HISTORY
An Italian researcher : Bizzozero was the first, in 1893, to describe a spiral bacterium from the stomach of a dog, followed by Salomon in Germany in 1896. The bacteria probably correspond to what we now call H. heilmannii. Morphologically, they are more helical than spiral, but these two types have always been mixed up. Research on dogs has brought a renewed interest to these micro-organisms. Lockard, in particular, describes three types of organism he thought corresponded to three forms of a same species: a helical form without fibrillae (H. heilmannii), a helical form with fibrillae (H. felis) and a straight form with fibrillae ("Flexispira rappini").

The first spiral bacterium observed in the human stomach was identified undoubtedly by Krienitz in 1906. The work was not repeated until 1939 by Doenges. He found H. pylori type bacteria in the stomach of 101/242 subjects killed in road accidents and, in two cases, bacteria of the same type as those observed in the dog. Later, several authors observed H. pylori before the first culture in April 1982 which paved the way to understanding this particular group of micro-organisms.

Did intestinal mucus bacteria undergo a similar trajectory ? In 1886, Escherich described spiral bacteria isolated from human diarrhoeic stools, which one may consider to be Campylobacter jejuni. These bacteria were not cultured from diarrhoeic stools until 1973. Human cases were merely anecdotes until Butzler put forward a new method for isolating them from stools.

PROPERTIES

Bacteria which colonise the mucus share a certain number of characteristics that we shall now describe:

Spiral and flagellar morphology

• The spiral shape seems to be the best for mobility in mucus. The amplitude of the spirals increases as a function of viscosity to reach a large surface/volume ratio.
• Similarly, the existence of polar flagella is another mobility factor in mucus. All flagellar bacteria increase their velocity when the viscosity of their environment is slightly higher than that of water. Nevertheless, the velocity of bacilliform bacteria quickly decreases when the viscosity goes beyond 2 to 5 centipoise, cancelling out, with E. coli for example, at 20 centipoise. The viscosity of the environment causes configurational changes in the flagellum, which reduces the effectiveness of the propulsion. The Spirochaetes have solved the problem by using a mode of locomotion based on endocellular organelles. The spiral bacteria, like H. pylori or C. jejuni, have a flagellar system adapted to this particular environment and can move even when the viscosity reaches 200 centipoise. It appears that a change in the flagellum's configuration affects the impulse produced. The flagella may be single or multiple. Upon division, they are found at both ends of the bacterium. For certain bacteria such as H. felis, they are extensions of fibrillar structures surrounding the bacterial body, in the same way as is observed in Spirochaetes.
• It has been shown that reduced-mobility strains of both C. jejuni and H. pylori could not colonise their hosts.

Microaerophily

• The atmosphere at the mucus level covering the cells is microaerobic. Oxygen diffuses from the underlying tissues. Nearly all bacteria colonising the mucus have the property of growing at low oxygen pressures and, equally, many microaerophilic bacteria are mucosal bacteria.
• Microaerophilic bacteria are bacteria for which oxygen is both necessary (they use it for their respiratory energy) and harmful (they are killed by oxygen by-products and oxygen itself). They use oxygen as an electron acceptor but, virtually without exception, cannot grow at the air concentration of 21% oxygen. The optimum oxygen concentration depends on the species and even the strain in question. Certain microaerophilic bacteria can however grow anaerobically by using a different electron acceptor to oxygen such as fumarate, for example.
• Various mechanisms have been put forward to explain this microaerophily without knowing which one specifically applies to H. pylori : abnormally high sensitivity to, or excessive generation of, the toxic forms of oxygen (H2O2, O2, OH). The cause could be insufficient catalase or superoxide dismutase production or activity.

Urease production (stomach bacteria)

• The stomach is a special part of the digestive tract because of the acid production that goes on. Bacteria living in the stomach mucus have developed a system to fight against acidity. In practice, since a minuscule quantity of urea diffuses from the blood compartment, the bacteria use a urease that they produce in abundant quantities in order to release ammonia from the urea and thus buffer their environment.
• In addition to the phenotypic particularities described, there are genetic reasons for setting mucus bacteria apart. Vandamme has proposed classifying them as the "superfamily VI" of Gram-negative bacteria.
  The proposal is based on the results of molecular hybridisation data, largely between ribosomal RNA and DNA. It has since been confirmed by ribosomal RNA sequencing data.

