Streptococci

Streptococci belong to the family Streptococcaceae (genus Streptococcus). T. Bilrot was first discovered in 1874 with a face; L. Pasteur - in 1878 with postpartum sepsis; isolated in pure culture in 1883 by F. Feleisen.

Streptococci (Greek streptos - chain and coccus - grain) - Gram-positive, cytochrome-negative, catalase-negative cells of globular or ovoid form 0.6-1.0 μm in diameter, grow in the form of chains of various lengths or in the form of tetracocci; immovable (except for some representatives of serogroup D); the content of G + C in DNA is 32-44 mol% (for the family). The dispute does not form. Pathogenic streptococci form a capsule. Streptococci are facultative anaerobes, but there are strict anaerobes. The temperature optimum is 37 ° C, the optimum pH is 7.2-7.6. On ordinary nutrient media, pathogenic streptococci either do not grow, or grow very poorly. For their cultivation usually use sugar broth and blood agar, containing 5% defibrinated blood. The medium should not contain reducing sugars, since they inhibit hemolysis. On the broth, the growth is near-parietal in the form of a tiny precipitate, the broth is transparent. Streptococci, forming short chains, cause turbidity of the broth. On dense media, streptococcus serogroup A form colonies of three types:

• mucoid - large, shiny, resemble a drop of water, but have a viscous consistency. Such colonies form freshly isolated virulent strains having a capsule;
• rough - larger than mucoid, flat, with uneven surface and scalloped edges. Such colonies form virulent strains having M antigens;
• smooth, less large colonies with even edges; form non-virulent cultures.

Streptococci ferment glucose, maltose, sucrose and some other carbohydrates to form an acid without gas (except S. Kefir, which forms acid and gas), milk is not coagulated (except for S. Lactis), proteolytic properties do not possess (except for some enterococci).

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

The main factors of pathogenicity of streptococci

Protein M is the main factor of pathogenicity. M-proteins of streptococcus are fibrillar molecules that form fimbriae on the surface of the cell wall of group A streptococci. M-protein determines adhesive properties, inhibits phagocytosis, determines antigenic type-specificity and has the properties of superantigen. Antibodies to the M-antigen have protective properties (antibodies to T- and R-proteins do not possess such properties). M-like proteins are found in streptococci of groups C and G and, possibly, are factors of their pathogenicity.

Capsule. It consists of hyaluronic acid, similar to that of tissue, so phagocytes do not recognize streptococci that have a capsule, like foreign antigens.

Erythrogenin - scarlet toxin, superantigen, causes STS. There are three serotypes (A, B, C). In patients with scarlet fever, it causes the appearance of a bright red rash on the skin and mucous membrane. Possesses pyrogenic, allergenic, immunosupressive and mitogenic action, destroys platelets.

Hemolysin (streptolysin) O destroys erythrocytes, possesses cytotoxic, including leukotoxic and cardiotoxic, action, it is formed by most streptococcus serogroups A, C and G.

Hemolysin (streptolysin) S has a hemolytic and cytotoxic effect. Unlike streptolysin O, streptolysin S is a very weak antigen, it also produces streptococcus serogroups A, C and G.

Streptokinase is an enzyme that converts the preactivator into an activator, and it - plasminogen into plasmin, the latter and hydrolyses fibrin. Thus, streptokinase, by activating fibrinolysin of blood, increases the invasive properties of streptococcus.

The factor inhibiting chemotaxis (aminopeptidase) suppresses the mobility of neutrophilic phagocytes.

Hyaluronidase is an invasion factor.

The turbidity factor is hydrolysis of serum lipoproteins.

Proteases - the destruction of various proteins; perhaps, tissue toxicity is associated with them.

DNA-ase (A, B, C, D) - hydrolysis of DNA.

The ability to interact with the Fc fragment of IgG by means of the receptor I - suppression of the complement system and activity of phagocytes.

Expressed allergenic properties of streptococci, which cause sensitization of the body.

Resistance of streptococci

Streptococci well tolerate low temperatures, are fairly resistant to drying, especially in protein environments (blood, pus, mucus), retain their vitality for several months on objects and dust. When heated to a temperature of 56 ° C die after 30 minutes, except group D Streptococcus, which can withstand heating to 70 ° C for 1 hour, 3-5% solution of carbolic acid and lysol kill them within 15 minutes.

Postinfectious immunity

The main role in its formation is played by antitoxins and type-specific M-antibodies. Antitoxic immunity after scarlet fever has a lasting, lasting character. Antimicrobial immunity is also strong and lasting, but its effectiveness is limited to the type-specificity of M antibodies.

