Helicobacter is a genus of Gram-negative bacteria possessing a characteristic helix shape. They were initially considered to be members of the Campylobacter genus, but since 1989 they have been grouped in their own genus. The Helicobacter genus belongs to class Epsilonproteobacteria, order Campylobacterales, family Helicobacteraceae and already involves >35 species.
Some species have been found living in the lining of the upper gastrointestinal tract, as well as the liver of mammals and some birds. The most widely known species of the genus is H. pylori which infects up to 50% of the human population. Some strains of this bacterium are pathogenic to humans as it is strongly associated with peptic ulcers, chronic gastritis, duodenitis, and stomach cancer. It also serves as the type species of the genus.
Helicobacter spp. are able to thrive in the very acidic mammalian stomach by producing large quantities of the enzyme urease, which locally raises the pH from ~2 to a more biocompatible range of 6 to 7. Bacteria belonging to this genus are usually susceptible to antibiotics such as penicillin, are microaerophilic (optimal oxygen concentration between 5 - 14%) capnophiles, and are fast-moving with their flagella.
Comparative genomic analysis has led to the identification of 11 proteins which are uniquely found in members of the family Helicobacteraceae. Of these proteins, 7 are found in all species of the family, while the remaining 4 are not found in any Helicobacter strains and are unique to Wollinella. Additionally, a rare genetic event has led to the fusion of the RpoB and RpoC genes in this family, which is characteristic of this family.
Non-pylori Helicobacter species
Recently, new gastric (Helicobacter suis and Helicobacter baculiformis) and enterohepatic (Helicobacter equorum) species have been reported. Helicobacter pylori is of primary importance for medicine; however, non-pylori Helicobacter species (NPHS), which naturally inhabit mammals (except humans) and birds, have been detected in human clinical specimens. NPHS encompass two (gastric and enterohepatic) groups, showing different organ specificity. Importantly, some species such as Helicobacter hepaticus, Helicobacter mustelae and, probably, Helicobacter bilis exhibit carcinogenic potential in animals. NPHS harbour many virulence genes and may cause diseases not only in animals but also in humans. Gastric NPHS such as H. suis (most often), Helicobacter felis, Helicobacter bizzozeronii and Helicobacter salomonis have been associated with chronic gastritis and peptic ulcers in humans and, importantly, with higher risk for MALT lymphoma compared to H. pylori. Enterohepatic species e.g., H. hepaticus, H. bilis and Helicobacter ganmani have been detected by PCR in but still not isolated from specimens of patients with hepatobiliary diseases. Moreover, NPHS may be associated with Crohn's disease, inflammatory bowel disease and ulcerative colitis. The significance of avian helicobacters (Helicobacter pullorum, Helicobacter anseris and Helicobacter brantae) also has been evaluated extensively. NPHS such as Helicobacter cinaedi and Helicobacter canis can cause severe infections, mostly in immunocompromised patients with animal exposure. Briefly, the role of NPHS in veterinary and human medicine is increasingly recognised. However, despite the growing interest in the possible association between NPHS and the chronic hepatobiliary or intestinal diseases in humans, more studies are still required to prove the suggested association. Several other topics such as isolation of still uncultured species, antibiotic resistance and treatment regimens for NPHS infections and, last but not least, NPHS pathogenesis and possible carcinogenesis in humans should be additionally evaluated.
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