Helicobacter Pylori its pathogenicity, virulence and treatment

  Characteristics of Helicobacter genus 

i) These bacteria are classified as Gram-negative organisms.

ii) They exhibit a helical, curved, or straight unbranched morphology.

iii) Their distinctive mode of rapid motility is achieved through the presence of sheathed flagella, which can be located at either one or both ends of the cell.

iv) these bacteria produce an external glycocalyx when cultured in liquid environments.

v) Notably, their major fatty acid profiles do not contain hexadecanoic acids.

vi) These bacteria exhibit optimal growth at 37°C, with varying growth rates observed between 30-42°C.

vii) They are microaerophilic, indicating a preference for low oxygen levels in their environment.

viii) These organisms are susceptible to several antibiotics, including penicillin, ampicillin, amoxicillin, erythromycin, gentamicin, kanamycin, rifampin, and tetracycline. However, they show resistance to nalidixic acid, cephalothin, metronidazole, and polymyxin.

ix) The chromosomal DNA of these bacteria is characterized by a GC content ranging from 35% to 44%.

Helicobacter Species Associated with Human Disease

Approximately 50% of the global population carries an H. pylori infection, and it is responsible for persistent chronic infections. Notably, it has been classified as a Group 1 carcinogen, with a significant association with the development of peptic ulcers (approximately 20% of cases) and gastric carcinoma (about 1% of cases).

Among the Helicobacter genus, H. pylori is the most extensively characterized and studied species.

Helicobacter Species Associated with Human Disease

•  Non-H. pylori helicobacters have recently been linked to various diseases.

For instance, H. bilis, which is commonly found in many mouse facilities, has been linked to conditions such as gastroenteritis, hepatitis, inflammatory bowel disease (IBD), and gallbladder cancers in multiple host species.

The prevalence of gastroenteritis, a condition often associated with bacterial infections, suggests a shared pathogenic mechanism in helicobacter infections. 

   

Helicobacter genus virulence

 Common virulence factors:

•       Adhesion factors

•       Cytolethal distending toxin (CDT)

•       Urease

•       gamma-glutamyl transpeptidase (gGT)

Helicobacter pylori virulence factors

            

Factors affecting gastric pathology during infection

Pathogenesis of Helicobacters

Diagnosis

•   Urea breathe test
•    histology
•    serology

Resistance to Oxidative stress by H pylori

pylori is microaerophilic to endure oxidative stress it expresses certain enzymes to combat superoxide stress defense mediated via the iron-cofactor superoxide dismutase (SodB) and the peroxide stress defense mediated via catalase (KatA) and alkyl hydroperoxide reductase

•       (AhpC).

•       In addition, two thioredoxins and their respective reductases, and the thiol-peroxidase Tcp mediate resistance to both nitrosative and oxidative stresses.

•       The neutrophil-activating protein (HP-NAP) or NapA protects bacterial DNA from the detrimental effects of reactive oxygen species. HP-NAP is also implicated in the activation of neutrophils, leading to the formation of reactive oxygen species.

•       The formation of reactive oxygen species is connected with iron metabolism, as oxygen radicals can be produced via iron oxides. So proteins involved in iron metabolism, like global iron-responsive regulator Fur, the FeoB iron transporter, the iron-storage protein ferritin, and the iron-cofactors SodB, are involved in oxidative stress resistance 

H. pylori is considered microaerophilic, allowing it to thrive in environments with lower oxygen levels. To cope with oxidative stress, the bacterium employs specific enzymes. It defends against superoxide stress through the iron-cofactor superoxide dismutase (SodB) and peroxide stress through catalase (KatA) and alkyl hydroperoxide reductase (AhpC).

Furthermore, the bacterium employs two thioredoxins and their respective reductases, as well as the thiol-peroxidase Tcp, to resist both nitrosative and oxidative stresses.

A protein known as neutrophil-activating protein (HP-NAP) or NapA plays a role in safeguarding bacterial DNA from the harmful effects of reactive oxygen species. HP-NAP also contributes to the activation of neutrophils, leading to the production of reactive oxygen species.

The generation of reactive oxygen species is intricately connected to iron metabolism, as oxygen radicals can be produced through interactions with iron oxides. Proteins involved in iron metabolism, including the global iron-responsive regulator Fur, the FeoB iron transporter, the iron-storage protein ferritin, and the iron-cofactors SodB, are crucial components of oxidative stress resistance.

Acid Resistance and Metabolism in H pylori 

H. pylori produces significant quantities of urease, an enzyme that facilitates the breakdown of urea into alkaline ammonia gas. This ammonia gas plays a crucial role in neutralizing the acidic pH of the stomach. Urease is an enzyme containing nickel and is composed of 12 UreA and 12 UreB subunits. The molecular masses of these subunits are 27 kDa and 62 kDa, respectively. The genes encoding UreA and UreB are part of an operon containing both ureA and ureB.

