Shigella

Dysentery - an infectious disease characterized by general intoxication of the body, diarrhea and a peculiar lesion of the mucous membrane of the large intestine. It is one of the most frequent acute intestinal diseases in the world. Dysentery is known from ancient times under the name of "bloody diarrhea", but its nature turned out to be different. In 1875 the Russian scientist f. A. Lesch singled out the amoeba Entamoeba histolytica from a patient with bloody diarrhea, in the next 15 years the independence of this disease was established, behind which the name amebiasis was preserved.

The causative agents of dysentery proper are a large group of biologically similar bacteria, united in the genus Shigella. The causative agent was first discovered in 1888 by A. Chantemes and F. Vidal; in 1891, he was described by AV Grigoriev, and in 1898 K. Shiga, using the serum he had obtained from the patient, identified the causative agent in 34 patients with dysentery, finally proving the etiological role of this bacterium. However, in the following years, other causative agents of dysentery were also discovered: in 1900 - by S. Flexner, in 1915 - by K. Sonne, in 1917 by C. Stutzer and K. Schmitz, in 1932 - by J. Boyd , in 1934 - D. Larjem, in 1943 - A. Saxom.

Currently, the genus Shigella includes more than 40 serotypes. All of them are short fixed gram-negative rods that do not form spores and capsules that grow well on common nutrient media, do not grow on a starving medium with citrate or malonate as the sole source of carbon; do not form H2S, do not have urease; the Foges-Proskauer reaction is negative; glucose and some other carbohydrates are fermented to produce an acid without gas (except for some biotypes of Shigella flexneri: S. Manchester and S. Newcastle); as a rule, do not ferment lactose (except for Sonne shigella), adonite, salicin and inositol, do not liquefy gelatin, usually form a catalase, do not have lysine-decarboxylase and phenylalanine deaminase. The content of G + C in DNA is 49-53 mol%. Shigella - facultative anaerobes, temperature optimum for growth 37 ° C, at a temperature above 45 ° C do not grow, the optimal pH of the medium is 6.7-7.2. Colonies on dense media are round, convex, translucent, in the case of dissociation, R-shaped rough colonies are formed. Growth on the MPB in the form of uniform opacity, rough forms form a precipitate. Freshly isolated Shigella Sonne cultures usually form colonies of two types: small round convex (I phase), large flat (II phase). The nature of the colony depends on the presence (phase I) or absence (phase II) of the plasmid with a mass of 120 MD, which also determines the virulence of shigella Sonne.

The international classification of shigellas was constructed taking into account their biochemical characteristics (mannitol-non-fermenting, mannitizing, fermenting, slowly fermenting shigella lactose) and features of the antigenic structure.

Shigella have different in specificity O-antigens: common for the family Enterobacteriaceae, generic, species, group and type-specific, as well as K-antigens; H-antigens they do not.

Classification takes into account only group and type-specific O-antigens. In accordance with these signs, the genus Shigella is divided into 4 subgroups, or 4 species, and includes 44 serotypes. In subgroup A (Shigella dysenteriae species) shigella not fermenting mannitol are included. The species includes 12 serotypes (1-12). Each serotype has its own specific type of antigen; antigenic links between serotypes, as well as with other species of shigella are poorly expressed. B group B (Shigella flexneri species) includes shigella, usually fermenting mannitol. Shigella of this species are serologically related to each other: they contain type-specific antigens (I-VI), which are divided into serotypes (1-6 / 'and group antigens, which are found in different formulations in each serotype and according to which the serotypes are subdivided into sub-types. Moreover, this species includes two antigenic variants - X and Y, which do not have typical antigens, they differ in groups of group antigens.Serotype S.flexneri 6 does not have any sub-serotypes, but it is divided into 3 biochemical types according to the peculiarities of glucose fermentation, mannitol and dulcitol.

Lipopolysaccharide antigen O in all Flexner shigella contains group antigen 3, 4 as the main primary structure, its synthesis is controlled by a chromosomal gene localized near his-locus. Type-specific antigens I, II, IV, V and group antigens 6, 7, 8 are the result of modification of antigens 3, 4 (glycosylation or acetylation) and are determined by the genes of the corresponding converting prophages whose integration site is located near the lac-pro chromosome of shigella.

