Escherichia is the name given to a genus of gram-negative, rod-shaped bacteria. Its most important representative and most relevant to human pathogens is Escherichia coli (E. coli). Escherichia belong to the enterobacteria and make up a small proportion of the normal flora of the intestine.
What are Escherichia?
Escherichia are gram-negative rod-shaped bacteria that are physiologically present in the intestinal flora of humans. They grow facultatively anaerobically, meaning that they can grow and multiply both with and without the presence of oxygen. In addition, they are oxidase-negative. Escherichia are flagellated bacteria, so they are motile. Selective cultivation of Escherichia is possible on culture media containing bile salts such as McConkey agar. E. coli as a species of Escherichia is the most common pathogen of bacterial infection and also serves as an indicator germ for contaminated drinking and bathing water. Research on E. coli has earned numerous scientists the Nobel Prize in Physiology and Medicine. Other species of Escherichia, such as E. hermanii or E. vulneris, are known, but infections with them are very rare.
Occurrence, distribution, and characteristics
Escherichia belong to the group of enterobacteria, which means that they are mainly found in the intestines of mammals. For human medicine, it is mainly E. coli that plays a role. If a person comes into contact with substances from its intestine, it can contaminate drinking water or food, for example, which can subsequently infect other people. That is why the E. coli is considered as a fecal indicator, no E. coli should be present in 100 ml of drinking water. In addition, inadequate hygiene in public toilets favors urinary tract infection, especially in women. Different antigenic structures can be detected on the surface of Escherichia by various agglutination reactions with known antisera, which is called serotyping. This results in an individual antigen pattern. A distinction is made between O antigens (surface antigens, which corresponds to lipopolysaccharides), H antigens (flagellin of the flagella, a thermostable protein), K antigens (carbohydrates of the outermost membrane) and F antigens (fimbriae). The fimbriae are there to attach to the mucosa in the gastrointestinal tract. Escherichia also do not have a capsule and are flagellated peritrichously (completely around the whole cell), so they are motile. This is especially important for E. coli because when in the stomach, it cannot be exposed to aggressive gastric acid and therefore moves away into the protective mucus. Different subtypes of E. coli are distinguished, each of which produces different virulence factors and causes different diseases. These are also referred to as pathovars:
EPEC (= enteropathogenic E. coli) attaches to the intestinal mucosa and can inject a toxin into the cells via a so-called type 3 secretion system. This toxin causes flattening of the intestinal epithelium. They primarily affect infants and are responsible for the rare infant diarrhea. ETEC (= enterotoxic E. coli) also produces two enterotoxins. It is the causative agent of traveler’s diarrhea, which is caused by fecal-oral contaminated food, especially in the tropics. The clinical picture is similar to that of cholera, as the two toxins correspond to each other. EHEC (= enterohemorrhagic E. coli) possesses the protein intimin, which promotes a firm binding of the bacteria to the intestinal mucosa. The pathogen also produces a toxin similar to the shiga toxin produced by shigella. This leads to inhibition of protein synthesis in the affected cells. They are also called STEC (= shigatoxin-producing E. coli). EAEC (= enteroaggregative E. coli) are able to form aggregates with other bacteria, which remain on the intestinal mucosa. UPEC (= uropathogenic E. coli) expresses on its surface the P-fimbriae, which serves specifically to bind to the epithelium of the urogenital tract. EIEC (= enteroinvasive E. coli) directly invades the intestinal epithelial cell and spreads to neighboring cells by directly invading them.
Diseases and symptoms
Escherichia intestinal infections, that is, diseases of the gastrointestinal tract (which are always caused by exogenous infections), are distinguished from extraintestinal diseases, which are usually caused by endogenous infections.E. coli are the most common pathogens of bacterial infection. The different subtypes cause different diseases:
EPEC is responsible for infant diarrhea, which is characterized by massive diarrhea and the risk of dehydration. In the Third World, the pathogen is the cause of high infant mortality. The causative agent of chronic persistent diarrhea is EAEC. The diarrhea is mucopurulent because it induces the intestinal mucosa to secrete more mucus. The causative agent of traveler’s diarrhea is ETEC, which are very similar to cholera. Rice water-like diarrhea of up to 20 liters per day is not uncommon. The EHEC, which is also the best known subtype, is responsible for watery to bloody diarrhea, which can be responsible for hemolytic uremic syndrome (HUS), especially in young children, which can lead to kidney failure. Additionally, fever, stomach cramps, and vomiting may be considered. Further complication may be intestinal perforation. EIEC is the causative agent of dysentery-like colitis with bloody-mucous diarrhea. UPEC as the causative agent of extraintestinal infection causes urinary tract infections when the bacterium passes from the intestine to the genitourinary tract. This is especially the case in women due to the anatomical proximity of the anus to the urethra. In addition, they can cause meningitis in the newborn because the birth canal is also close to the anus and thus can infect the child during birth.