Epidemiology of streptococcal infection

The source of exogenous streptococcal infection is patients with acute streptococcal diseases (angina, scarlet fever, pneumonia), as well as convalescent after them. The main way of infection - airborne, in other cases - direct contact and very rarely nutritional (milk and other foods).

[6], [7], [8], [9], [10]

Symptoms of Streptococcal Infection

Streptococci are the inhabitants of the mucous membranes of the upper respiratory tract, digestive and genitourinary tracts, so the diseases caused by them can be endogenous or exogenous, that is, they are caused either by their own cocci or by infection from the outside. Penetrating through the damaged skin, streptococci spread from the local hearth through the lymphatic and circulatory system. Infection by airborne or airborne dust leads to the defeat of lymphoid tissue ( tonsillitis ), regional lymph nodes are involved in the process, where the pathogen spreads through the lymphatic vessels and hematogenously.

The ability of streptococci to cause various diseases depends on:

• places of the entrance gates (wound infections, puerperal sepsis, erysipelas, etc., respiratory infections - scarlet fever, sore throats);
• the presence of various pathogenicity factors in streptococci;
• the state of the immune system: in the absence of antitoxic immunity, the infection with toxigenic streptococci of serogroup A leads to the development of scarlet fever, and in the presence of antitoxic immunity, angina arises;
• sensitizing properties of streptococci; they largely determine the peculiarity of the pathogenesis of streptococcal diseases and are the main cause of complications such as nephro-nephritis, arthritis, cardiovascular damage, etc .;
• pyogenic and septic functions of streptococci;
• The presence of a large number of serovarians of streptococcus serogroup A on the M-antigen.

Antimicrobial immunity, which is due to antibodies to the protein M, is of a type-specific nature, and since there are many serovariants for the M-antigen, repeated diseases of sore throat, erysipelas and other streptococcal diseases are possible. More complex is the pathogenesis of chronic infections caused by streptococci: chronic tonsillitis, rheumatism, nephritis. The following facts serve as confirmation of the aetiological role of serogroup A streptococci in them:

• these diseases, as a rule, arise after the transfer of acute streptococcal infections (angina, scarlet fever);
• In such patients streptococci or their L-forms and antigens in the blood are often found, especially with exacerbations, and, as a rule, hemolytic or greening streptococci on the mucous throat;
• constant detection of antibodies to various antigens of streptococci. Especially valuable diagnostic value is the detection in patients with rheumatism during exacerbation in the blood of anti-O-streptolysins and anti-hyaluronidase antibodies in high titres;
• development of sensitization to various streptococcal antigens, including the heat-stable component of erythrogenin. It is possible that in the development of rheumatism and nephritis the role of autoantibody plays a role in connective and renal tissue, respectively;
• the obvious therapeutic effect of the use of antibiotics against streptococci (penicillin) during rheumatic attacks.

Scarlet fever

Scarlatina (latelate scarlatium - bright red color) is an acute infectious disease that is clinically manifested by angina, lymphadenitis, small-dot bright red rash on the skin and mucous membrane, followed by desquamation, as well as general intoxication of the body and a tendency to purulent-septic and allergic complications.

The causative agents of scarlet fever are beta-hemolytic streptococci of group A, having M-antigen and producing erythrogenin. Etiological role in scarlet fever was attributed to different microorganisms - the simplest, anaerobic and other cocci, streptococci, filtering forms of streptococcus, viruses. The decisive contribution to the clarification of the true cause of scarlet fever was made by Russian scientists G. N. Gabrichevsky, I. G. Savchenko and American scientists Dick (GF Dick and GH Dick). IG Savchenko as early as 1905-1906. Showed that scarlet latin streptococcus produces toxin, and the antitoxic serum obtained by it has a good curative effect. Proceeding from the works of IG Savchenko, the wife of Dick in 1923-1924. Showed that:

• the administration of a small dose of toxin to a person who does not have scarlet fever, intradermally, causes a positive local toxic reaction in the form of redness and swelling (Dick's reaction);
• in persons who have had scarlet fever, this reaction is negative (the toxin is neutralized by the antitoxin available);
• the administration of large doses of toxin subcutaneously to persons who did not suffer from scarlet fever, causes symptoms typical of scarlet fever.

Finally, by infecting volunteers with streptococcal culture, they were able to reproduce scarlet fever. Currently, the streptococcal etiology of scarlet fever is universally recognized. The peculiarity here is that scarlet fever causes not one of any streptococcal serotype, but any of the beta-hemolytic streptococci, which has an M-antigen and produces erythrogenin. However, in the epidemiology of scarlet fever in different countries, in different regions and at different times, the main role is played by streptococci having different serotypes of M-antigen (1, 2,4 or other) and producing erythrogenins of different serotypes (A, B, C). It is possible to change these serotypes.

