Virulence factors help a pathogen to evade the immune response of the host, and to successfully
colonize. The many virulence factors of
H. pylori include its flagella, the production of urease, adhesins,
serine protease HtrA (high temperature requirement A), and the major exotoxins
CagA and
VacA. The presence of VacA and CagA are associated with more
advanced outcomes. CagA is an oncoprotein associated with the development of gastric cancer. It has been shown that expression of two DNA repair proteins,
ERCC1 and
PMS2, was severely reduced once
H. pylori infection had progressed to cause
dyspepsia. Dyspepsia occurs in about 20% of infected individuals. Epigenetically reduced protein expression of DNA repair proteins
MLH1,
MGMT and
MRE11 are also evident. Reduced DNA repair in the presence of increased DNA damage increases carcinogenic mutations and is likely a significant cause of gastric carcinogenesis. These
epigenetic alterations are due to
H. pylori-induced
methylation of CpG sites in promoters of genes and involves enhancement of the transformed host cell phenotype by means of alterations in cell proteins, such as
adhesion proteins.
H. pylori has been proposed to induce inflammation and locally high levels of
tumor necrosis factor (TNF), also known as tumor necrosis factor alpha (TNFα)), and/or
interleukin 6 (IL-6). According to the proposed perigenetic mechanism, inflammation-associated signaling molecules, such as TNF, can alter gastric epithelial cell adhesion and lead to the dispersion and migration of mutated epithelial cells without the need for additional mutations in
tumor suppressor genes, such as genes that code for cell adhesion proteins.
Flagellum The first virulence factor of
Helicobacter pylori that enables colonization is its
flagellum.
H. pylori has from two to seven flagella at
the same polar location which gives it a high motility. The flagellar filaments are about 3 μm long, and composed of two copolymerized
flagellins, FlaA and FlaB, coded by the genes
flaA, and
flaB. Occasionally the bacteria are found inside the epithelial cells themselves. The use of
quorum sensing by the bacteria enables the formation of a biofilm which furthers persistent colonisation. In the layers of the biofilm,
H. pylori can escape from the actions of antibiotics, and also be protected from host-immune responses. In the biofilm,
H. pylori can change the flagella to become adhesive structures.
Urease enzyme diagram In addition to using
chemotaxis to avoid areas of high acidity (low pH),
H. pylori also produces large amounts of
urease, an
enzyme which breaks down the
urea present in the stomach to produce
ammonia and
bicarbonate, which are released into the bacterial cytosol and the surrounding environment, creating a neutral area. The decreased acidity (higher pH) changes the mucus layer from a gel-like state to a more viscous state that makes it easier for the flagella to move the bacteria through the mucosa and attach to the gastric epithelial cells. 10% of the cell is of
nitrogen, a balance that needs to be maintained. Any excess is stored in urea excreted in the urea cycle. Ammonia reduces stomach acidity, allowing the bacteria to become locally established. Arginase promotes persistent infection by consuming arginine; macrophages use arginine to produce nitric oxide, which has a strong antimicrobial effect. The ammonia produced to regulate
pH is toxic to epithelial cells.
Adhesins H. pylori must make attachment with the epithelial cells to prevent its being swept away with the constant movement and renewal of the mucus. To give them this adhesion,
bacterial outer membrane proteins as virulence factors called
adhesins are produced. BabA (blood group antigen binding adhesin) is most important during initial colonization, and SabA (sialic acid binding adhesin) is important in persistence. BabA attaches to glycans and mucins in the epithelium. Adherence via BabA is acid sensitive and can be fully reversed by a decreased pH. It has been proposed that BabA's acid responsiveness enables adherence while also allowing an effective escape from an unfavorable environment, such as a low pH, that is harmful to the organism. SabA (coded for by the
sabA gene) binds to increased levels of
sialyl-Lewis X antigen expressed on gastric mucosa.
Cholesterol glucoside The outer membrane contains
cholesterol glucoside, a sterol glucoside that
H. pylori glycosylates from the
cholesterol in the gastric gland cells, and inserts it into its outer membrane. This cholesterol glucoside is important for membrane stability, morphology and immune evasion, and is rarely found in other bacteria. The enzyme responsible for this is
cholesteryl α-glucosyltransferase (αCgT or Cgt), encoded by the
HP0421 gene. A major effect of the depletion of host cholesterol by Cgt is to disrupt cholesterol-rich
lipid rafts in the epithelial cells. Lipid rafts are involved in cell signalling and their disruption causes a reduction in the immune-inflammatory response, particularly by reducing
interferon gamma. Cgt is also secreted by the type IV secretion system, and is secreted selectively so that gastric niches where the pathogen can thrive are created.
