Escherichia Coli
Classification/Diagnostic Characteristics
E.Coli is in the domain Bacteria that consists of single celled prokaryotes.
E.Coli is an endotoxin, meaning that it is released when Gram-negative bacteria grow or lyse. Endotoxins are also lipopolysaccharides, meaning they consist of a polysaccharide and a lipid component that forms the outer bacterial membrane.
E.Coli is a proteobacteria.
Domain: Bacteria
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Enterobacteriales
Family: Enterobacteriaceae
Genus: Escherichia
Species: Escherichia coli (E. coli)
(SP)
Used with Permission from: http://www.denniskunkel.com/ (Link To)
Used with Permission from: http://www.denniskunkel.com/ (Link To)
The photo above is an electron microscopy picture (10). (SP)
E. Coli is prokaryotic, as the diagram below shows, E. Coli do not have a nucleus and simply store their DNA within the cytoplasm. (SL 15)
cell-ecoli.gif


Relationship to Humans
E.Coli is an endotoxin, which is a type of bacterial toxin that is rarely fatal to the host. When humans are the hosts of E.Coli, the effects in the human are fever, vomiting, and diarrhea.
E.Coli is frequently used for biotechnology as well. Scientists use restriction enzymes to cut sequences from two E.Coli plasmids containing antibiotic resistance genes. Then they use DNA ligase to join fragments. The resulting plasmids are inserted into new E.Coli cells, which then are given a resistance to antibiotics.

Though most E. coli strains are harmless, some variations of the bacteria can cause food poisoning in humans, a common cause for food recalls due to contamination. E coli. represents around 0.1% of the gut bacteria population found in most humans, and its harmless variations are actually important in the synthesis of vitamin K2, as well as blocking the settlement of harmful bacteria. The pathogenic forms of E. coli are transmitted between humans through the fecal-oral route, and can survive for extended period of times outside of a host. (3) (4) (YR)

E.coli is also widely used in several fields of study because they are model organisms, or organisms that has been intensely studied and genetically mapped and whose biological information can apply to other organisms. Scientists use E.coli as a basis for testing because the thorough understanding of the organism provides comprehensible and useful experiments that can often be applied to other similar life-forms.

Protein production, one of biology's burgeoning fields, also makes use of E.coli. Using recombination DNA technology (the process of creating instructions for a protein using several different genes), genes for human proteins can be inserted inside E.coli. The E.coli can then be grown in large bio-reactors, mass producing essential proteins such as insulin, and human growth hormone. (SF)

Although the name E. coli may have a negative connotation, it has actually been found that E. coli has the potential to manage nuclear waste. E. coli break down phytic acid-often found in seeds-thus freeing phosphate molecules. These phosphate molecules can then bind to Uranium atoms, forming Uranium phosphate. This product compound is a precipitate-or a solid generated by a reaction- which forms on the bacteria. The solid Uranium particles can then be removed from the environment, reducing the risks associated with radioactive elements like Uranium. (S.S-8).

Habitat and Niche
The E.Coli virus inhabits the human intestine.

The primary habitat of E. Coli is in the gastrointestinal tract of humans and many warm blooded animals. E. Coli is the most abundant facultatively anaerobic microorganism found in the gastrointestinal tract of humans and mammals. The E. Coli forms a mutualistic relationship with its host. E. Coli usually adhere to the mucus or the epithelium on the wall of the intestines of the gastrointestinal tract. A single strand of this bacteria can last for months or even years. Freter's Nutrient-Niche theory describes E. Coli's niche: "the ecological niches of E. Coli depend on the nutrient availability within the intestines of the host organism"(1). There are so many different nutrient defined niches where the E. Coli can colonize because intestines are a balanced ecosystem and have so many nutrients. The area where E. Coli colonize in the gastrointestinal tract is dependent on the preferred nutrient for that particular population of E. Coli occupying the niche. The velocities of movement of chyme is much higher in the small intestine than in the large intestine. In order for E. Coli to be successful in the large intestine it must be able to adhere to the wall of the small intestine or else it will be swept away. If E. Coli is not able to grow on the intestinal wall it usually will colonize in the colon. Different regions in the gastrointestinal tract represent different niches too. E Coli can also be found outside the body in a faecally-contaminated environments like mud or water (1). (SM)
E. Coli in intestine (2) (SM)
E. Coli in intestine (2) (SM)

Predator Avoidance
E.Coli contains a restriction enzyme that gets rid of viruses, called bacteriophages, that infect E.Coli. The restriction enzymes bond to the bacteriophage at two active sites and cleave its DNA at a specific sequence. Then the double-stranded DNA molecules are cut into smaller, noninfectious fragments. The bonds of the bacteriophages DNA backbone are broken. The enzyme is named EcoRI after E.Coli.

Nutrient Acquisition
E.Coli is a chemoheterotroph meaning that it obtains its energy and carbon from complex organic compounds that have been synthesized by other organisms.