As the main factors of pathogenicity of streptococci in scarlet fever exotoxin (erythrogenin), pyogenic-septic and allergenic properties of streptococcus and its erythrogenin. Erythrogenin consists of two components - a thermolabile protein (actually a toxin) and a thermostable substance with allergenic properties.

Infection with scarlet fever occurs mainly by airborne droplets, but the entrance gates can be any wound surfaces. The incubation period is 3-7, sometimes 11 days. In the pathogenesis of scarlet fever, there are three main points that are associated with the properties of the pathogen:

• the action of scarlet toxin, which causes the development of toxicosis - the first period of the disease. It is characterized by damage to the peripheral blood vessels, the appearance of a small-dotted rash of bright red color, as well as an increase in temperature and general intoxication. The development of immunity is associated with the appearance and accumulation of antitoxin in the blood;
• the action of streptococcus itself. It is non-specific and manifests itself in the development of various purulent-septic processes (otitis, lymphadenitis, nephritis appear on the 2-3 nd week of illness);
• sensitization of the body. It is reflected in the form of various complications such as nephro-nephritis, polyarthritis, cardiovascular diseases, etc., for 2-3 weeks. Disease.

In the clinic, scarlet fevers also distinguish between I (toxicosis) and stage II, when there are purulent-inflammatory and allergic complications. In connection with the use of antibiotics (penicillin) for treatment of scarlet fever, the frequency and severity of complications decreased significantly.

Postinfectious immunity

Strong, long-lasting (repeated diseases are observed in 2-16% of cases), is caused by antitoxins and cells of immune memory. Those who have recovered also have an allergic condition to the scarlatinous allergen. It is detected by intradermal injection of killed streptococci. In patients who had recovered at the injection site - redness, swelling, tenderness (Aristovsky-Fanconi test). To test for the presence of antitoxic immunity in children, Dick's reaction is used. With its help, it is established that passive immunity in children of the first year of life persists for the first 3-4 months.

Laboratory diagnostics of scarlet fever

In typical cases, the clinical picture of scarlet fever is so clear that bacteriological diagnosis is not carried out. In other cases, it consists in the isolation of a pure culture of beta-hemolytic streptococcus, which in all patients with scarlet fever is found on the mucous membrane of the throat.

Aerobic Gram-positive cocci, referred to the genera Aerococcus, Leuconococcus, Pediococcus and Lactococcus, are characterized by poor pathogenicity. The diseases that they cause in humans are rare and mostly in people with immune system disorders.

Classification of streptococci

The genus of streptococci includes about 50 species. Among them are 4 pathogenic (S. Pyogenes, S. Pneumoniae, S. Agalactiae and S. Equi), 5 opportunistic and more than 20 opportunistic species. For convenience, the whole genus is divided into 4 groups using the following signs: growth at a temperature of 10 ° C; growth at 45 ° C; growth on medium containing 6.5% NaCl; growth in a medium with a pH of 9.6; growth on medium containing 40% bile; growth in milk with 0.1% methylene blue; growth after heating at a temperature of 60 ° C for 30 minutes.

Most pathogenic streptococci belong to the first group (all of the listed signs, as a rule, are negative). Enterococci (serogroup D), which also cause various human diseases, belong to the third group (all of these signs are usually positive).

The most simple classification is based on the ratio of streptococci to erythrocytes. Distinguish:

• b-hemolytic streptococci - with growth on the blood agar around the colony a clear hemolysis zone;
• a-hemolytic streptococci - around the colony, greenish staining and partial hemolysis (greening is caused by the conversion of oxyhemoglobin to methemoglobin);
• a1-hemolytic streptococci in comparison with b-hemolytic streptococci form a less pronounced and unclear hemolysis zone;
• a- and al-streptococci are called S. Viridans (green streptococci);
• y-non-hemolytic streptococci do not cause hemolysis on a dense nutrient medium. Serological classification was of great practical importance.

Streptococci have a complex antigenic structure: they have a common antigen for all genus and various other antigens. Among them, group-specific polysaccharide antigens localized in the cell wall are of particular importance for the classification. For these antigens, at the suggestion of R. Lansfeld, streptococci are divided into serological groups, denoted by the letters A, B, C, D, F, G, etc. Now 20 serological groups of streptococci (from A to V) are known. Pathogenic for human streptococci belong to group A, to groups B and D, less often to C, F and G. In connection with this, the definition of the group affiliation of streptococci is a crucial moment in the diagnosis of the diseases caused by them. Group polysaccharide antigens are determined by the appropriate antisera in the precipitation reaction.

