PROTEOBACTERIA: CHARACTERISTICS, SUBPHILES, PATHOGENESIS - BIOLOGY - 2025

The proteobacteria are bacterial phylum larger, complex and diverse among prokaryotes. It comprises about 384 genera and 1,300 species of gram-negative bacteria with a cell wall consisting mainly of lipopolysaccharides.

In humans, proteobacteria are present on the skin, oral cavity, tongue, and vaginal tract, in addition to the intestine and feces. Proteobacteria are one of the most abundant phyla in the human intestinal microbiota.

Group of E. coli (Gammaproteobacteria). By Photo by Eric Erbe, digital colorization by Christopher Pooley, both of USDA, ARS, EMU., via Wikimedia Commons

The increase in the normal proportions of bacteria of this phylum compared to others (Bacteroidetes and Firmicutes) is associated with intestinal and extraintestinal diseases, mainly with an inflammatory phenotype.

Proteobacteria include a wide variety of pathogens, such as the Brucella and Rickettsia genera belonging to the Alphaproteobacteria class, Bordetella and Neisseria from the Betaproteobacteria class, Escherichia, Shigella, Salmonella and Yersinia from the Gammaproteobacteria class and, finally, Helicobacter from the class Epsilonproteobacteria.

In addition to pathogens, the phylum proteobacteria includes mutualistic species such as obligate endosymbionts of insects, including the genera Buchnera, Blochmannia, Hamiltonella, Riesia, Sodalis, and Wigglesworthia.

Recent studies have concluded that symbiotic proteobacteria have evolved in most cases from parasitic ancestors, which is consistent with the paradigm that bacterial mutualists often evolve from pathogens.

characteristics

The bacteria of this phylum are diverse morphologically, physiologically and ecologically. Its name is derived from the ancient Greek god of the sea Proteus, who had the ability to assume many different forms, alluding to the great diversity of forms of the bacteria gathered in this taxa.

Cells can be in the form of bacilli or cocci, with or without prosteca, flagellated or not, and only some species can form fruiting bodies. They can be nutritionally phototrophic, heterotrophic, and chemolithotrophic.

Subphiles

Based on the phylogenetic analysis of the 16S rRNA gene, the proteobacteria phylum is divided into 6 classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Epsilonproteobacteria and Zetaproteobacteria.

All classes are monophyletic, except the gammaproteobacteria which are paraphyletic with the Betaproteobacteria.

Alphaproteobacteria

The Alphaproteobacteria class includes 13 orders of bacteria. They can adopt various morphologies such as stalked, starry and spiral. They can also form stems and buds, which allows them to increase their surface-to-volume ratio, allowing them to survive in environments with few nutrients.

Alphaproteobacteria exhibit a great diversity of metabolic strategies such as photosynthesis, nitrogen fixation, ammonia oxidation, and methylotrophy. The most abundant marine cellular organisms are included in this group.

Many species of this class of bacteria tend to adopt an intracellular lifestyle as mutualists of plants or pathogens of plants or animals, such as Rhizobim, which forms with the roots of some species of plants or Wolbachia, a parasite of the common mosquito.

Alphaproteobacteria have also been associated with the ancestral group that gave rise to mitochondria, the Rickettsiales. Other genera, such as Rickettsia, are pathogens.

Betaproteobacteria

Betaproteobacteria are formed by 14 orders of bacteria that present a diversity of forms and metabolisms. They can be strict or facultative aerobic.

Some species can be chemoautotrophic, such as the genus Nitrosomonas, which is an oxidizer of ammonia. Others are phototrophs like Rhodocyclus and Rubrivivax, which use light as an energy source.

The betaproteobacteria intervene in the fixation of nitrogen, through the oxidation of ammonia, producing nitrite, a very important compound in plant physiology.

Other species may be pathogenic within this group, such as Neisseriaceae (which cause gonorrhea and meningitis), Ralstonia, a plant pathogen of nightshades (tomato, potato), and Burkholderia glumae, which causes damage to the panicle in the rice cultivation.

Deltaproteobacteria

Deltaproteobacteria group 7 orders of gram-negative bacteria. They are anaerobic and are commonly isolated in sediments of lakes, swamps and sea beds. They are sulfate reducers and participate in the natural sulfur cycle.

This class includes bacteria that predate other bacteria, such as the species of the genera Bdellovibrio and Myxococcus. Myxobacteria emit spores and group in multicellular fruiting bodies in food-limited environments. These constitute the most complex group of bacteria

Epsilonproteobacteria

The epsilonproteobacteria include only one order of gram-negative bacteria. They are shaped like thin helical or curved bars. Some species are symbionts of the digestive tract of animals, others are parasites of the stomach (Helicobacter spp.) Or the duodenum (Campylobacter spp.).