E. coli uses solute transport proteins, which are dedicated nutrient uptake systems. It uses passive diffusion which allows molecules such as oxygen, nitrogen, carbon dioxide, and water to pass through its cytoplasmic membrane. E. coli also employs facilitated diffusion to transport larger molecules such as glycerol. By first entering a carrier protein, glycerol can then enter the cell.
Most commonly, E. coli uses active transport systems to acquire nutrients. This type of transport requires ATP. Through processes on the cytoplasmic membrane, molecules that were originally unable to enter the cell through simple or facilitated diffusion can now pass through. Long chains of fatty acids are transported into E. coli using group translocation systems, which chemically modify cells for active transport. Generally, PEP (phosphoenolpyruvate) or ATP is used to modify the cells. Fatty acids, in particular, first enter the perplasm via a membrane protein called FadL. Then, the fatty acids gain an H+ ion, which allows them to diffuse into the cytoplasmic membrane. Lastly, a FadD protein binds to the fatty acid molecular and attaches Co-enzyme A by ATP hydrolysis, which allows for delivery into the cell's cytoplasm. (5) (RS)

Reproduction and Life Cycle
E.Coli uses asexual reproduction and therefore the resulting offspring are clones of the parent organism. E.Coli uses binary fission to reproduce. Each cell of E.Coli is the whole organism, and therefore each cell that divides forms two new cells.
E.Coli has 4x10^6 base pairs of DNA. The rate of replication is about 1,000 base pairs per second, therefore it takes 40 minutes to replicate the chromosomes. But new rounds of replication begin at each new chromosome before the first chromosome has divided, enabling E.Coli cells to divide every 20 minutes
E.Coli contains plasmids which are small, circular DNA molecules that replicate in prokaryotic cells. The plasmids in E.Coli are small and easy to manipulate in labs.

The genetic information of E.Coli is preserved in the cytosol of the cell and is known as the nucleoid. The absence of a nuclear membrane makes it so that the nucleoid is one long strand of DNA which binds its ends together. (AC) (17)
biowiki.jpg
A diagram of an E.Coli bacteria and its components. (AC) (16)


Growth and Development
The bacteria, E.Coli, exchanges genetic material by conjugation. Most reproduced cells are clones copies of their parent cell, but the cells are able to exchange genes just by physical contact during a process called bacterial conjugation. The contact is started with a sex pilus, a thin projection, that extends from one cell to another. Genetic material then passes from the donor cell to the recipient through a conjugation tube, a thin cytoplasmic bridge. The DNA gets inside the recipient cell's genome and then recombines with the cells genes. The recipients genetic constiution is then altered.

E. coli can grow in temperatures ranging from 7ºC to 50ºC, but the optimal growth temperature is 37ºC. The growth of this bacterium is also dependent on the amount of nutrients available, as it will grow best when the proper nutrients and amount of each nutrient is available. The optimal pH for E. coli growth is at 7, but it can survive anywhere between 4.4 and 9.7. Under optimal conditions, E. coli can double in 20 minutes, but in the colon, where E. coli must compete with other bacteria for nutrients, it can take 12 hours for it to double. (14) (CM)

Integument
E.Coli have cell walls located outside their plasma membrane. The cell wall contains peptidoglycan, a polymer of amino sugars that are linked by covalent bonds to peptides, forming a giant molecule that surrounds the cell. Some E.Coli species also have capsules that protect the bacteria from attack and help keep the cells from drying out.

Movement
E.coli use flagella for movement. The flagella are made of a protein called flagellin. A motor protein spins each flagellum like a propeller, moving the cell.

The motors are located in the cell wall, and spin the flagella into bundles that rotate counter-clockwise, creating a twist that causes the bacterium to rotate clockwise. (7) (NU)


(NU)

Bacterial organisms like Escherichia coli have developed mechanisms that allow them to detect and direct cell movement toward nourishment when starved. Such behavior is known as chemotaxis. When the concentration of the food attractants increases, a signal is transmitted from the chemoreceptors, the chemical receptors, to the flagellar motor, which controls the flagellum and influences the random movement of the bacterium. Only some nutrients, such as specific amino acids and sugars, can be sensed by the chemotaxis signal transduction system, the process that converts a chemical stimulus into a specific cellular response. There are two modes of swimming behavior that are employed by the flagellar motor rotation: counter clockwise rotation, which makes the cell ‘run,’ and clockwise rotation, which makes the cell ‘tumble.’ Both modes are used by Escherichia coli to be as close as possible to nutrient rich environments, with cells being capable of running four times longer than tumbling. (SM) [6]




This video shows a simulated image of E. Coli moving in order to show how the flagella and other hairs actually move to create motion. As you can see by this clear simulation, the motor protein must be acting on all the flagellum the same way because they all move together in order to propel the E. Coli in the most efficient way. (AG) (12)