In addition to group antigens in hemolytic streptococci, type-specific antigens were found. In streptococcus group A they are proteins M, T and R. The protein M is thermally stable in an acidic medium, but is destroyed by trypsin and pepsin. It is found after hydrochloric acid hydrolysis of streptococci by a precipitation reaction. The T protein breaks down when heated in an acidic medium, but is resistant to the action of trypsin and pepsin. It is determined by the agglutination reaction. R-antigen is also found in streptococcus serogroups B, C and D. It is sensitive to pepsin, but not to trypsin, destroyed by heating in the presence of acid, but is stable with moderate heating in a weak alkaline solution. M-antigen hemolytic streptococcus serogroup A is divided into a large number of serovariants (about 100), their definition has epidemiological significance. By T-protein, streptococcus serogroup A is also subdivided into several dozen serovariants. In group B there are 8 different serovariants.

Streptococci also have cross-reacting antigens, common to antigens of the cells of the basal layer of skin epithelium and epithelial cells of the cortical and medullary zones of the thymus, which may be the cause of autoimmune disorders caused by these cocci. In the cell wall of streptococci, an antigen (receptor I) is detected, to which their ability, as well as staphylococci, having protein A is associated, interact with the Fc fragment of the IgG molecule.

Diseases caused by streptococci are divided into 11 classes. The main groups of these diseases are:

• various suppurative processes - abscesses, phlegmon, otitis, peritonitis, pleurisy, osteomyelitis, etc .;
• erysipelas - wound infection (inflammation of the lymphatic vessels of the skin and subcutaneous tissue);
• suppurative complications of wounds (especially in wartime) - abscesses, phlegmon, sepsis, etc .;
• angina - acute and chronic;
• sepsis: acute sepsis (acute endocarditis); chronic sepsis (chronic endocarditis); postpartum (puerperal) sepsis;
• rheumatism;
• pneumonia, meningitis, creeping corneal ulcer (pneumococcus);
• scarlet fever;
• tooth decay - the causative agent of it most often is S. Mutatis. Cariogenic streptococci genes responsible for the synthesis of enzymes that provide colonization of the surface of teeth and gums with these streptococci are isolated and studied.

Although most streptococcal pathogens belong to serogroup A, streptococci of serogroups D and B play an important role in human pathology. Streptococci of serogroup D (enterococci) are recognized as causative agents of wound infections, various purulent surgical diseases, purulent complications in pregnant women, puerperas and gynecological patients , infect the kidneys, bladder, cause sepsis, endocarditis, pneumonia, food toxic infections (proteolytic variants of enterococci). Streptococci of serogroup B (S. Agalactiae) often cause diseases of newborns - respiratory infections, meningitis, septicemia. Epidemiologically, they are associated with the carriage of this type of streptococcus in the mother and the staff of the maternity hospitals.

Anaerobic streptococci (Peptostreptococcus), which are found in healthy people in the microflora of the respiratory tract, mouth, nasopharynx, intestine and vagina, may also be responsible for purulent-septic diseases - appendicitis, postpartum sepsis, etc.

[11], [12], [13], [14]

Laboratory diagnostics of streptococcal infection

The main method of diagnosing streptococcal diseases is bacteriological. The material for the study is blood, pus, mucus from the pharynx, plaque from the tonsils, detachable wounds. The decisive stage in the study of the isolated pure culture is the determination of its serogroup. Two methods are used for this purpose.

• Serological - definition of the group polysaccharide by the precipitation reaction. For this purpose, use the appropriate group-specific sera. If the strain is beta-hemolytic, its polysaccharide antigen is extracted with HCl and tested with antisera serogroups A, B, C, D, F and G. If the strain does not cause beta-hemolysis, its antigen is extracted and tested with only B and D antisera. Antisera of groups A, C, F and G often give cross-reactions with alpha-hemolytic and non-hemolytic streptococci. Streptococci that do not cause beta-hemolysis and do not belong to groups B and D are identified by other physiological tests. Group D streptococci are isolated into an independent genus of Enterococcus.
• The grouping method is based on the ability of aminopeptidase (an enzyme that produces streptococcus serogroups A and D) to hydrolyze pyrrolidine naphthylamide. For this purpose, commercial kits of the necessary reagents are prepared for the determination of Group A streptococci in blood and broth cultures. However, the specificity of this method is less than 80%.

Serotyping of streptococcus serogroup A is produced by reaction or precipitation (determine M-serotype), or agglutination (determine T-serotype) only for epidemiological purposes.

Among the serological reactions for the detection of streptococcus serogroups A, B, C, D, F and G, coagglutination and latex agglutination reactions are used. Determination of the titer of anti-hyaluronidase and anti-O-streptolysin antibodies is used as an auxiliary method for the diagnosis of rheumatism and for evaluating the activity of the rheumatic process.

For the detection of streptococcal polysaccharide antigens, it is also possible to use IFM.