Helicobacter spp

1.6.1. Species characteristics

Helicobacter was first described in 1984, by Marshall & Warren, who

demonstrated the presence of unidentified curved bacilli in the gastric epithelium of 58 patients with active chronic gastritis or ulceration. Initially, the bacterium was classified as a new species in the Campylobacter genus; however, in 1989 the new genus „Helicobacter‟ was established (Goodwin et al., 1989). Since

classification, the Helicobacter genus now includes 18 different species (Table 1.1).

Helicobacter spp. are gram-negative, spiral-shaped, actively motile bacteria with

flagella that colonize the gastrointestinal tract of both humans and animals (De Bock et al., 2006). Helicobacter spp. are urease positive, enabling them to survive the highly acidic environment in the stomach (Kusters et al., 2006). H. pylori has complex growth requirements and is therefore often hard to culture. It requires specialised growth medium, supplemented with blood or serum, and a microaerophilic environment, with optimal growth at 2-5% O2/5-10% CO2 and

high humidity levels (Kusters et al., 2006). In older cultures, H. pylori tends to transform from its normal helical bacillary morphology into a coccoid form (Owen, 1998).

Species Major Host

H. pylori Human

H. canis Dog, Human

H. felis Cat, Dog, Human

H. heilmannii Cat, Dog, Human

H. bizzozeronii Dog

H. fennelliae Human

H. pullorum Poultry, Human

H. pametensis Wild birds, Pig

H. cholecystus Hamster

H. hepaticus Mice

H. muridarum Rat, Mice

H. trogontum Rat

H. bilis Mice, Dog

Flexispira rappini Dog, Pig, Sheep

H. cinaedi Human, Hamster

H. acinonyx Cheetah

H. mustelae Ferret

H. nemestrinae Macaque monkey

H. suis Pig

Table 1.1- The Helicobacter species and their major hosts

1.6.2. Clinical importance and pathogenicity

Nearly half of the world population are estimated to be infected with Helicobacter

pylori, however many patients remain asymptomatic (Go, 2002). Helicobacter is

thought to be transmitted via person-person contact, as clustering of infection has been found within families (Drumm et al., 1990) and institutionalised young people (Laporte et al., 2004). H. pylori organisms colonise the gastric epithelium and rarely penetrate into cells, tending to cluster around intracellular junctions (Peterson, 1991). More recently, intracellular localisation of Helicobacter spp. has been described in the parietal cells and macrophages of laboratory Beagle dogs (Lanzoni et al., 2011). Gastric colonisation with H. pylori stimulates cytokine secretion from gastric epithelial cells, leading to activation of immune and inflammatory cells and the development of chronic, active gastritis (Ernst & Gold, 2000), which can ultimately lead to gastric ulceration. In cases of acute gastritis,

H. pylori infection leads to the degeneration of surface epithelial cells, resulting in decreased mucin secretion and an infiltration of neutrophils. In chronic cases of gastritis, as well as epithelial degeneration and neutrophil infiltration, an influx of lymphocytes and plasma cells leads to chronic inflammation (Dixon, 1995), however, even when gastritis is present, only a minority of patients go on to develop clinical signs of disease (Kusters et al., 2006). Acute Helicobacter

infections result in antral inflammation and hypochlorhydria, thought to be caused by inhibition of parietal cell function. H. pylori infection has been shown to inhibit H+/K+ATPase α-subunit gene expression in AGS cells (Göoz et al., 2000).

H. pylori associated gastritis may also result in gastric acid hypersecretion,

through the stimulation of gastrin release (McColl et al., 1998), thus patients colonised with H. pylori may present with either increased or decreased acid secretion (Schubert & Peura, 2008).

