Streptococci

Streptococci belong to the Streptococcaceae family (genus Streptococcus). They were first discovered by T. Bilroth in 1874 during erysipelas; by L. Pasteur in 1878 during postpartum sepsis; isolated in pure culture in 1883 by F. Feleisen.

Streptococci (Greek streptos - chain and coccus - grain) are gram-positive, cytochrome-negative, catalase-negative cells of spherical or ovoid shape with a diameter of 0.6-1.0 μm, growing in the form of chains of various lengths or as tetracocci; non-motile (except for some representatives of serogroup D); the content of G + C in DNA is 32-44 mol % (for the family). They do not form spores. Pathogenic streptococci form a capsule. Streptococci are facultative anaerobes, but there are also strict anaerobes. The temperature optimum is 37 °C, the optimal pH is 7.2-7.6. Pathogenic streptococci either do not grow or grow very poorly on ordinary nutrient media. Sugar broth and blood agar containing 5% defibrinated blood are usually used for their cultivation. The medium should not contain reducing sugars, as they inhibit hemolysis. On the broth, the growth is bottom-parietal in the form of a crumbly sediment, the broth is transparent. Streptococci that form short chains cause turbidity of the broth. On dense media, serogroup A streptococci form colonies of three types:

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

Streptococci ferment glucose, maltose, sucrose and some other carbohydrates to form acid without gas (except for S. kefir, which forms acid and gas), do not curdle milk (except for S. lactis), and do not have proteolytic properties (except for some enterococci).

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The main factors of pathogenicity of streptococci

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

Capsule. It consists of hyaluronic acid, similar to that found in tissue, so phagocytes do not recognize streptococci with a capsule as foreign antigens.

Erythrogenin is a scarlet fever toxin, a superantigen, that causes TSS. There are three serotypes (A, B, C). In patients with scarlet fever, it causes a bright red rash on the skin and mucous membrane. It has a pyrogenic, allergenic, immunosuppressive and mitogenic effect, and destroys platelets.

Hemolysin (streptolysin) O destroys erythrocytes, has a cytotoxic effect, including leukotoxic and cardiotoxic, and is produced by most streptococci of 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 is also produced by streptococci of serogroups A, C and G.

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

Chemotaxis inhibitory factor (aminopeptidase) inhibits the motility of neutrophil phagocytes.

Hyaluronidase is an invasion factor.

The turbidity factor is the hydrolysis of serum lipoproteins.

Proteases - destruction of various proteins; possibly associated with tissue toxicity.

DNases (A, B, C, D) - DNA hydrolysis.

The ability to interact with the Fc fragment of IgG via the I receptor - inhibition of the complement system and phagocyte activity.

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

Streptococcal resistance

Streptococci tolerate low temperatures well, are quite resistant to drying, especially in a protein environment (blood, pus, mucus), and remain viable for several months on objects and dust. When heated to a temperature of 56 °C, they die after 30 minutes, except for group D streptococci, which can withstand heating to 70 °C for 1 hour. A 3-5% solution of carbolic acid and lysol kills them within 15 minutes.

Post-infectious immunity

Antitoxins and type-specific M-antibodies play the main role in its formation. Antitoxic immunity after scarlet fever is strong and long-lasting. Antimicrobial immunity is also strong and long-lasting, but its effectiveness is limited by the type-specificity of M-antibodies.

Epidemiology of streptococcal infection

The source of exogenous streptococcal infection are patients with acute streptococcal diseases (tonsillitis, scarlet fever, pneumonia), as well as convalescents after them. The main way of infection is airborne, in other cases - direct contact and very rarely alimentary (milk and other food products).

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Symptoms of streptococcal infection

Streptococci are inhabitants of the mucous membranes of the upper respiratory tract, digestive and genitourinary tracts, therefore the diseases they cause can be endogenous or exogenous, i.e. caused either by their own cocci or as a result of infection from the outside. Having penetrated through damaged skin, streptococci spread from the local focus through the lymphatic and circulatory systems. Infection by airborne droplets or airborne dust leads to damage to the lymphoid tissue ( tonsillitis ), the process involves regional lymph nodes, from where the pathogen spreads through the lymphatic vessels and hematogenously.

The ability of streptococci to cause various diseases depends on:

• entry points (wound infections, puerperal sepsis, erysipelas, etc.; respiratory tract infections - scarlet fever, tonsillitis);
• the presence of various pathogenicity factors in streptococci;
• states of the immune system: in the absence of antitoxic immunity, infection with toxigenic streptococci of serogroup A leads to the development of scarlet fever, and in the presence of antitoxic immunity, tonsillitis occurs;
• sensitizing properties of streptococci; they largely determine the peculiarity of the pathogenesis of streptococcal diseases and are the main cause of complications such as nephrosonephritis, arthritis, damage to the cardiovascular system, etc.;
• purulent and septic functions of streptococci;
• the presence of a large number of serovariants of serogroup A streptococci according to M-antigen.

