Here is a fun fact: right now, inside your intestines, there are millions of E. coli bacteria. Sound scary? Well, don’t worry too much. Despite being the main antagonist of a recent outbreak in romaine lettuce, the vast majority of E. coli bacteria are completely harmless to humans. In fact, humans and E. coli share an important symbiotic relationship that benefits both organisms.
Strains of the E. coli bacteria are found in the lower intestine of most warm-blooded mammals, humans included. E. coli benefit their hosts by producing vitamin K and preventing the colonization of other pathogenic bacteria. E. coli, along with 1,000 other species of bacteria, constitute the normal microbiota of the mammalian gut. These bacteria are harmless to their hosts and are a part of the normal functioning of the human body.
Biology Of E. Coli
E. coli (Escherichia coli) are a small, Gram-negative species of bacteria. Most strains of E. coli are rod-shaped and measure about 2.0 μm long and 0.2-1.0 μm in diameter. They typically have cell volume of 0.6-0.7 μm, most of which is filled by the cytoplasm. Since it is a prokaryote, E. coli don’t have nuclei; instead, their genetic material floats uncovered, localized to a region called the nucleoid.
E. coli are Gram-negative bacteria, meaning that they do not retain the crystal violet stain commonly used to differentiate bacteria. Their status as Gram-negative bacteria is due to their thin cell walls. E. coli has cell walls made out of two thing peptidoglycan layers, an inner and outer membrane. The Gram-negative outer membrane explains why many strains of E. coli are resistant to penicillin; the mechanism of action is disrupted by the thin cell walls. Many serotypes also have an external, flagella extending from the cell wall that is used to motility. In the mammalian gut, E. coli use their flagella to cling to the microvilli of the intestines.
E. coli are small rod-shaped bacteria. Credit: Public Domain
Metabolism
E. coli is a facultative anaerobe meaning that it primarily breathes oxygen, but can anaerobically respirate when oxygen is not available. Specifically, when oxygen is not present, E. coli derives its nutrients from the process of fermentation. During fermentation, E. coli breaks down carbohydrates into pyruvate in the absence of oxygen. This process produces ethanol and carbon dioxide.
Cell Cycle
Like all bacteria, E. coli reproduces through binary fission, in which one cell splits into a genetically identical copy. E. coli’s cell cycle is divided into three periods that roughly mirror the phases of eukaryotic mitosis. The B period occurs directly after cell division. The B period is the “normal” period in the life cycle in which the cell is functioning normally, similar to interphase in mitosis. Once DNA begins to replicate, the cell enters the C period which lasts until chromosomal replication is complete. The D period takes place after chromosomal replication and is the point when the cell splits in two. Once the D period is finished, the new cell proceeds into the B period, starting the cycle over again.
The exact length of the B period depends upon the available nutrients. The more food, the quicker E. coli goes through its normal phase and the quicker it begins copying chromosomes. However, the lengths of the C and D periods remain constant. When food sources are very high, cells will begin replicating before the previous round of replication is complete, resulting in a very fast growth rate. This fast growth rate is one reason why E. coli are often used as a model organism in laboratory research. In ideal conditions, E. coli can achieve a duplication rate of 22 minutes.
E. coli are known to engage in horizontal gene transfer, a process in which one bacterium inserts a section of its genetic code directly into the DNA of another. Horizontal gene transfer in bacteria serves an analogous function to sexual reproduction in eukaryotes in that it provides a source of genetic diversity. Some strains of E. coli inherit their pathogenic features from having their DNA altered by other bacteria.
The growth of an E. coli colony over time. Credit: WikiCommons CC-BY SA 4.0
The Role Of E. coli In The Human Body And Disease
Most strains of E. coli are actually completely harmless to humans. In fact, colonies of E. coli form a natural part of the microbiome of the mammalian gut. In mammals, E. coli facilitates the absorption of iron via the production of enterobactin, a siderophore (Greek: “iron carrier”) that strongly binds to iron ions. It is thought that enterobactin produced by E. coli binds to ATP-synthase which helps it draw iron into the intestines. Additionally, E. coli produces vitamin K2, an important nutrient that assists with blood clotting. The presence of E. coli in the gut also prevents the colonization of harmful bacteria.