Appeared in the country in the 80's. XX century. And the widely distributed new subgenotype S.flexneri 4 (IV: 7, 8) differs from the sub-serotype 4a (IV; 3,4) and 4b (IV: 3,4,6), originated from S.flexneri Y (IV: 3, 4) due to its lysogenization by converting prophages IV and 7, 8.

The subgroup C (Shigella boydix) includes shigella, usually fermenting mannitol. The members of the group are serologically different from each other. Antigenic bonds within the species are poorly expressed. The species includes 18 serotypes (1-18), each of which has its main type antigen.

In subgroup D (Shigella sonnet species) shigella, usually fermenting mannitol and slow (after 24 hours incubation and later) ferment lactose and sucrose. Type 5. Sonnei includes one serotype, however, colonies I and II phases have their type-specific antigens. For intraspecific classification of shigella Sonne, two methods are proposed:

• dividing them into 14 biochemical types and subtypes by their ability to ferment maltose, rhamnose and xylose;
• division into phagotypes by sensitivity to a set of corresponding phages.

These methods of typing are mainly of epidemiological importance. In addition, Shigella Sonne and Shigella Flexner are also typed for the same purpose by the ability to synthesize specific colicins (colicino-genotyping) and sensitivity to known colicins (colicinotyping). To determine the type of colicins produced by Shigella, J. Abbot and R. Chenon, sets of typical and indicative strains of shigella are proposed, and to determine the sensitivity of shigella to known types of colicins, use a set of reference colicinogenic strains by P. Frederic.

[1], [2], [3], [4], [5], [6],

Shigella Resistance

Shigella have a fairly high resistance to environmental factors. They survive on cotton cloth and on paper up to 0-36 days, in dried feces - up to 4-5 months, in the soil - up to 3-4 months, in water - from 0.5 to 3 months, on fruits and vegetables - up to 2 weeks, in milk and dairy products - up to several weeks; at a temperature of 60 C perished in 15-20 minutes. Sensitive to chloramine solutions, active chlorine and other disinfectants.

Factors of shigella pathogenicity

The most important biological property of shigellas, which determines their pathogenicity, is the ability to penetrate into epithelial cells, multiply in them and cause their death. This effect can be detected with the keratoconjunctival test (introduction of a single loop of shigella culture (2-3 billion bacteria) into the lower eyelid of guinea pig causes the development of serous-purulent keratoconjunctivitis), as well as by infection of cell cultures (cytotoxic action) or chick embryos (their death), or intranasally white mice (development of pneumonia). The main factors of shigella pathogenicity can be divided into three groups:

• factors that determine the interaction with the epithelium of the mucosa;
• factors that provide resistance to humoral and cellular mechanisms for protecting the macroorganism and the ability of shigella to multiply in its cells;
• the ability to produce toxins and toxic products that cause the development of the pathological process itself.

The first group includes factors of adhesion and colonization: their role is played by saws, outer membrane proteins and LPS. Adhesion and colonization are facilitated by enzymes that destroy mucus, - neuraminidase, hyaluronidase, mucinase. The second group includes invasion factors that promote the penetration of shigella into enterocytes and their multiplication in them and in macrophages with simultaneous manifestation of cytotoxic and (or) enterotoxic effect. These properties are controlled by plasmid genes with a mass of 140 MD (it codes for the synthesis of outer membrane proteins that cause invasion) and chromosome genes of shigella: ksr A (causes keratoconjunctivitis), cyt (responsible for cell destruction), and other genes not yet identified. Protection of shigella from phagocytosis is provided by surface K antigen, antigens 3,4 and lipopolysaccharide. In addition, lipid A endotoxin shigell has an immunosuppressive effect: it suppresses the activity of immune cells.