CLASSIFICATION GENUS HELICOBACTER

Since the creation of the Helicobacter genus, numerous organisms have been discovered and added to it, and others have been reclassified. The stomach Helicobacter can be differentiated from those of the intestine.

Stomach Helicobacter
H. pylori

• H. pylori can be considered the number-one mucus bacterium because of its extensive distribution and its importance in human pathological disease.
  H. pylori officially entered the medical world once it was cultured. It took place in April 1982 in Perth, Australia. Associated with the discovery which contested the sterile-stomach dogma then revolutionised our ideas on stomach diseases are the names of Marshall and Warren.
• In vivo, H. pylori is strictly host-specific (humans), organ-specific (the stomach) and even tissue-specific (mucus-producing cells). Its presence is always accompanied by chronic inflammation : gastritis. Consequently, it could be said that it is probably the most widespread pathogenic bacterium in the world since an estimated 50% of all humans harbour it. Prevalence, however, is considerably higher in developing than in developed countries.
• Chronic gastritis is the precursor to most gastroduodenal diseases, ulcers especially, but also to gastric cancer and certain types of non-ulcer dyspepsia. It is a necessary condition although not a cause on its own. Other factors as well as the infection, essentially environmental or host-related, must be included in the global picture. Here, we find the known causes for these diseases : smoking habits, stress, certain blood group associations for ulcer disease, and a diet too rich in salt and low in vitamins for gastric cancer. It is equally possible that not all strains have the same pathogenic properties. Greater inflammation seems to be linked to the presence of a marker known as CagA. H. pylori lives in the mucus, with about 20% of the bacterial bodies adhering to the mucous cells. Adherence disturbs exocytosis of mucus granules. In the event of infection, the mucus thickness decreases and its properties, hydrophobicity in particular, are altered. This seems to be due to phospholipases. The qualitative and quantitative changes do not seem to alter the ecological niche that gastric mucus represents for H. pylori.
• Nevertheless, if the pH changes (through antisecretory treatment for example), a redistribution of the bacteria may be observed; they leave the too-alkaline antral mucus for the now-more-hospitable fundus.

The physiological response to the secretagogues is a sequence of events: an external signal acts on a specific receptor which acts on regulation G-proteins, on secondary messages such as cyclic AMP or calcium which brings about the movement of mucus granules towards the luminal cell membrane. The pharmacological agents act directly on the intracellular messengers. Dotted line indicates H. pylori inhibitory action.

H. felis

• This bacterium is mainly found in the stomach of cats and corresponds to one of the morphological aspects described in the past. It can be grown on culture medium. It can also be transferred into the stomach of mice where it elicits gastritic lesions of varying intensity according to the species of mouse involved. Under the electron microscope, fibrillae surrounding the spiral bacterial body prolonging themselves by flagella may be seen. H. felis produces a urease.
• Although the bacterium has not been encountered in man, it is an important species because the mouse infected by this bacterium is used as a model for anti-H. pylori vaccination trials.

H. heilmannii

• In the past, this bacterium has been described in the stomach of cats, dogs and monkeys. Its morphology is similar to that of H. felis but it does not have fibrillae. It was also observed by Doenges in man before being rediscovered by Dent et al. (three cases) and baptised Gastrospirillum hominis. It is urease-positive.
• The bacterium is not cultivable, but methods of molecular biology (16S ribosomal RNA sequencing) have allowed its classification among bacteria of the genus Helicobacter.
• A very similar bacterium, named Gastrospirillum suis first of all, has also been described in the pig. It seems to be present in most of these animals. It was recently shown that strains of this species found in humans most often corresponded to G. suis and, occasionally, to the bacterium found in the cat or dog. The frequency of isolation of this bacterium is perhaps underestimated. Mazzuchetti et al. demonstrated H. heilmannii to be present in 1.1% of asymptomatic subjects.
• Research by systematically seeding gastric biopsies into the mouse stomach should offer a clearer picture.
• The clinical cases described show that H. heilmannii was the cause of one gastritis and that it could sometimes cause ulcers. Similarly, H. heilmannii-infected pigs developed ulcers, serving as the first model of ulcerogenesis to be described.

