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Laboratory Diagnostics of Shigellosis

What is Shigellosis (bacillary dysentery)? Acute antroponosis disease with fecal-oral mechanism of transfer of the causative agents, which is bacteria of genus Shigella and characterized by a defect of the intestine (mainly distal section of large intestine), general intoxication, frequent watery stool with admixture of mucus and blood, and tenesmus.
Taxonomic position of the causative agents of shigellosis. Family Enterobacteriaceae Genus Shigella Species: S. dysenteriae, S. flexneri, S. boydii, S. sonnei.
Features of shigellosis. The heaviest motion has Shigellosis caused by S. dysenteriae, which produces Shiga toxin (exotoxin), less difficult; – Shigellosis is caused by S. sonnei.
Morphological and tinctorial features of the shigellosis causative agents. Short lines with rounded ends Gram-negative immobile monobacteria. Spores and capsules are not formed.
Type of breathing of shigellae. Facultative anaerobes.
Biological properties of shigellae, describing their pathogenicity.   1/Abilities to adhesion, penetration into the epithelium of mucus membrane of the large intestine and intracellular duplication in them and in macrophages without their destruction; 2/external membrane of bacteria protects it from the action of acid of gastric juice; 3/ K-antigen, antigens 3, 4 lipopolysacharide- protects from phagocytosis; 4/ lipid A endotoxin of shigellae has immunosupressive action (represses activity of cells of immune memory).
Basic biotope of causative agents. Sigmoid colon and rectum.
Factors of shigellae pathogenicity. 1/Toxins and the toxic substances providing development of pathological process (basic factor of pathogenicity); 2/ factors of adhesion and colonization (provide interaction with epitheliocyte); 3/ factors of invasion (resistance to the humoral and cellular defence mechanisms of macroorganism and ability to be made multiple copies in cells of a macroorganism).
Factors provide the cooperation of the shigellaewith epitheliocyte. Pili, lipopolysacharide, enzymes: mucinaze, plasmocoagulaze, hyaluronidaze, fibrinolysin.
Invasive factors of shigellae, essence invasion and its consequences.   Protein of external membrane (invasine), protein of endocellular distribution: provide penetration of Shigella in enterocyte, reproduction in enterocyte and macrophage, cause apoptosis of macrophage, lysis of membranes of cells, providing endocellular and intercellular distribution of Shigella.
Toxins of shigellae. Exotoxins are Shiga toxin and Shiga-like toxins, enterotoxin (LT), endotoxin (lipopolysacharide which is linked with O-antigen of cell wall).
Features of the Shiga toxin which is synthesized by S. dysenteriae 1 (Shigella of Grigoriev’s-Shiga). It has neurotoxic property. Enterotoxical (amplification diarrheal syndrome is a result of adenylcyclase activation, increases of excretion of water in a gleam of intestines) and citotoxic activity (breaks synthesis of proteins, suction of Na+ and water causing death of enterocyte and increases liquid in the center of an inflammation). A toxin strikes endothelial of submucosal shell of bowel (causes a diarrhea with blood), glomeruli of kidney (resulting in the hemolytic syndrome of uremia with development of kidney insufficiency).
Features of endotoxin of shigellae. 1/ It has special tropism to the epithelium of mucus shell of large intestine; 2/ it protects Shigella from action of bile; 3/ lipid A has immunosupressive activity (represses activity of cells of immune memory).
Stages of pathogeny of acute shigellosis. Adhesion, colonization, inversion of shigella into the cytoplasm of enterocyte, their endocellular reproduction destruction of enterocyte and tearing away of epithelium, output of exciter into the intestinal, forming of Delayed Hypersensitivity.
A source of infection of shigellosis. Patients with clinical manifestations (especially in the first 3 days of disease), subclinical signs diseases, infected workers or products of food retail industries, public food consumption, equipments for water-supply.
The transmission of the infection. Through contaminated water (prevails at shigellosis Flexneri), contaminated food: especially through milk and milk products (S. sonnei it is long is kept and made multiple copies), contact (especially for species S. dysenteriae). Insects-carriers (flies, cockroaches) transfer causative agents on foodstuff.
Features of the shigellosis epidemiology. Change of specific composition of causative agents, biotype of S. sonnei and serotype of S. flexneri in certain regions, probably, related to the change of immunity and with change of properties of shigella.
Reasons of change of species composition of causative agents of shigellosis Changes of collective immunity and properties of shigellae.
Terms of transition of acute shigellosis are in chronic. Disbacteriosis, metabolic disturbance in an organism, hypovitaminosis. Weak imunogene causative agents. Development of partial tolerance is to the causative agents. Insufficient activity of immune mews is purchased. Delayed Hypersensitivity. Endocellular parasitism.