Antimicrobial immunity, which is caused by antibodies to the M protein, is type-specific, and since there are many serovariants for the M antigen, repeated cases of tonsillitis, erysipelas and other streptococcal diseases are possible. The pathogenesis of chronic infections caused by streptococci is more complex: chronic tonsillitis, rheumatism, nephritis. The following circumstances confirm the etiological role of serogroup A streptococci in them:

• These diseases usually occur after acute streptococcal infections (tonsillitis, scarlet fever);
• In such patients, streptococci or their L-forms and antigens are often found in the blood, especially during exacerbations, and, as a rule, hemolytic or greening streptococci on the mucous membrane of the pharynx;
• constant detection of antibodies to various streptococcal antigens. Of particular diagnostic value in patients with rheumatism during an exacerbation is the detection of anti-O-streptolysins and antihyaluronidase antibodies in high titers in the blood;
• development of sensitization to various streptococcal antigens, including the heat-stable component of erythrogenin. It is possible that autoantibodies to connective and renal tissue, respectively, play a role in the development of rheumatism and nephritis;
• obvious therapeutic effect of the use of antibiotics against streptococci (penicillin) during rheumatic attacks.

Scarlet fever

Scarlet fever (late Latin scarlatium - bright red color) is an acute infectious disease that clinically manifests itself as tonsillitis, lymphadenitis, small-point bright red rash on the skin and mucous membrane with subsequent peeling, as well as general intoxication of the body and a tendency to purulent-septic and allergic complications.

Scarlet fever is caused by beta-hemolytic streptococci of group A, which have M-antigen and produce erythrogenin. The etiologic role in scarlet fever was attributed to various microorganisms - protozoa, anaerobic and other cocci, streptococci, filterable forms of streptococci, viruses. A 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). I. G. Savchenko showed back in 1905-1906 that scarlet fever streptococcus produces a toxin, and the antitoxic serum obtained by him has a good therapeutic effect. Based on the work of I. G. Savchenko, the Dick spouses showed in 1923-1924 that:

• the intradermal administration of a small dose of toxin to individuals who have not had scarlet fever causes a positive local toxic reaction in the form of redness and swelling (Dick reaction);
• in people who have had scarlet fever, this reaction is negative (the toxin is neutralized by the antitoxin they have);
• The introduction of large doses of the toxin subcutaneously into individuals who have not had scarlet fever causes them to develop symptoms characteristic of scarlet fever.

Finally, by infecting volunteers with a streptococcus culture, they were able to reproduce scarlet fever. At present, the streptococcal etiology of scarlet fever is generally recognized. The peculiarity here is that scarlet fever is caused not by one particular serotype of streptococci, but by any of the beta-hemolytic streptococci that have an M-antigen and produce 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 that have different serotypes of the M-antigen (1, 2, 4 or another) and produce erythrogenins of different serotypes (A, B, C). A change of these serotypes is possible.

The main factors of streptococcal pathogenicity in scarlet fever are exotoxin (erythrogenin), pyogenic-septic and allergenic properties of streptococcus and its erythrogenin. Erythrogenin consists of two components - a heat-labile protein (the toxin itself) and a heat-stable substance with allergenic properties.

Scarlet fever is transmitted mainly by airborne droplets, but any wound surface can also be the entry point. The incubation period is 3-7, sometimes 11 days. The pathogenesis of scarlet fever reflects 3 main points related to the properties of the pathogen:

• the action of scarlet fever toxin, which causes the development of toxicosis - the first period of the disease. It is characterized by damage to peripheral blood vessels, the appearance of a small-point 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 the streptococcus itself. It is non-specific and manifests itself in the development of various purulent-septic processes (otitis, lymphadenitis, nephritis appear on the 2-3rd week of the disease);
• sensitization of the body. It is reflected in the form of various complications such as nephrosonephritis, polyarthritis, cardiovascular diseases, etc. on the 2nd-3rd week of illness.

In the clinic of scarlet fever, stage I (toxicosis) and stage II are also distinguished, when purulent-inflammatory and allergic complications are observed. Due to the use of antibiotics (penicillin) for the treatment of scarlet fever, the frequency and severity of complications have significantly decreased.

Post-infectious immunity

Strong, long-lasting (recurrent diseases are observed in 2-16% of cases), caused by antitoxins and immune memory cells. Those who have recovered from the disease also retain an allergic condition to the scarlet fever allergen. It is detected by intradermal injection of killed streptococci. Those who have recovered from the disease have redness, swelling, and soreness at the injection site (Aristovsky-Fanconi test). The Dick reaction is used to check for antitoxic immunity in children. With its help, it was established that passive immunity in children of the first year of life is maintained during the first 3-4 months.

Laboratory diagnostics of scarlet fever

In typical cases, the clinical picture of scarlet fever is so clear that bacteriological diagnostics are not performed. In other cases, it consists of isolating a pure culture of beta-hemolytic streptococcus, which is found on the mucous membrane of the pharynx in all patients with scarlet fever.

Aerobic gram-positive cocci, classified in the genera Aerococcus, Leuconococcus, Pediococcus and Lactococcus, are characterized by weak pathogenicity. The diseases they cause in humans are rare and mainly occur in individuals with impaired immune systems.