The third group of pathogenicity factors include endotoxin and two types of exotoxins found in shigella - Shiga exotoxins and shiga-like (SLT-I and SLT-II), whose cytotoxic properties are most pronounced in S. Dysenteriael. Shiga- and shiga-like toxins are also found in other serotypes of S. Dysenteriae, they are also formed by S.flexneri, S. Sonnei, S. Boydii, EHEC and some salmonella. Synthesis of these toxins is controlled by the tox-genes of the converting phages. LT enterotoxins are found in Shigella Flexner, Sonne and Boyd. Synthesis of LT in them is controlled by plasmid genes. Enterotoxin stimulates the activity of adenylate cyclase and is responsible for the development of diarrhea. Shiga Toxin, or neirotoksin, does not react with the adenylate cyclase system, but has a direct cytotoxic effect. Shiga and Shiga-like toxins (SLT-I and SLT-II) have a m. 70 kD and consist of subunits A and B (the last of 5 identical small subunits). The receptor for toxins is the glycolipid of the cell membrane. The virulence of Shigella Sonne also depends on the plasmid with a mass of 120 MD. It controls the synthesis of about 40 polypeptides of the outer membrane, seven of them are associated with virulence. Shigella Sonne, having this plasmid, form colonies of the I phase and possess virulence. Cultures that lost the plasmid form colonies of the second phase and are devoid of virulence. Plasmids see m. 120-140 MD were found in shigella Flexner and Boyd. Lipopolysaccharide shigella is a strong endotoxin.

[7], [8], [9], [10], [11], [12], [13],

Postinfectious immunity

As observations on monkeys have shown, after the transferred dysentery the durable and rather long immunity remains. It is due to antimicrobial antibodies, antitoxins, increased activity of macrophages and T-lymphocytes. A significant role is played by local immunity of the intestinal mucosa, mediated by IgAs. However, immunity is of a type-specific nature, there is no lasting cross-immunity.

Epidemiology of dysentery

The source of infection is only a person. No animals in nature have dysentery. Under experimental conditions, dysentery can only be reproduced in monkeys. The method of infection is fecal-oral. Transmission routes - water (prevailing for Shigella Flexner), food, especially important role belongs to milk and dairy products (the prevailing path of infection for shigella Sonne), and contact-household, especially for the species S. Dysenteriae.

A special feature of the epidemiology of dysentery is the change in the species composition of the pathogens, as well as the biotypes of Sonne and the Flexner serotypes in certain regions. For example, until the late 30's. XX century. S. Dysenteriae 1 accounted for up to 30-40% of all cases of dysentery, and then this serotype began to occur less and less frequently and almost disappeared. However, in the 1960s-1980s, S. Dysenteriae reappeared in the historical arena and caused a series of epidemics that led to the formation of its three hyperendemic foci - in Central America, Central Africa and South Asia (India, Pakistan, Bangladesh and other countries). The reasons for the change in the species composition of the causative agents of dysentery are probably related to changes in collective immunity and changes in the properties of dysentery bacteria. In particular, the return of S. Dysenteriae 1 and its wide spread, which caused the formation of hyperendemic foci of dysentery, is associated with the acquisition of plasmids, which caused multiple drug resistance and increased virulence.

[14], [15], [16], [17], [18], [19], [20],

Symptoms of dysentery

The incubation period of dysentery is 2-5 days, sometimes less than a day. The formation of an infectious focus in the mucosa of the descending part of the large intestine (sigmoid and rectum), where the causative agent of dysentery penetrates, has a cyclic character: adhesion, colonization, introduction of shigella into the cytoplasm of enterocytes, their intracellular multiplication, destruction and rejection of epithelial cells, exit of pathogens into the lumen intestines; After this, the next cycle begins - adhesion, colonization, etc. The intensity of the cycles depends on the concentration of pathogens in the parietal layer of the mucous membrane. As a result of repeated cycles, the inflammatory focus grows, ulcers formed, joining, increase the nakedness of the intestinal wall, as a result of which blood, mucopurulent lumps, and polymorphonuclear leukocytes appear in the feces. Cytotoxins (SLT-I and SLT-II) cause cell destruction, enterotoxin-diarrhea, endotoxins-common intoxication. The clinic of dysentery is largely determined by what type of exotoxins are more produced by the pathogen, the degree of its allergic effect and the immune status of the organism. However, many questions of the pathogenesis of dysentery remain unclear, in particular: the features of the course of dysentery in children of the first two years of life, the causes of the transition of acute dysentery into chronic dysentery, the significance of sensitization, the mechanism of local immunity of the intestinal mucosa, etc. The most typical clinical manifestations of dysentery are diarrhea, frequent desires: in severe cases up to 50 or more times a day, tenesmus (painful spasms of the rectum) and general intoxication. The nature of the stool is determined by the degree of defeat of the large intestine. The most severe dysentery caused by S. Dysenteriae 1, most easily - dysentery Sonne.