H. pylori can adhere to the surface of the phagocytes and impede their action. Phagocytes respond by generating and releasing oxygen metabolites into the surrounding space.
H. pylori can survive this response by the activity of
catalase at its attachment to the phagocytic cell surface. Catalase decomposes hydrogen peroxide into water and oxygen, protecting the bacteria from toxicity. Catalase has been shown to almost completely inhibit the phagocytic oxidative response.
Tipα TNF-inducing protein alpha (Tipα) is a carcinogenic protein encoded by
HP0596 unique to
H. pylori that induces the expression of
tumor necrosis factor. Tipα enters gastric cancer cells where it binds to cell surface
nucleolin, and induces the expression of
vimentin. Vimentin is important in the
epithelial–mesenchymal transition associated with the progression of tumors.
CagA CagA (cytotoxin-associated antigen A) is a major
virulence factor for
H. pylori, an
oncoprotein that is encoded by the
cagA gene. Bacterial strains with the
cagA gene are associated with the ability to cause ulcers, MALT lymphomas, and gastric cancer. The
cagA gene codes for a relatively long (1186-
amino acid) protein. The
cag pathogenicity island (PAI) has about 30 genes, part of which code for a complex
type IV secretion system (T4SS or TFSS). The low
GC-content of the
cag PAI relative to the rest of the
Helicobacter genome suggests the island was acquired by
horizontal transfer from another bacterial species. The virulence of
H. pylori may be increased by genes of the
cag pathogenicity island; about 50–70% of
H. pylori strains in Western countries carry it. Western people infected with strains carrying the
cag PAI have a stronger inflammatory response in the stomach and are at a greater risk of developing peptic ulcers or stomach cancer than those infected with strains lacking the island. The type-IV
secretion apparatus also injects the
cag PAI-encoded protein CagA into the stomach's epithelial cells, where it disrupts the
cytoskeleton, adherence to adjacent cells, intracellular signaling,
cell polarity, and other cellular activities. Once inside the cell, the CagA protein is
phosphorylated on
tyrosine residues by a host cell membrane-associated
tyrosine kinase (TK). CagA then allosterically activates
Shp2, a
protein tyrosine phosphatase and
protooncogene. These proteins are directly toxic to cells lining the stomach and signal strongly to the immune system that an invasion is underway. As a result of the bacterial presence, neutrophils and macrophages establish residence in the tissue to fight the bacterial assault. Pathogenic strains of
H. pylori have been shown to activate the
epidermal growth factor receptor (EGFR), a
membrane protein with a TK
domain. Activation of the EGFR by
H. pylori is associated with altered
signal transduction and
gene expression in host epithelial cells that may contribute to pathogenesis. A
C-terminal region of the CagA protein (amino acids 873–1002) has also been suggested to be able to regulate host cell
gene transcription, independent of protein tyrosine phosphorylation. All strains of
H. pylori carry this gene but there is much diversity, and only 50% produce the encoded cytotoxin. VacA is an oligomeric protein complex that causes a progressive vacuolation in the epithelial cells leading to their death. VacA has been shown to increase the levels of
COX2, an up-regulation that increases the production of a
prostaglandin indicating a strong host cell inflammatory response.
Outer membrane proteins and vesicles About 4% of the genome encodes for
outer membrane proteins that can be grouped into five families. The largest family includes
bacterial adhesins. The other four families are
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. A
Helicobacter pylori virulence factor
DupA is associated with the development of duodenal ulcers.
Mechanisms of tolerance The need for survival has led to the development of different mechanisms of tolerance that enable the persistence of
H. pylori. These mechanisms can also help to overcome the effects of antibiotics. All organisms encode genetic programs for response to stressful conditions including those that cause DNA damage.
H. pylori is naturally competent for transformation. While many organisms are competent only under certain environmental conditions, such as starvation,
H. pylori is competent throughout logarithmic growth.
Transformation (the transfer of DNA from one bacterial cell to another through the intervening medium) appears to be part of an adaptation for
DNA repair. An overall response to multiple stressors can result from an interaction of the mechanisms. Similarly, RecN protein plays an important role in DSB repair. An
H. pylori recN mutant displays an attenuated ability to colonize mouse stomachs, highlighting the importance of recombinational DNA repair in survival of
H. pylori within its host. By changing the shape of the bacterium to a coccoid form, the exposure of
LPS (targeted by antibiotics) becomes limited, and so evades detection by the immune system. It has also been shown that the
cag pathogenicity island remains intact in the coccoid form. Some of these antibiotic-resistant cells may remain in the host as
persister cells. Following eradication, the persister cells can cause a recurrence of the infection. Bacteria can detach from the biofilm to relocate and colonize elsewhere in the stomach to form other biofilms. ==Diagnosis==