Escherichiosis (genus Escherichia, E. Coli)

The main representative of the genus Escherichia - E. Coli - was first discovered in 1885 by T. Esherich, in honor of which this genus of bacteria was given its name. Key signs of this genus are: peritrichi (or immobile ones), ferment lactose to form acid and gas (or lactose-negative), do not grow on a starving environment with citrate, the Foges-Proskauer reaction is negative, the sample with MR is positive, does not have phenylalanine deaminase, does not grow on medium with KCN, the content of G + C in DNA is 50-51 mol%.

The genus Escherichia includes at least 7 species; Of particular importance in medicine is the form of E. Coli, in particular those of its variants that cause human diseases. They are divided into 2 main groups: causing extraintestinal diseases and agents of acute intestinal diseases (OKZ). Representatives of the first are divided into three pathogroups:

1. meningeal (MENEC - meningitis E. Coli);
2. septicemia (SEPEC - septicemia E. Coli) and
3. uropathogenic (UPEC - uropathogenic E. Coli).

In turn, the variants of E. Coli that cause OKZ, were initially divided into 4 following categories: enterotoxigenic E. Coli (ETEC); Enteric-invasive E. Coli (EIEC); Enteropathogenic E. Coli (EPEC) and enterohaemorrhagic E. Coli (EHEC). Subsequently, two more categories were identified: enteroaggregative E. Coli (EAEC) and diffusive-aggregative E. Coli (DAEC).

In addition, E. Coli is used in international standards as an indicator of the degree of fecal contamination of water, especially drinking water, and food products.

A standard strain of E. Coli (E. Coli K-12) is widely used in laboratories in many countries of the world to study the genetics of bacteria.

[1], [2], [3],

Morphology

E. Coli - facultative anaerobic, grows well on ordinary nutrient media - colonies on agar are round, convex, translucent. Growth on broth in the form of diffuse turbidity. The temperature optimum for growth of 37 ° С, grows in a range from 10 to 45 ° С, the optimum pH is 7.2-7.5. On all differential diagnostic environments of colony E. Coli. Degrading lactose. Painted in the color of the indicator (on the environment of Endo - dark crimson with metallic luster).

Biochemical properties

The E. Coli can in most cases ferment the following carbohydrates to form acid and gas: glucose, lactose, mannitol, arabinose, galactose, sometimes sucrose and some other carbohydrates; forms an indole; as a rule, does not form H ₂ S; restores nitrates to nitrites, does not dilute gelatin, does not grow on a hungry environment with citrate, gives a positive reaction with MR and a negative one - Foges-Proskauer. By these signs, it can easily be distinguished from pathogens of a number of diseases (dysentery, typhoid, salmonella, etc.). However, pathogenic E. Coli, in terms of both cultural and biochemical properties, often do not differ from non-pathogenic ones.

[4], [5], [6], [7], [8], [9], [10], [11]

Pathogenicity factors of Escherichia coli

The ability of E. Coli to cause various diseases is due to the presence of the following factors of pathogenicity:

The factors of adhesion and colonization. They are necessary for attachment to tissue cells and their colonization. Three variants of the colonization factor were found: a) CFA / I-CFA / VI (English colonization factor) - they have a fimbrial structure; b) EAF (English enteropathogenic E. Coli adherence factor) - intimin is the outer membrane protein, is encoded by the eAeA gene. Found in 4 and EHEC, revealed by the ability of bacteria to attach to the cells of Hep-2; c) Adhesion Henle-407 - fimbrial structures, are detected by the ability of bacteria to attach to Henle-407 cells. They are all encoded by plasmid genes. In addition to these, other factors of colonization are described, in the role of which bacterial lipopolysaccharides can also act.

Factors of invasion. With their help, EIEC and EHEC, for example, penetrate the epitheliocytes of the intestine, multiply in them and cause their destruction. The role of the invasion factors is performed by the outer membrane proteins.

