Staphylococci

Staphylococcus was discovered in 1878 by R. Koch and in 1880 by L. Pasteur in purulent material. L. Pasteur, having infected a rabbit, finally proved the role of staphylococcus as a causative agent of purulent inflammation. The name "staphylococcus" was given in 1881 by A. Ogston (due to the characteristic arrangement of cells), and its properties were described in detail in 1884 by F. Rosenbach.

Staphylococci are gram-positive, geometrically regular, spherical cells with a diameter of 0.5-1.5 μm, usually located in clusters, catalase-positive, reduce nitrates to nitrites, actively hydrolyze proteins and fats, ferment glucose under anaerobic conditions to form acid without gas. They can usually grow in the presence of 15% NaCl and at a temperature of 45 °C. The G + C content in DNA is 30-39 mol %. Staphylococci do not have flagella and do not form spores. They are widespread in nature. Their main reservoir is the skin of humans and animals and their mucous membranes communicating with the external environment. Staphylococci are facultative anaerobes, only one species (Staphylococcus saccharolyticus) is a strict anaerobe. Staphylococci are not demanding to nutrient media, grow well on ordinary media, the temperature optimum for growth is 35-37 "C, pH 6.2-8.4. Colonies are round, 2-4 mm in diameter, with smooth edges, convex, opaque, painted in the color of the formed pigment. Growth in liquid culture is accompanied by uniform turbidity, over time, a loose sediment falls out. When growing on ordinary media, staphylococci do not form a capsule, however, when sowing by injection into semi-liquid agar with plasma or serum, most strains of S. aureus form a capsule. Acapsular strains in semi-liquid agar grow in the form of compact colonies, capsular strains form diffuse colonies.

Staphylococci have high biochemical activity: they ferment glycerol, glucose, maltose, lactose, sucrose, mannitol with the release of acid (without gas); they form various enzymes (plasmacoagulase, fibrinolysin, lecithinase, lysozyme, alkaline phosphatase, DNase, hyaluronidase, telluride reductase, proteinase, gelatinase, etc.). These enzymes play an important role in the metabolism of staphylococci and largely determine their pathogenicity. Such enzymes as fibrinolysin and hyaluronidase cause high invasiveness of staphylococci. Plasmacoagulase is the main factor of their pathogenicity: it protects against phagocytosis and converts prothrombin into thrombin, which causes fibrinogen coagulation, as a result of which each cell is covered with a protein film that protects against phagocytes.

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Pathogenicity factors of staphylococci

Staphylococcus is a unique microorganism. It can cause more than 100 different diseases, related to eleven classes according to the International Classification of 1968. Staphylococci can affect any tissue, any organ. This property of staphylococci is due to the presence of a large complex of pathogenicity factors.

Adhesion factors - the attachment of staphylococci to tissue cells is due to their hydrophobicity (the higher it is, the stronger the adhesive properties are), as well as the adhesive properties of polysaccharides, possibly also protein A, and the ability to bind fibronectin (a receptor for some cells).

Various enzymes that play the role of “aggression and defense” factors: plasmacoagulase (the main pathogenicity factor), hyaluronidase, fibrinolysin, DNase, lysozyme-like enzyme, lecithinase, phosphatase, proteinase, etc.

Complex of secreted exotoxins:

• membrane-damaging toxins - a, p, 8 and y. Previously, they were described as hemolysins, necrotoxins, leukocidins, lethal toxins, i.e. by the nature of their action: hemolysis of erythrocytes, necrosis when administered intradermally to a rabbit, destruction of leukocytes, death of a rabbit when administered intravenously. However, it turned out that such an effect is caused by the same factor - a membrane-damaging toxin. It has a cytolytic effect on various types of cells, which is manifested in the following way. Molecules of this toxin first bind to as yet unknown receptors of the target cell membrane or are non-specifically absorbed by lipids contained in the membrane, and then form a mushroom-shaped heptamer from 7 molecules, consisting of 3 domains. The domains that form the "cap" and "edge" are located on the outer surface of the membranes, and the "foot" domain serves as a transmembrane channel-pore. It is through this that small molecules and ions enter and exit, which leads to swelling and death of cells with a nucleus and osmotic lysis of erythrocytes. Several types of membrane-damaging (pore-forming) toxins have been discovered: a-, b-, s- and y-hemolysins (a-, b-, S- and y-toxins). They differ in a number of properties. Hemolysin a is most often found in staphylococci isolated from humans; it lyses human, rabbit and ram erythrocytes. It causes a lethal effect in rabbits after 3-5 minutes of intravenous administration. Hemolysin b is most often found in staphylococci of animal origin; it lyses human and ram erythrocytes (better at a lower temperature). Hemolysin S lyses human and many animal erythrocytes. The lethal effect on a rabbit when administered intravenously occurs within 16-24-48 hours. Very often, staphylococci contain a- and 8-toxins simultaneously;
• exfoliative toxins A and B are distinguished by their antigenic properties, temperature sensitivity (A is thermostable, B is thermolabile), and the localization of the genes that control their synthesis (A is controlled by a chromosomal gene, B by a plasmid gene). Often, both exfoliatins are synthesized in the same strain of S. aureus. These toxins are associated with the ability of staphylococci to cause pemphigus in newborns, bullous impetigo, and scarlet fever-like rash;
• true leukocidin is a toxin that differs from hemolysins in its antigenic properties and selectively acts on leukocytes, destroying them;
• an exotoxin that causes toxic shock syndrome (TSS). It has superantigen properties. TSS is characterized by fever, decreased blood pressure, skin rashes followed by peeling on the hands and feet, lymphopenia, sometimes diarrhea, kidney damage, etc. More than 50% of S. aureus strains are capable of producing and secreting this toxin.

Strong allergenic properties, which are possessed by both the components of the cell structure and exotoxins and other waste products secreted by bacteria. Staphylococcal allergens are capable of causing hypersensitivity reactions of both the delayed type (DTH) and immediate type (IT) - Staphylococci are the main culprits of skin and respiratory allergies (dermatitis, bronchial asthma, etc.). The peculiarity of the pathogenesis of staphylococcal infection and its tendency to become chronic are rooted in the DTH effect.

Cross-reacting antigens (with isoantigens of erythrocytes A and B, kidneys and skin - induction of autoantibodies, development of autoimmune diseases).

Factors that inhibit phagocytosis. Their presence can manifest itself in inhibition of chemotaxis, protection of cells from absorption by phagocytes, providing staphylococci with the ability to reproduce in phagocytes and blocking the "oxidative burst". Phagocytosis is inhibited by the capsule, protein A, peptideglycan, teichoic acids, toxins. In addition, staphylococci induce the synthesis of phagocytic activity suppressors by some cells of the body (for example, splenocytes). Inhibition of phagocytosis not only prevents the body from clearing staphylococci, but also disrupts the function of processing and presentation of antigens to T- and B-lymphocytes, which leads to a decrease in the strength of the immune response.

The presence of a capsule in staphylococci increases their virulence for white mice, makes them resistant to the action of phages, does not allow typing with agglutinating serums and masks protein A.

Teichoic acids not only protect staphylococci from phagocytosis, but apparently play a significant role in the pathogenesis of staphylococcal infections. It has been established that in children suffering from endocarditis, antibodies to teichoic acids are detected in 100% of cases.

Mitogenic action of staphylococci on lymphocytes (this action is exerted by protein A, enterotoxins and other products secreted by staphylococci).

Enterotoxins A, B, CI, C2, C3, D, E. They are characterized by antigen specificity, thermal stability, resistance to formalin (do not turn into anatoxins) and digestive enzymes (trypsin and pepsin), and are stable in the pH range from 4.5 to 10.0. Enterotoxins are low-molecular proteins with a molecular weight of 26 to 34 kDa with superantigen properties.

It has also been established that there are genetically determined differences in susceptibility to staphylococcal infection and the nature of its course in people. In particular, severe staphylococcal purulent-septic diseases are more often found in people with blood groups A and AB, less often - in people with blood groups 0 and B.

The ability of staphylococci to cause food poisoning of the intoxication type is associated with the synthesis of enterotoxins. Most often they are caused by enterotoxins A and D. The mechanism of action of these enterotoxins is poorly understood, but it differs from the action of other bacterial enterotoxins, which disrupt the function of the adenylate cyclase system. All types of staphylococcal enterotoxins cause a similar picture of poisoning: nausea, vomiting, pain in the pancreas, diarrhea, sometimes headache, fever, muscle spasm. These features of staphylococcal enterotoxins are due to their superantigenic properties: they induce excessive synthesis of interleukin-2, which causes intoxication. Enterotoxins excite the smooth muscles of the intestine and increase the motility of the gastrointestinal tract. Poisoning is most often associated with the consumption of staphylococcal-infected dairy products (ice cream, pastries, cakes, cheese, cottage cheese, etc.) and canned goods with butter. Infection of dairy products can be associated with mastitis in cows or with purulent-inflammatory diseases of people involved in food production.