Bacteria in this group inhabit microaerophilic or anaerobic environments, such as deep-sea hydrothermal vents. They are chemolytotrophic, because they obtain their energy from the oxidation of reduced sulfur or hydrogen coupled to the reduction of nitrate or oxygen. Others are autotrophic and use the reverse Krebs cycle to fix carbon dioxide in biomass.

Pathogeny

Because proteobacteria are the phylum of bacteria with the largest number of species and the most complex and diverse, it includes a wide variety of pathogens.

Escherichia coli

These bacteria are excreted in the feces of infected animals and can survive in the environment for up to three days.

E. coli colonizes a new host through the fecal-oral route, by ingesting raw food or contaminated water, adhering to intestinal cells and causing diarrhea in affected people.

Fecal bacteria can colonize the urethra and spread through the urinary tract to the bladder and kidneys or prostate in men, causing urinary tract infection.

When a specific strain of E. coli, which contains a capsular antigen called K1, colonizes the newborn's intestines through the contaminated mother's vagina, bacteremia occurs, leading to neonatal meningitis.

In rarer cases, virulent strains are also responsible for hemolytic-uremic syndrome, peritonitis, mastitis, septicemia, and pneumonia.

Salmonella

Once S. enterica enters a new host, it begins its cycle of infection through lymphoid tissue. The bacteria adhere to the intestinal epithelial cells of the ileum and the M cells, inducing in them a rearrangement of their cytoskeleton that triggers the formation of large undulations on the surface allowing non-selective endocytosis, for which the bacteria manage to enter the cell.

Likewise, Salmonella produces cytotoxic effects that destroy M cells and induce apoptosis in activated macrophages and phagocytosis in non-activated macrophages, for which they are transported to the liver and spleen, where they multiply.

In humans S. enterica can cause two diseases: typhoid fever, caused by S. enterica sub. enterica Paratyphi serotypes or salmonellosis caused by other serotypes.

Vibrio

Most Vibrio infections are associated with gastroenteritis, but they can also infect open wounds and cause septicemia. These bacteria can be carried by marine animals and their ingestion causes fatal infections in humans.

Y. cholerae (the causative agent of cholera) is usually spread by contaminated water. Other pathogenic species such as V. parahaemolyticus and V. vulnificus are transmitted by contaminated food, generally associated with the consumption of undercooked shellfish.

Outbreaks of V. vulnificus are deadly and commonly occur in hot climates. After Hurricane Katrina, in New Orleans, an outbreak of this species occurred.

Helicobacter

Some Helicobacter species live in the upper gastrointestinal tract and liver of mammals and some birds. Some strains of these bacteria are pathogenic to humans and are strongly associated with peptic ulcers, chronic gastritis, duodenitis, and stomach cancer.

Species of the genus Helicobacter can thrive in the stomach of a mammal, producing large amounts of urease, which locally raises the pH from 2 to 6 or 7, making it a more compatible medium.

Y. pylori infects up to 50% of the human population. It is found in mucus, on the inner surface of the epithelium, and occasionally within the epithelial cells of the stomach.

Colonization of the stomach by H. pylori can lead to chronic gastritis, an inflammation of the stomach lining at the site of infection.

Yersinia

The genus Yersinia includes 11 species, of which only Y. pestis, Y. pseudotuberculosis, and certain strains of Y. enterocolitica are of pathogenic importance to humans and some warm-blooded animals.

Y. pestis is the causative agent of pneumonic, septicemic and bubonic plague. The type of plague depends on the form of infection, either through the bite of infected fleas (bubonic plague and septicemic plague) or from person to person by coughing, vomiting and sneezing, when the disease has progressed to the pneumonic form. (pulmonary or pneumonic plague).

Pneumonic plague occurs when bacteria infect the lungs, while bubonic plague occurs when bacteria enter the body through the skin from a flea bite and travel through the lymphatic vessels to a lymph node, causing inflammation. Finally, septicemic plague occurs due to blood infection, after the bite of infected fleas

Y. pseudotuberculosis is acquired by contact with infected animals or by consuming contaminated food and water. It is the cause of a disease similar to tuberculosis, called scarlet fever, which affects the lymph nodes. It can cause localized tissue necrosis, granulomas in the spleen, liver and lymph nodes.

Y. enterocolitica infections generally occur from consumption of undercooked pork or from contaminated water, meat, or milk. Acute infections generally lead to self-limited entero colitis or terminal ileitis and adenitis in humans. Symptoms may include watery or bloody diarrhea and fever, similar to appendicitis or salmonellosis or shigellosis.

References

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