Sensing the Environment
E.Coli inhabits the human intestine and the chemical composition of the environment changes when the human host ingests different foods. E.Coli undergoes specific processes to adjust to the new environment, such as when lactose is ingested by the host. E.Coli changes the expression of its genes for the uptake and metabolism of lactose. There are three proteins involved: B-galactose permease, which is a carrier protein in the plasma membrane that moves sugar into the cell, B-galactosidase, an enzyme that hydrolyzes lactose to glucose and galactose, and B-galactoside transacetylase, which transfers acetyl groups from acetyl CoA to certain B-galactosides. The number of these enzymes are lower if the E.Coli is in an environment without B-galactosides, but then if E.Coli is put in an environment with lactose as the predominant sugar, then the E.Coli cells make these enzymes. The expression of the genes that make these proteins are switched on by an inducer called allolactose.

In order to perform either aerobic or anaerobic respiration, E Coli uses a variety of redox pairs. Compounds such as pyruvic acid, hydrogen, and amino acids are oxidized and substrates such oxygen, nitrate, and fumarate are reduced. This versatile aspect of E Coli's respiratory system helps it adapt to the conditions of its anaerobic or aerobic environment. The organism also adapts to signals such pH, temperature, and osmolarity, a measure of solute concentration. To accomodate larger molecules or exclude harmful substances, it is also able to vary the diameter of its outer membrane porins.(13) (MT)

Gas Exchange
E.Coli have cell walls made of peptidogylcan. Some species of E.Coli have capsules that keep the cells from drying out.

Peptidogylcan, also known as murein, is a substance forming the cell walls of many bacteria, consisting of glycosaminoglycan chains and short peptides (11). (SP)

E. Coli is a facultative anaerobe. This means that it performs aerobic respiration if oxygen is present, but switches to fermentation if oxygen is not present. (JM 9)

Waste Removal
Waste products pass directly through the plasma membrane.

Environmental Physiology
E.Coli reacts to its environment, the human intestine. It inhabits the human intestine and is forced to adjust to sudden changes in its environment as the foods consumed by the host change. E.Coli responds by changing the expression of his genes.

Internal Circulation
The E.Coli cells are so small that they do not need a complex circulation system. Most of their circulation occurs by simple diffusion through their membrane.

Chemical Control
E.Coli has no endocrine system for chemical control, but its selectively permeable cell wall determines and regulates what chemicals enter and leave the cell.


REVIEW QUESTIONS:

1. What makes E. Coli so important in laboratory work? What makes them model organisms? (CM)
2. E. Coli can exist in many different areas of the human digestive tract. What does this tell you about their success as organisms? (RG)
3. Explain what restriction enzymes can do to the viruses inside of E. Coli? (SM)
4. Knowing that E. Coli inhabit the human intestine, what can you infer about their osmoregulation? What can you infer about their temperature regulation? (SL)
5. What is binary fission? How are E. Coli able to reproduce using binary fission? How is the offspring similar to the parent E. Coli? (SL)



Sources
1. Moder, Justine. "Escherichia Coli." Escherichia Coli. N.p., n.d. Web. 22 Nov. 2013.
2. E. Coli inside the gastrointestinal tract picture
3. http://www.sciencemag.org/content/308/5728/1635
4. http://gut.bmj.com/content/49/1/47
5. "Nutrient Transport." E. Coli Student Portal. UCLA, n.d. Web. 18 Nov. 2013.
6. http://en.wikipedia.org/wiki/Chemotaxis#Bacterial_chemotaxis
7. http://www.microbeworld.org/component/jlibrary/?view=article&id=6885
8. http://www.sciencedaily.com/releases/2009/09/090907013804.htm
9. Todar, Kenneth, Ph.D. "Pathogenic E. Coli." //Textbookofbacteriology.net//. Todar's Online Textbook of Bacteriology, n.d. Web. 24 Nov. 2013.
10. Moder, Justine. "Escherichia Coli." Escherichia Coli. N.p., Apr. 2008. Web. 25 Nov. 2013.
11. "Peptidoglycan." Peptidoglycan. CC-BY-SA, n.d. Web. 25 Nov. 2013.
12. http://www.youtube.com/watch?v=dtHHKegLI9I
13. Laroussi, M. //Plasma Medicine: Applications of Low-temperature Gas Plasmas in Medicine and Biology//. Cambridge: Cambridge UP, 2012. Web. 25 Nov. 2013.
14. Hartsock, Angela. "Growth Requirements of E. Coli. and Auxotrophs." //Education Portal//. N.p., n.d. Web. 25 Nov. 2013.
15.http://static.ddmcdn.com/gif/cell-ecoli.gif
16. Brain, Marshall. "How Cells Work." How Stuff Works. N.p., n.d. Web. 25 Nov. 2013.
17. Kaiser, Gary E. "Prokaryotic Cell Structure: Structures Within the Cytoplasm - The Nucleoid." Doc Kaiser's Microbiology Page. N.p., n.d. Web. 25 Nov. 2013.