Exotoxins. In pathogenic E. Coli, exotoxins, damaging membranes (hemolysin), inhibiting protein synthesis (Shiga toxin), activating secondary messengers (English messenger), toxins CNF, ST, CT, CLTD, EAST have been detected.

Hemolysins produce different pathogens, including E. Coli. Hemolysin is a pore-forming toxin. It first contacts the membrane of the target cell, and then forms a pore through which small molecules and ions enter and exit, leading to cell death and erythrocyte lysis.

Shiga Toxin (STX) was first detected in Shigella dysenteriae, and then a similar toxin (shiga-like toxin) was detected in the EHEC. The toxin (N-glycosidase) blocks protein synthesis by interacting with 28S rRNA, as a result of which the cell perishes (cytotoxin). There are two types of shiga-like toxin: STX-1 and STX-2. STX-1 antigenic properties are almost identical to Shiga toxin, and STX-2 differs from Shiga toxin in antigenic properties and, unlike STX-1, is not neutralized by antiserum to it. Synthesis of cytotoxins STX-1 and STX-2 is controlled in E. Coli by the genes of moderate converting prophages 9331 (STX-1) and 933W (STX-2).

• Toxin L (thermolabile toxin) - ADP-ribosyltransferase; binding to G-protein, causes diarrhea.
• Toxin ST (thermostable toxin), interacting with the receptor of guanylate cyclase, stimulates its activity and causes diarrhea.
• CNF (cytotoxic necrotic factor) - a protein deamidase, damages the so-called RhoG-proteins. This toxin is found in UPEC, causing urinary tract infections.
• CLTD-toxin is a cytoletic loosening toxin. The mechanism of action is poorly understood.
• The toxin EAST is a thermostable toxin of enteroaggregative E. Coli (EAEC), probably similar to a thermostable toxin (ST).

Endotoxins-lipopolysaccharides. They determine the antigenic specificity of bacteria (which is determined by the repeated side chain of sugars) and the shape of the colonies (the loss of side chains leads to the transformation of S-colonies into the R-colony).

Thus, the pathogenicity factors of E. Coli are controlled not only by the chromosomal genes of the host cell, but also by genes introduced by plasmids or by moderate converting phages. All this indicates the possibility of pathogenic variants of E. Coli as a result of the spread of plasmids and moderate phages among them. Below is a brief description of the four categories of E. Coli that cause OKZ; information on the recently identified categories of DAEC and EAEC in the sources available to us was not found.

ETEC includes 17 serogroups. Adhesion and colonization factors of the fimbrial structure of the CFA type and enterotoxins (LT or ST, or both) are encoded by the same plasmid (plasmids). Colonize the villi without damaging them. Enterotoxins cause a violation of water-salt metabolism. Localization of the process is the area of the small intestine. The infecting dose of 108-1010 cells. The disease proceeds according to the type of cholera-like diarrhea. Type of epidemics - water, less often food. Children aged 1 to 3 years and adults are ill.

EIEC includes 9 serogroups, pathogenicity is associated with the ability to penetrate the epithelial cells of the intestinal mucosa and multiply within them, causing their destruction. These properties are encoded, in addition to chromosomal genes, by plasmid genes (140 MD). Plasmid encodes the synthesis of outer membrane proteins, which determine the invasion. Both the plasmid itself and the proteins it encodes are related to those of the causative agents of dysentery, which explains the similarity between EIEC and shigella. The infecting dose of 10s of cells. Localization of the process - the lower ileum and large intestine. The disease proceeds according to the type of dysentery: first watery diarrhea, then a colitis syndrome. Children are sick for 1,5-2 years, adolescents and adults. Type of outbreaks - food, water.

Epidemiology

E. Coli is a representative of the normal microflora of the intestinal tract of all mammals, birds, reptiles and fish. Therefore, in order to find out which variants of E. Coli and why they cause escherichiosis, it was required to study the antigenic structure, to develop the serological classification necessary for the identification of pathogenic serovariants, and to find out what pathogenicity factors they possess, ie, why they are capable of causing various forms of escherichiosis .