Within the urease gene cluster, there is a second operon located downstream of ureAB, which encodes the UreIEFGH proteins. These accessory proteins, UreEFGH, are responsible for aiding in the assembly of urease subunits and the incorporation of nickel into the active sites of urease. Additionally, the UreI protein serves as an acid-activated urea channel.

Another enzyme, arginase (RocF), plays a vital role in acid tolerance by converting L-arginine into L-ornithine and urea. Interestingly, mutations in the rocF gene do not impact urease activity but do significantly reduce the acid resistance of H. pylori.

The lipopolysaccharide (LPS) of H. pylori contains antigens with fucosylated oligosaccharides that closely resemble human blood group antigens in terms of their structural and immunological characteristics. These bacterial antigens, known as Lewis antigens, exhibit substantial antigenic variation and are believed to play a role in evading the host immune response.

Cag Pathogenicity island (cag PAI)  dependent pathogenesis 

 CagA protein, which has a molecular weight of 140 kDa, is a highly immunogenic protein encoded by the cagA gene. This gene is found in approximately 50 to 70% of H. pylori strains and serves as an indicator for the presence of a genomic Pathogenicity Island (PAI) of around 40 kb. Depending on the specific strain, this PAI encodes between 27 to 31 proteins.

Strains that carry the cag PAI are commonly referred to as CagA strains. They are frequently identified in patients who exhibit a more robust inflammatory response and are at a significantly higher risk of developing symptomatic conditions, such as peptic ulcers or gastric cancer, especially in Western populations, though less so in Asian populations.

The proteins encoded by the cag PAI play a crucial role in the formation of a type IV secretion apparatus, which takes on a syringe-like structure. This apparatus is capable of penetrating gastric epithelial cells, facilitating the translocation of CagA, peptidoglycan, and potentially other bacterial factors into host cells.

The CagA protein undergoes phosphorylation at tyrosine residues located in EPIYA motifs through Src family kinases. Once phosphorylated, CagA interacts with various host signaling molecules, such as the tyrosine phosphatase SHP-2, leading to morphological changes in epithelial cells.

Strains carrying CagA with a higher number of repeats induce more pronounced morphological changes in cultured epithelial cells and have been associated with an increased risk of gastric carcinogenesis.

Additionally, the cag PAI has implications for the immune response due to its capacity to induce apoptosis in T cells.

cag PAI dependant pathogenesis

H pylori vacuolating toxins (Vac A)

Approximately 50% of all H. pylori strains are known to secrete VacA, a highly immunogenic protein with a molecular weight of 95 kDa. In vitro, VacA induces significant vacuolization in epithelial cells. This protein plays a crucial role in the pathogenesis of both peptic ulcers and gastric cancer. While not essential for H. pylori growth in vitro, VacA has been reported to substantially contribute to the colonization of the murine stomach by H. pylori.

VacA's effects include the induction of membrane channel formation, disruption of endosomal and lysosomal activities, alterations in integrin receptor-induced cell signaling, interference with cytoskeleton-dependent cell functions, induction of apoptosis in gastric cells, and immune modulation.

Notably, there is considerable sequence heterogeneity within the vacA gene, particularly in the signal region (s) and the middle region (m). The s region encodes the signal peptide and occurs in two types, s1 and s2. Meanwhile, the m region, housing the p58 cell-binding domain, comes in two types as well, m1 and m2.

The vacuolating activity varies, being high in s1/m1 genotypes, intermediate in s1/m2 genotypes, and absent in s2/m2 genotypes. Correspondingly, vacA s1/m1 genotypes are more frequently associated with conditions like peptic ulceration and gastric carcinoma.

It's important to note that VacA has the ability to penetrate deeper tissues, allowing interaction with other relevant cell types such as granulocytes, monocytes, B cells, and T cells. This interaction with immune cells leads to the inhibition of antigen presentation and T-cell proliferation.

Oxidative stress caused by Hpylori

Immune modulation by H pylori

Immune modulation by H pylori

Immune evasion is mainly accomplished through:

•       Low immunogenic antigenic epitopes (O antigen in LPS, sheathed flagella, altering structures through phase variation)

•       Promoting tolerance through the inactivation of APCs and increase in T regs

•       Low T cell proliferation

• Anti-inflammatory responses

Treatment strategies

•       Previously, symptom control using antacids, proton pump inhibitors

•       The standard first-line therapy is a one-week "triple therapy"

proton pump inhibitors such as omeprazole 

antibiotics clarithromycin and amoxicillin.

•       Variations of the triple therapy, using a different proton pump inhibitor, as with pantoprazole or rabeprazole, or replacing amoxicillin with metronidazole for people who are allergic to penicillin.

•       quadruple therapy adds a bismuth colloid, such as bismuth subsalicylate.

•       lactic acid bacteria exerts a suppressive effect on Helicobacter pylori infection in both animals and humans