However, some serotypes of E. coli are highly infectious and can cause a range of symptoms, including gastroenteritis, UTIs, colitis, and, in extreme cases, meningitis and Chron’s disease. Most infections by harmful E. coli are caused by ingestion of tainted food, in particular, beef, milk, and fresh produce. E. coli can also be spread by contact with contaminated water sources and fecal matter. Symptoms normally manifest about 3-4 days after exposure, though it can take as long as 10 days. Generally, infections are mild to moderate, though can become very severe in young children and the elderly. Outbreaks of E. coli normally result in wide-spread food recalls to isolate and remove the contaminated products from circulation.
The exact mechanism of virulence differs from strain to strain. For example, some E. coli produce the Shiga toxin, a powerful biotoxin that is classified as a bioterrorism agent. The Shiga toxin destroys red blood cells which then clog the kidneys, causing severe renal failure and strokes. Other strains cause urinary tract infections and if they leak out of the GI tract, peritonitis. In general, pathogenic strains of E. coli cause their bad effects by producing toxins that damage cells or that the body cannot process properly.
Pathogenic E. coli can be differentiated into three types, O, K, and H groups. The division into these three groups is based on the chemical properties of the antigens embedded in the bacterial cell wall. Different antigens produce a different immune response which is relevant to how that type of E. coli is treated. Their status as Gram-negative bacteria means that they cannot be treated with several antibiotics that are effective against Gram-positive bacteria. The most common antibiotics used are fluoroquinolones or azithromycin. Recent years have seen the growth of antibiotic-resistant strains of E. coli, exacerbated by overexposure to antibiotics used on humans and growth promoters in animal feed. There is much current research on developing a vaccine for E. coli that can inoculate from future infections.
Credit: M. Jones via WikiCommons CC-BY SA 3.0
Genetics of E. coli
E. coli are some of the most genetically diverse known bacteria. It is estimated that only 20% of the genes in a typical E. coli strand are shared between strands. Many of the differences in E. coli are molecular, though these small differences often lead to changes in the physiology and life cycle of the cell. Escherichia and the closely related genus Salmonella diverged about 102 million years ago. This split was caused by the divergence of their hosts into mammals and reptiles/birds, which explains why E. coli is found in the former and Salmonella in the latter.
Over 300 strains of E. coli have had their complete genome sequenced. This has revealed that only 20% of gene sequences are shared between all strains, the other 80% varies considerably among strains. Further, it has been revealed that approximately two-thirds of the overall genome of all know strains of E. coli has been introduced by other bacteria via horizontal gene transfer.
In general, E. coli DNA is stored in a single chromosomal ring that contains about 4.6 million base pairs. It is very coding dense, with an average of 118 base pairs separating distinct genes, and has 4,288 protein-coding genes. The extremely dense coding contains several redundancies and remnants of RNA viruses.
The dense coding and fast growth rates of E. coli make them extremely useful in the lab for bioengineering applications. Recent trends in research have sought to engineer strains of E. coli to produce materials that have a high carbon footprint. For example, recent work has engineered E. coli that produce polymer plastics. It is believed that bioengineered E. coli and other bacteria could provide a virtually fossil fuel free way of producing materials.
Discovery of E. coli.
E. coli was first identified in 1885 by German-Austrian pediatrician and microbiologist Theodor Escherich. Escherich first discovered the bacteria in the feces of his patients as a part of his research identifying the role of gut flora in early infantile and childhood development. Escherich demonstrated that gut flora, including E. coli, is a critical part of digestion and that imbalances in gut flora can cause pathologies. He was also well known for his pioneering work in using X-rays as a diagnostic tool in children.