulosome contains nearly 20 catalytic enzymes that are encoded by over 100 genes. Some of the genes are play a role in synthesizing the enzymes while others are responsible for regulating cellulosome activity with inducers and repressors. The cellulases are each responsible in the cellulose breakdown process. This also suggests that the cellulose degrading bacteria has extracellular appendages. In order to perform cellulolysis, C. thermocellum must adhere to the surfaces of plants [2]. To be able to stick onto the surfaces of plants suggests that the bacteria has pili which are structures that provide it attachment and motility capabilities.

In terms of energy production, since C. thermocellum is an anaerobic organism, it obtains most of its energy through the metabolic process of fermentation. In fermentation, the bacterial cell does not consume oxygen to produce energy because there is a lack of good electron acceptors. Rather, the bacteria utilizes smaller sugar molecules obtained from cellulose breakdown of plants during fermentation to produce pyruvate. Along with pyruvate, some waste products are also generated such as hydrogen, carbon dioxide, acetate, and primarily ethanol [3].

Ecology

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Pathology

Most bacterial species in the Clostridium genus generate toxins that are harmful and malevolent to other host cells. For instance, one of the most deadly toxins is produced by Clostridium botulinum which causes paralysis by stopping all motor and neural activities. Paralysis usually occurs from the head, then affects the upper body, and then gradually affects the lower body of humans because the cranial nerves are affected first by the toxins. In the worst case scenario, the respiratory muscles are infected by the toxins of C. botulinum leading to the inability to breathe and finally death. Infection by C. botulinum usually occurs from overdose of botox injection leading to toxification of the human body or from ingestion of bad, spoiled food products. The other prominent Clostridium species that exhibits pathogenic effects is Clostridium tetani. C. tetani primarily infect young children by producing toxins in their opened, unclean wounds. The production of the neurotoxins leads to the disease called tetanus which is usually causes stiffening of the muscles, swelling, sweating, and muscle spasms. Unlike its bacterial relatives, C. thermocellum does not produce or secrete any kind of toxins that will induce any pathological effects on human, animal, or plant hosts.

Application to Biotechnology

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Current Research

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References

National Center for Biotechnology Information.

Bayer, E., Kenig, R., and Lamed, R. "Adherence of Clostridium thermocellum to Cellulose". Journal of Bacteriology. 1983. Volume 156. p. 818-827.

Weimer, P. and Zeikus, J. "Fermentation of Cellulose and Cellobiose by Clostridium thermocellum in the Absence and Presence of Methanobacterium thermoautotrophicum". Applied and Environmental Microbiology. 1977. Volume 33. p. 289-297.


Edited by Kenny Lam