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H. pylori is a microaerophilic organism and a carcinogenic gastric pathogen(x) that lives in the mucosal membrane in the human stomach. It is the primary cause of gastritis, peptic ulcer disease, ulceration of the duodenum and stomach cancer. It is estimated that over 50% of the world's population is infected by H. pylori. Symptoms of infection range from acid reflux, oesophagitis and gastrointestinal discomfort to vomiting.
DNA (deoxyribonucleic acid) stores and transmits hereditary information. It is capable of replicating and directing RNA (ribonucleic acid) synthesis and, through RNA, controls protein synthesis. DNA is arranged into a double helix structure. Four types of nucleotides make up a DNA sequence: adenine, cytosine, guanine, and thymine (A,C,G,T)
How H. pylori Satisfies the Conditions for Life (Reproduction)
H. pylori is a living organism because it is capable of reproducing itself. It does this by a process called binary fission. Due to an increase in cell dimensions, septa appear at the circumference and proceed inwards, until two daughter cells separate.
The H. pylori bacterium can grow in the acidic environment of the human stomach. It achieves this by producing an enzyme- urease- which breaks down a waste product found in all human fluids called urea into its chemical components, ammonia and carbon dioxide. Ammonia, a strong base, reduces acidity in the stomach lining, thereby making its environment more habitable by achieving homeostasis.(x)
H. pylori has a high mutation rate and very high recombination frequency.(x)
It has been shown that H. pylori cells growing in broth media do consume oxygen(x), and therefore respire.
Ten strains of H. pylori require amino acids to grow, such as arginine, histidine, leucine, methionine, phenylalanine and valine. Five of the strains require serine. In the presence of glucose, alanine significantly enhances growth, which is consistent with its use as either a nitrogen source or possibly a carbon source.
Optimal growth occurs in the presence of 5-15% oxygen. Incubation in air reduces its rate of survival and it grows poorly under anaerobic conditions. The presence of 5% carbon dioxide provides optimal conditions, and 10% carbon dioxide led to a loss in cultivability in one study (79(5)455.pdf) Glucose is not necessary for growth, although survival is enhanced by the presence of pyruvate, succinate and citrate. Its optimal growth temperature is 30-37 degrees Celsius. Humans are the perfect growth medium for this pathogen, in particular, the stomach. There are no animal or environmental reservoirs for strains infecting humans.
H. pylori can survive below 30 degrees Celsius in some foods.
H. pylori is corkscrew shaped (although its morphology is not constant) perfectly designed for burrowing into the epithelium of the stomach. It is highly motile, and the reason for this is its flagellae, which propels it through the gastric acid. It has an outer cellular membrane that is impermeable to bile and digestive enzymes.
Blaser Lab Group
NYU School of Medicine
Above: H. pylori adhering to the gastric mucosa epithelium.
H. pylori Responds to Changes in its Environment
Epidemiological studies show that secondary risk factors affect the incidence of some gastric illnesses linked with H. pylori infection. In these studies, dietary sodium chloride intake increased the risk of severe disease outcomes. Supporting this claim, experiments conducted whereby animals were infected with H. pylori show that there is a synergistic effect between H. pylori and salt in terms of disease progression. The bacterium senses changes in salt concentration as a result of dietary intake and alters its gene expression and growth accordingly.
H. pylori Can Keep its Internal Environment Unchanged
H. pylori achieves homeostasis in the presence of urea, and can survive for a period of hours in this environment. The bacterium maintains its cytoplasmic pH at a value that is almost neutral. (pubmed) In order to live in the acidic environment of the stomach, the bacterium constantly synthesizes the enzyme urease catalyzing the hydrolysis of urea to ammonia and carbamate to elevate pH.
DNA is found in the nuclei of the cells in living organisms, with the exception of some RNA viruses. Deoxyribonucleic acid is a macromolecule built from chains of repeating units of the sugar deoxyribose and phosphate linked to four different bases: A, T, G, C. It is the genetic information that makes each individual unique.
DNA grows in a 5' to 3' direction. DNA polymerase parental strand in a 3' to 5' direction, and the new DNA strand grows in a 5' to 3' direction. The DNA polymerase directs this growth by forming hydrogen bonds between the newly added bases and the existing bases on the parental strand as well as by catalyzing the addition of a phosphodiester bond between the 5' phosphate group of a new nucleotide and the 3' hydroxyl that is free on reads along the the nascent strand.
By itself, DNA is a large molecule that contains genetic information that parents pass on to their children. It does not grow in the same way a child will grow to become an adult. DNA is not an organism. It is essential to life; however, it is not alive because it exists at the molecular level and not at the cellular level.
DNA is self- replicating, however, this does not mean that it reproduces like bacteria and other organisms. DNA does not reproduce asexually or sexually, another reason why it cannot be classified as a living organism. An enzyme called helicase plays a significant role in separating the DNA strands so that replication of a single strand can occur.
DNA can become damaged due to environmental toxins or exposure to radiation, and if the damage is not repaired this can lead to mutations that can cause disease. An example of the link between environmental- induced DNA damage and disease is skin cancer. This disease is caused by over-exposure to UV radiation from the sun.
The double helix Helicobacter pylori
H. pylori is a good example of a living organism because it reproduces and is perfectly adapted to its living conditions, evolving to live in the human stomach in the presence of hydrochloric acid. It is a tough pathogen to kill; it took Robin Warren a long time to recover after he self-infected with the bacterium (Warren & Marshall, 2005 Nobel Prize in Physiology or Medicine). H. pylori has even evolved a strategy to evade a host immune response. A bacterial cytotoxin, VacA is present in all H. pylori strains. Evidence suggests that VacA may be the reason why H. pylori can evade the adaptive immune response.
DNA cannot be considered a live organism for the reasons outlined in the text. Although it is not completely inert, it is not alive by the accepted definition of biology-
-all living things grow
-living things reproduce (DNA makes copies of all living things)
-they have the capacity to evolve
-they acquire materials and energy from the environment and convert it for use
-they maintain a constant environment e.g., body temperature, sweating, using food for heat
-respond to stimuli
DNA is important for every cell in a living organism; however, it does not make the cell alive, it transmits information.