H. mustelae

• H. mustelae is a bacterium colonising the ferret stomach where it induces gastritic lesions. It seems to be closely host-specific like H. pylori. It also has the particularity of adhering strongly to the epithelial cells. H. mustelae can be cultivated on the usual media. Its morphology is a relative exception because it sometimes occurs as a bacillus shape.
• Although the bacterium has never been isolated in man, it is still of value because the H. mustelae- infected ferret can also be used as a model for vaccination against a stomach bacterium, as with H. felis in the mouse.

H. nemestrinae

• This bacterium has been isolated from the stomach of a macaque monkey (M. nemestrinae) in which it did not seem to elicit gastritis. It is very close to H. pylori. Very few strains are available.

H. acinonyx

• Close to H. pylori, this bacterium has been isolated from the stomach of cheetahs suffering from gastritis.

Intestinal Helicobacter
H. muridarum

• This is without doubt the first Helicobacter to be isolated; it was cultured from a mouse intestine. At first, the bacterium was not classified. It was only after the discovery of H. pylori and description of the new genus that it was included in the Helicobacter genus. The bacterium has a urease whose optimal activity is not at stomach pH, unlike the species described above. Nevertheless, it can invade the mouse stomach when its acidity decreases.

H. hepaticus

• This is another mouse intestine bacterium, but one which can also colonise the bile ducts, and cause hepatitis complicated with a carcinogenic process. The bacterium was discovered recently. Infection by H. hepaticus is a very interesting model of hepatic carcinogenesis.

H. cinaedi

• H. cinaedi was first classified among the Campylobacter. The bacterium is part of the hamster's intestinal flora.
• H. cinaedi was isolated in human stools and in hemocultures. Interestingly, affected subjects were virtually always homosexual, and sometimes immunodepressed.
• The bacterium does not produce urease.

H. fennelliae

• H. fennelliae is similar to the preceding, as were the circumstances of its isolation in man, but no reservoir has yet been identified.

H. canis

• H. canis lives in the canine intestine and can cause diarrhoea in humans.

H. pametensis

• This recently described species of Helicobacter has been isolated from wild birds such as terns and seagulls. Its medical importance is unknown.

"Flexispira rappini"

• This bacterium has still not been rebaptised Helicobacter despite the fact that genome studies classify it in this genus. It has been isolated from sheep and mice, and causes diarrhoea.

GENUS CAMPYLOBACTER

• Campylobacter spp. were the first known mucus bacteria, even though nobody realised the particularity of their ecological niche at the time. The bacteria live in the digestive tract mucus of man and animals.
• The species C. jejuni and C. coli seem to be part of the normal flora of birds, poultry in particular, without being obligatory. They can also colonise farm animals and man. C. coli has a porcine reservoir as well. C. lari is essentially found in seagulls from which dissemination is possible. C. upsaliensis is a bacterium found in the intestine of dogs and C. fetus in birds and cattle.
• Several species of Campylobacter are also physiologically present in the buccal cavity of man : C. sputorum, C. concisus, C. rectus, C. curvus. These species may, on rare occasions, cause infections in man.

GENUS ARCOBACTER

• Arcobacter are close relatives of Campylobacter. They have been grouped off and, along with Campylobacter, form the family Campylobacteraceae.
• A. butzleri is without doubt the most commonly isolated bacterium of this group in man.

GENUS WOLINELLA

• W. succinogenes is currently the only species. It is found in the rumen of cattle. It is also urease +.

TO CONCLUDE

The great interest in H. pylori these past years has taught us that there is a wide variety of often-unknown bacteria. We are at the beginning of a new age, one running alongside the development of sophisticated analytical methods of molecular biology. The foreseeable future will certainly furnish us with the opportunity of seeing this group of bacteria expand even further.


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