171 variants of O-antigens (01-0171), 57 variants of H-antigens (H1-H57) and 90 variants of surface (capsular) K antigens were found in E. Coli. However, in fact there are 164 groups of O-antigen and 55 serovariants for the H-antigen, since some of the former 0: H-serogroups were excluded from the E. Coli species, but the ordinal numbers of the O and H antigens remained unchanged. The antigenic characteristic of diarrheogenic E. Coli includes the numbers of O and H antigens, for example, 055: 116; 0157: H7; O-antigen means belonging to a particular serogroup, and the H-antigen is its serovariant. In addition, with a more in-depth study of O and H antigens, the so-called factorial O and H antigens are identified, that is, their antigenic subvariants, for example: H2a, H2S, H2C or O20, O20a, O20ab, etc. In total, 43 O-serogroups and 57 OH-serovarants are included in the list of diarrheogenic E. Coli. This list is replenished with all new serovariants.

[12], [13], [14]

Symptoms

The group includes 9 serogroups of class 1 and four serogroups of class 2. Serogroups of class 1 have a plasmid (60 MD), which controls the synthesis of the adhesion factor and colonization of the EAF type. It is represented by a protein localized in the outer membrane, and is identified by the ability of bacteria to attach to the cells of HEp-2. The protein has a mass of 94 kD. In serogroups of class 2, this plasmid is absent, their pathogenicity is due to some other factors. Some strains 4 of both classes have the ability to synthesize STX. 4 colonize the plasmolemma of enterocytes, cause damage to the surface of the epithelium with the formation of erosion and moderate inflammation. The infecting dose of 105-10 ¹² cells. The process is localized in the small intestine. The disease is characterized by watery diarrhea and marked dehydration. Mostly children of the first year of life are ill. The way of infection is contact-household, less often food.

Serogroups EIEC and 4 are the most frequent culprits of nosocomial outbreaks.

ЕНЕС produce cytotoxins STX-1 and STX-2. People cause hemorrhagic colitis with severe complications in the form of hemolytic uremia and thrombotic thrombocytopenic purpura. Toxins destroy endothelial cells of small blood vessels. The formation of blood clots and loss of fibrin leads to a violation of blood flow, bleeding, ischemia and necrosis in the cell wall. Uremical haemolytic syndrome can lead to death. EHEC are represented by many serotypes (-150), but the main epidemiological role is played by E. Coli 0157-H7 and its EA-free 0157: NM non-mutant mutant, since only they form STX. These strains of bacteria can only release one of the cytotoxins, or both. It is believed that the natural reservoir of ENOV serovars, including E. Coli 0157: H7, is cattle and sheep. The most frequent route of infection is food (meat, especially minced meat, milk). E. Coli 0157: H7 is unusually resistant to unfavorable factors. This contributes to its survival and reproduction in different products. Possible contamination by contact and household. The onset of the disease is acute: intestinal cramps, then diarrhea, initially watery, then with blood. Children and adults are ill. The sick person is contagious.

Laboratory diagnostics

It is based on the isolation of the pure culture of the pathogen and its identification, as well as on the testing of toxins by PCR. The causative agent of the escherichiosis is identified with a set of polyvalent OK sera and a set of adsorbed sera containing antibodies to only certain antigens. To identify the EIEC, a keratocon-active sample can be used. Some EIEC representatives are immobile, do not ferment lactose and salicin. Identification of E. Coli 0157: H7 is helped by its inability to ferment sorbitol (use Endo medium with sorbitol instead of lactose). But it is best to use the PCR test system to identify and differentiate pathogens of OKZ (all categories). If necessary, the selected pathogens determine the sensitivity to antibiotics.

Treatment of Escherichia coli

Various antibiotics are used. Oral saline solutions are used to restore the disturbed water-salt metabolism. They are produced in cellophane bags in the form of powders containing NaCl 3.5 g; NaHC03 - 2.5 g; KC1 - 1.5 g and glucose - 20.0 g and dissolved in 1 liter of water.