Laboratory diagnostics of dysentery

The main method is bacteriological. The feces serve as a material for the study. Scheme of pathogen isolation: seeding on the differential diagnostic environment of Endo and Ploskirev (parallel to the enrichment medium followed by inoculation on the Endomo and Ploskireva environments) to isolate isolated colonies, obtaining a clean culture, studying its biochemical properties, and, with the latter, identification with polyvalent and monovalent diagnostic agglutination sera. The following commercial sera are produced.

To Shigella, not fermenting mannitol:

• to S. Dysenteriae 1 and 2 (polyvalent and monovalent),
• to S. Dysenteriae 3-7 (polyvalent and monovalent),
• to S. Dysenteriae 8-12 (polyvalent and monovalent).

To shigella, fermenting mannitol: to the typical S. Flexneri antigens I, II, III, IV, V, VI, to the group antigens S.flexneri 3, 4, 6,7,8 - polyvalent, to S. Boydii antigens 1-18 (polyvalent and monovalent), to antigens of S. Sonnei I phase, II phase, to S. Flexneri antigens I-VI + S. Sonnei - polyvalent.

For rapid identification of shigella, the following method is recommended: a suspicious colony (lactose-negative on Endo medium) is transferred to TSI medium (English triple sugar iron) - three-sugar agar (glucose, lactose, sucrose) with iron to determine H2S production; or on a medium containing glucose, lactose, sucrose, iron and urea.

Any organism that cleaves urea after 4-6 hours of incubation is most likely related to the genus Proteus and can be excluded. The microorganism that forms H, S, or which has urease, or forms an acid on the jamb (ferments lactose or sucrose), can be excluded, although strains forming H2S should be investigated as possible members of the genus Salmonella. In all other cases, the culture grown on these media must be examined and, if the glucose is fermented (discoloration of the column), it is isolated in its pure form. Simultaneously, it can be studied in the agglutination reaction on glass with the corresponding antisera to the genus Shigella. If necessary, conduct other biochemical tests that verify belonging to the genus Shigella, and also study mobility.

To detect antigens in the blood (including the CEC), urine and feces, the following methods can be used: RPGA, RSK, coagglutination reaction (in urine and feces), IPM, RAGA (in blood serum). These methods are highly effective, specific and suitable for early diagnosis.

For serological diagnosis, the following can be used: RPGA with appropriate erythrocyte diagnosticums, immunofluorescence method (in indirect modification), Coombs method (determination of the titre of incomplete antibodies). Diagnostic value also has an allergic test with dysentrine (solution of protein fractions Shigella Flexner and Sonne). The reaction is taken into account after 24 hours. It is considered positive in the presence of hyperemia and infiltration with a diameter of 10-20 mm.

Treatment of dysentery

The main attention is paid to the restoration of normal water-salt metabolism, rational nutrition, detoxification, rational antibiotic therapy (taking into account the sensitivity of the pathogen to antibiotics). A good effect results from the early use of a polyvalent dysentery bacteriophage, especially pectin-coated with pectin, which protects the phage from the action of HC1 gastric juice; in the small intestine pectin dissolves, the phages are released and manifest their action. With prophylactic phage should be given at least once every three days (the period of its survival in the intestine).

Specific prophylaxis of dysentery

To create artificial immunity against dysentery, various vaccines were used: from killed bacteria, chemical, alcohol, but they were all ineffective and withdrawn from production. Vaccines against Flexner's dysentery from live (mutant, streptomycin-dependent) Shigella Flexner were created; ribosomal vaccines, but they also did not find wide application. Therefore, the problem of specific prevention of dysentery remains unsolved. The main way to combat dysentery is to improve the system of water supply and sanitation, to ensure strict sanitary and hygienic regimes in food enterprises, especially the dairy industry, in children's institutions, public places and in personal hygiene.