Classification of streptococci

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

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

The simplest classification is based on the ratio of streptococci to erythrocytes. A distinction is made between:

• b-hemolytic streptococci - when growing on blood agar, there is a clear zone of hemolysis around the colony;
• a-hemolytic streptococci - greenish coloration around the colony and partial hemolysis (greening is caused by the conversion of oxyhemoglobin to methemoglobin);
• a1-hemolytic streptococci, compared to b-hemolytic streptococci, form a less pronounced and cloudy zone of hemolysis;
• a- and al-streptococci are called S. viridans (green streptococci);
• y-non-hemolytic streptococci do not cause hemolysis on a solid nutrient medium. Serological classification has gained great practical importance.

Streptococci have a complex antigen structure: they have a common antigen for the entire genus and various other antigens. Among them, group-specific polysaccharide antigens localized in the cell wall are of particular importance for classification. According to these antigens, at the suggestion of R. Lansfeld, streptococci are divided into serological groups designated by the letters A, B, C, D, F, G, etc. Currently, 20 serological groups of streptococci are known (from A to V). Streptococci pathogenic for humans belong to group A, to groups B and D, less often to C, F and G. In this regard, determining the group affiliation of streptococci is a decisive moment in the diagnosis of the diseases they cause. Group polysaccharide antigens are determined using the corresponding antisera in the precipitation reaction.

In addition to group antigens, type-specific antigens have been found in hemolytic streptococci. In group A streptococci, these are the M, T, and R proteins. Protein M is heat-resistant in an acidic medium, but is destroyed by trypsin and pepsin. It is detected after hydrochloric acid hydrolysis of streptococci using a precipitation reaction. Protein T is destroyed by heating in an acidic medium, but is resistant to trypsin and pepsin. It is determined using an agglutination reaction. R-antigen has also been found in streptococci of serogroups B, C, and D. It is sensitive to pepsin, but not to trypsin, is destroyed by heating in the presence of acid, but is resistant to moderate heating in a weak alkaline solution. According to M-antigen, hemolytic streptococci of serogroup A are divided into a large number of serovariants (about 100), their determination is of epidemiological significance. According to the T-protein, serogroup A streptococci are also divided into several dozen serovariants. In group B, 8 serovariants are distinguished.

Streptococci also have cross-reacting antigens common to antigens of the basal layer cells of the 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. An antigen (receptor I) has been found in the cell wall of streptococci, which is associated with their ability, like staphylococci with protein A, to 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);
• purulent complications of wounds (especially in wartime) - abscesses, phlegmon, sepsis, etc.;
• tonsillitis - acute and chronic;
• sepsis: acute sepsis (acute endocarditis); chronic sepsis (chronic endocarditis); postpartum (puerperal) sepsis;
• rheumatism;
• pneumonia, meningitis, creeping corneal ulcer (pneumococcus);
• scarlet fever;
• dental caries - its causative agent is most often S. mutatis. Genes of cariogenic streptococci responsible for the synthesis of enzymes that ensure colonization of the surface of teeth and gums by these streptococci have been isolated and studied.

Although most of the streptococci pathogenic for humans belong to serogroup A, streptococci of serogroups D and B also 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, women in labor and gynecological patients, they infect the kidneys, bladder, cause sepsis, endocarditis, pneumonia, food poisoning (proteolytic variants of enterococci). Streptococci of serogroup B (S. agalactiae) often cause diseases in newborns - respiratory tract infections, meningitis, septicemia. Epidemiologically, they are associated with the carriage of this type of streptococci in the mother and the staff of maternity hospitals.

Anaerobic streptococci (Peptostreptococcus), which are found in healthy people as part of the microflora of the respiratory tract, mouth, nasopharynx, intestines and vagina, can also be the culprits of purulent-septic diseases - appendicitis, postpartum sepsis, etc.

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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, and wound discharge. The decisive stage of the study of the isolated pure culture is the determination of its serogroup. Two methods are used for this purpose.

• Serological - determination of the group polysaccharide using a precipitation reaction. For this purpose, the corresponding group-specific sera are used. If the strain is beta-hemolytic, its polysaccharide antigen is extracted with HCl and tested with antisera of serogroups A, B, C, D, F, and G. If the strain does not cause beta-hemolysis, its antigen is extracted and tested with antisera of groups B and D only. Antisera of groups A, C, F, and G often cross-react 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 as a separate genus, Enterococcus.
• The grouping method is based on the ability of aminopeptidase (an enzyme produced by serogroup A and D streptococci) to hydrolyze pyrrolidine-naphthylamide. For this purpose, commercial kits of the necessary reagents are produced for the determination of group A streptococci in blood and broth cultures. However, the specificity of this method is less than 80%.

Serotyping of serogroup A streptococci is performed using either a precipitation reaction (determines the M serotype) or an agglutination reaction (determines the T serotype) only for epidemiological purposes.

Among the serological reactions for detecting streptococci of 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 diagnosing rheumatism and for assessing the activity of the rheumatic process.

IFM can also be used to detect streptococcal polysaccharide antigens.