Features of immunity at shigellosis Type specific cellular and humoral (conditioned increase of activity of macrophage, T-lymphocyte, antimicrobial antibodies and antitoxins). Cross immunity does not arise. Local immunity of mucus membrane of the intestine (sIgA).
Mechanism of local immunity. Prevents adhesion of shigellae on epithelium, penetration of shigellae in a cell, the prevention of immunological damage of the intestine.
Material which is probed in shigellosis. For a bacteriological method: Feces (basic), foodstuff (especially milk, cheese, sour cream) at flashes of shigellosis, autopsy material (fragment of large colon, pieces of parenchymatous organs, mesenterial lymphatic nodes), blood and urine (at suspicion on bacteriemia). For serological method - blood.
Methods of microbiological diagnostics of shigellosis The basic - bacteriological, additional – serological (tube agglutination test, PHAT), express-methods of diagnostics of antigenes: IFT (research of feces and urine), ELISA (research of serum of blood), molecular-biological: PCR.
Purpose of bacteriological research of material. 1/ Diagnostics of shigellosis; 2/ control of etiotropic treatment; 3/ an exposure of persons is with the subclinical forms of disease; 4/ control after the contact persons; 5/ cont333rol after the workers of food retail industry, child's establishments, hospitals.
Media for primary cultivation of causative agents. Differential media: MacConkey, EMB or DCA (Desoxycholate citrate agar), selective media - Hektoen enteric agar, enrichment medium – selenite medium.
Cultural properties causative agents of shigellosis. It forms colourless (lactose-negative), shallow (1-1½mm), convex, circular transparent smooth (S-colonies) colonies with intact edges. S. sonnei forms also R-colonies - flat dim with a rough surface and unequal edges.
Features of shigellae fermentation of glucose. Causative agents of shigellosis ferment the glucose with formation of acid without gas. Biotypes of S. flexneri: S. manchester, S. newcastle – forms acid and gas.
Features of shigellae fermentation of lactose. Causative agents of shigellosis do not ferment the lactose (except S. sonnei that ferments it’s slowly (in 48-72 hours).
Biochemical features of S. dysenteriae. Has the least fermentative activity. A major feature is a negative reaction in mannitol test. It ferments glucose with formation of acid. It does not ferment lactose, sacrose, dulcit; indole – variability test.
Biochemical features of S. flexneri. It is more active species of shigellae: ferments mannitol, ferment glucose with formation of acid or acid and gas, it does not ferment lactose, sacrose, and indole – variability test.
Biochemical features of S. sonnei. The most active species of shigellae: slowly ferments lactose and sacrose, constantly ferments mannitol, maltose; glucose - with formation of acid; indole-negative, and does not ferment dulcit.
Antigen structure of causative agents of shigellosis. O-antigen of different specificity: general for family, genus, species, type, group. S. sonnei has the antigen of 1st phase, which is a K-antigen. K‑antigen type B is in S. dysenteriae, S. boydii and S. flexneri 6. Í-antigen is absent in shigellae.
Purpose of determination of antigen structure of shigellae. Identification of causative agents is by serotype (S. dysenteriae, S. boydii, S. sonnei) and subserotype (S. flexneri).
Purpose of application of serological method. A retrospective acknowledgement of the diagnosis of”shigellosis” at atypical and subclinical forms of disease.
Features of serological method of diagnostics of shigellosis. The reaction of agglutination must be dynamics, for revealing increase of titre of antibodies (on 2-3rd and 8-10 days of illness). A diagnostic value has fourfold and greater increase of titre of antibodies. For the children of 1st of life serologic research little informing.
Features of the shigellosis pathogenesis 1. The pathogenesis is bigining after penetration of the mucus membranes of the bowel. 2. Mixed with unchanged fuchsin active enterocyte, shigella compels them to itself by invagination of cell membrane and formation of the phagosome vacuoles. 3. Hemolysin, that is contained in the microcapsule of shigella, causes distention and tearing away of the infected microflagella of mucus shell, passively releasing it from an exciter. 4. Receptors of enterotoxin are in the thin bowel of enterocyte which draws watery, although an exciter has ability to submerge into the epithelium of thin bowel. Colocyte have receptors for aging and invasion of shigella. 5. Aging of shigella on enterocyte of thin bowel is repressed due to destruction parenchymatous enzymes their superficial citotoxic albumen.The colon of citotoxin of shigella regenerates. 6. The overwhelming defect of distal parts of colon is conditioned to the protracted stay in the intestinal membrane, toxins and bacteria, creating favourable condition for the massive invasion of exciter into colocyte.


Date: 2016-01-14; view: 711


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