Helicobacter pylori
is a Gram negative spiral rod, campylobacter like bacteria which was observed
in close apposition to the gastric mucosa in several cases of gastritis and
peptic ulcer. It is a microaerophilic
bacterium identified for the first time in 1983 by Barry Marshall and Robin
Warren. They were originally named Campylobacter
pylori, but they differed in many respects from campylobacters, and hence
they have been redesignated as Helicobacter
pylori. The name Helicobacter pylori means
helical or spiral bacteria which inhabits in the pylorus region of stomach. Helicobacters inhabit the stomachs of different
animals, each with its own helicobacter species. The only animal Helicobacter pylori infects is the
monkey. H pylori is adapted to the
human gastric mucosa and hence it is also linked to the development of duodenal
ulcers and stomach cancer. However,
over 80 percent of individuals infected with the bacterium are asymptomatic and it has been postulated that it may play an important
role in the natural stomach ecology. More than 50% of the world's population
harbor H. pylori in their upper gastrointestinal tract. About10% of the infected individuals may develop cancer.
Infected persons are also at a high risk of developing lymphoma. Infection is
more prevalent in developing countries, and incidence is decreasing in Western
countries. H. pylori's
helix shape is thought to have evolved to penetrate the mucoid lining of the stomach. The H. pylori bacteria burrow into the cells of the
stomach lining and cause low grade inflammation of the stomach and duodenum.
Interestingly, it has been observed the presence
of the bacteria may decrease the prevalence of esophagitis by decreasing the
amount of stomach acid that refluxes back into the esophagus. This in turn
leads to a decreased risk for esophageal cancer in those infected with Helicobacter pylori. Further, H. pylori seems to decrease the risk of
developing asthma and allergies. H. pylori bacteria are found most frequently in
underdeveloped countries; but with improved economic conditions, the rate of
infection in the population decreases. The infection rate in the United States
is between 20%-30%. However, it is higher in Hispanics, African Americans, and
the elderly person.
Classification of Helicobacter pylori:
Domain:
Eubacteria.
Kingdom:
Bacteria.
Phylum:
Proteobacteria.
Class:
Epsilonproteobacteria.
Order:
Camphylobacterales.
Family:
Helicobacteraceae.
Genus:
Helicobacter.
Species:
Helicobacter pylori.
Morphology and Cultural chacteristics:
H. pylori is
a gram negative spiral rod, about 3 micrometres long with a diameter of about 0.5 micrometres,
motile by a unipolar tuft of lophotrichous flagella. It grows on Chocolate agar
or Campylobacter media under microaerophilic conditions, with 5-10% carbon
dioxide, and grows best at pH 6-7. However when accompanied by urea, which is
found in the digestive tract of humans, it can tolerate much lower pH levels
and the bacterium typically grows best at oxygen level of 5%, which is the
oxygen level found in the stomachs mucous layer. At 37 degree centigrade,
colonies take 2-7 days to develop. Coccoid forms appear in old cultures. It
produces oxidase, catalase, phosphatase and hydrogen sulfide. It contains a hydrogenase which can be used to obtain energy by oxidizing molecular hydrogen (H2) produced by intestinal bacteria. The most distinctive feature of H. pylori
is the production of abundant urease,
and this property has been used as a rapid diagnostic test in gastric biopsy
samples. It does not metabolise carbohydrates or reduce nitrate. H. pylori possesses five major outer
membrane protein (OMP) families.
It includes the largest family known as putative adhesins. The other four families include porins, iron transporters, flagellum-associated proteins and proteins of unknown function. Like
other typical Gram-negative bacteria, the outer membrane of H. pylori consists of phospholipids and lipopolysaccharide (LPS). The O
antigen of LPS may be fucosylated
and mimic Lewis blood
group antigens found on the gastric epithelium. The outer membrane also contains cholesterol glucosides, which are found in few other bacteria. H.
pylori has four to six
lophotrichous flagella; all gastric and enterohepatic Helicobacter species are highly motile due to
flagella. The characteristic
sheathed flagellar filaments of Helicobacter are composed of two copolymerized
flagellins, FlaA and FlaB.