In 0.5-2% of H. pylori infections, patients go on to develop gastric cancer (Fritz & Van der Merwe, 2009) and both gastric lymphoma and adenocarcinomas are associated with H. pylori infection. Thus in 1994, H. pylori was classified as a type I carcinogen in humans by the International Agency for Research on Cancer (IARC, 1994). Clinical outcomes of H. pylori infection are mainly determined by the virulence factor of the bacterial strain and the most widely studied H. pylori

virulence factor is CagA, encoded for by the cagA gene. Infection with strains of

H. pylori carrying the cagA gene is associated with the development of gastric

carcinoma (Hatakeyama, 2004). Pathogenic strains of H. pylori may also produce the vacuolating cytotoxin, VacA, encoded for by the vacA gene (Wada et al., 2004), which has been shown to cause cytoplasmic vacuolation in MKN-28 cells, (Ricci et al., 2002) and in primary human gastric epithelial cells (Smoot et al., 1996).

1.6.3. Helicobacter spp. found in dogs

Several species of Helicobacter are capable of infecting dogs, for instance, H.

felis, H. bizzozeronii, H. salmonis, H. bilis, H. heilmannii and Flexispira rappini

have all been cultivated from canine stomachs (Eaton et al., 1996; Jalava et al., 1998; Neiger et al., 1999). However, H. pylori has not been found in dogs (Neiger & Simpson, 2000), thus suggesting that dogs may not present a zoonotic risk for

this common strain of the bacteria. Previous studies have shown that living conditions can impact on the prevalence of Helicobacter infection, as dogs housed in animal shelters or bred in laboratory colonies were shown to have a higher prevalence of Helicobacter infection than pet dogs (Eaton et al., 1996).

Helicobacter spp. are highly prevalent in both healthy and unwell dogs,

suggesting that there is not a clear link between the infection and gastric disease in dogs (Simpson et al., 2000). Helicobacter infected dogs usually present with mononuclear gastritis of mild to moderate severity and no association between

Helicobacter infection and gastric ulceration or neoplasia in dogs has been made

to date (Simpson et al., 2000).

1.6.4. Helicobacter infection and NSAID treatment

As Helicobacter infected patients can remain asymptomatic, clinicians may

unknowingly treat them for other conditions using NSAID therapy. Individually, both NSAIDs and Helicobacter infection are major causes of gastrointestinal disease, however, little is known about their potential interaction. In a study looking at long-term NSAID users, the risk of peptic-ulcer disease was significantly higher in H. pylori-positive patients than in H. pylori-negative patients (Huang et al., 2002), suggesting a possible interaction between the two. Several studies have shown that H. pylori infection leads to increased COX-2 expression and PGE2 synthesis (Fu et al., 1999; Romano et al., 1998; Chan et al.,

2001) that is reduced following eradication (McCarthy et al., 1999). COX-2 expression was found to be localised at higher levels in the gastric antrum where

H. pylori density is greatest (Fu et al., 1999). COX-1 expression also appears to be

higher in biopsies from H. pylori-positive patients compared to non-infected patients (Franco et al., 1999). As H. pylori infection and NSAID therapy have opposing effects on PGE2 production, an interaction between the two is likely and

warrants further study.

1.6.5.Helicobacter infection and cell migration

In addition to causing direct gastric mucosal damage, colonisation with H. pylori

may lead to changes in the modulation of gastric epithelial cell migration, and a number of different mechanisms for this effect have been studied. Focal adhesions

and the actin cytoskeleton both have a major role in cell migration, for instance, focal adhesion assembly and disassembly helps to coordinate cell movement across the ECM and actin polymerisation is essential for cellular protrusion and contraction (Ridley et al., 2003). H. pylori-infected gastric epithelial cells typically appear elongated, which may be caused by integrin/CagA signalling leading to stabilisation of focal adhesions at the rear of the cell (Wessler et al., 2011). H. pylori has also been shown to stimulate host cell motility via CagA interactions with SHP2, a protein-tyrosine phosphatase, and PAR1, a polarity- regulating kinase (Kikuchi et al., 2012). Furthermore, H. pylori VacA has been shown to inhibit re-epithelisation, possibly though the alteration of cytoskeleton associated proteins (Pai et al., 2000), while H. pylori adhesion to gastric epithelial cells has been associated with disruption of epithelial adhesion molecules, including a reduction in adherens junctions (Conlin et al., 2004) and induction of MMP-7 expression (Wroblewski et al., 2003).