Thus, the abundance of various pathogenicity factors in staphylococci and their high allergenic properties determine the features of the pathogenesis of staphylococcal diseases, their nature, localization, severity of the course and clinical manifestations. Avitaminosis, diabetes, decreased immunity contribute to the development of staphylococcal diseases.

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Staphylococcal resistance

Among non-spore-forming bacteria, staphylococci, like mycobacteria, have the greatest resistance to external factors. They tolerate drying well and remain viable and virulent for weeks and months in dry, fine dust, being a source of dust infection. Direct sunlight kills them only within many hours, and diffused light has a very weak effect. They are also resistant to high temperatures: they can withstand heating to 80 °C for about 30 minutes, dry heat (110 °C) kills them within 2 hours; they tolerate low temperatures well. Sensitivity to chemical disinfectants varies greatly, for example, a 3% phenol solution kills them within 15-30 minutes, and a 1% aqueous chloramine solution - in 2-5 minutes.

Epidemiology of staphylococcal infections

Since staphylococci are permanent inhabitants of the skin and mucous membranes, the diseases caused by them can be either autoinfections (with various damages to the skin and mucous membranes, including microtraumas) or exogenous infections caused by contact-household, airborne, airborne dust or alimentary (food poisoning) methods of infection. Of particular importance is the carriage of pathogenic staphylococci, since carriers, especially in medical institutions (various surgical clinics, maternity hospitals, etc.) and in closed groups, can cause staphylococcal infections. The carriage of pathogenic staphylococci can be temporary or intermittent, but people who have it permanently (resident carriers) pose a particular danger to others. In such people, staphylococci persist for a long time and in large quantities on the mucous membranes of the nose and throat. The reason for the long-term carriage is not entirely clear. It may be a consequence of weakening of local immunity (lack of secretory IgA), disruption of the functions of the mucous membrane, increased adhesive properties of staphylococcus, or caused by some of its other properties.

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Symptoms of Staph Infections

Staphylococci easily penetrate the body through the smallest damage to the skin and mucous membranes and can cause a variety of diseases - from acne to severe peritonitis, endocarditis, sepsis or septicopyemia, in which the mortality rate reaches 80%. Staphylococci cause furuncles, hidradenitis, abscesses, phlegmon, osteomyelitis; in wartime - frequent culprits of purulent complications of wounds; staphylococci play a leading role in purulent surgery. Possessing allergenic properties, they can cause psoriasis, hemorrhagic vasculitis, erysipelas, nonspecific polyarthritis. Infection of food products with staphylococci is a common cause of food poisoning. Staphylococci are the main culprits of sepsis, including in newborns. Unlike bacteremia (bacteria in the blood), which is a symptom of a disease and is observed in many bacterial infections, sepsis (septicemia - putrefaction of the blood) is an independent disease with a specific clinical picture, which is based on damage to the organs of the reticuloendothelial system (mononuclear phagocyte system - MPS). In sepsis, there is a purulent focus from which the pathogen periodically enters the blood, spreads throughout the body and affects the reticuloendothelial system (MSP), in the cells of which it multiplies, releasing toxins and allergens. At the same time, the clinical picture of sepsis depends weakly on the type of pathogen, but is determined by damage to certain organs.

Septicopyemia is a form of sepsis in which the pathogen causes purulent foci in various organs and tissues, i.e. it is sepsis complicated by purulent metastases.

Bacteremia in sepsis and septicopyemia can be short-term and long-term.

Post-infectious immunity exists, it is caused by both humoral and cellular factors. Antitoxins, antimicrobial antibodies, antibodies against enzymes, as well as T-lymphocytes and phagocytes play an important role in it. The intensity and duration of immunity against staphylococci have not been studied enough, since their antigen structure is too diverse, and there is no cross-immunity.

Classification of staphylococci

The genus Staphylococcus includes more than 20 species, which are divided into two groups - coagulase-positive and coagulase-negative staphylococci. Various features are used to differentiate the species.

Coagulase-positive staphylococci are mainly pathogenic for humans, but many coagulase-negative ones are also capable of causing diseases, especially in newborns (neonatal conjunctivitis, endocarditis, sepsis, urinary tract diseases, acute gastroenteritis, etc.). S. aureus, depending on who is its main carrier, is divided into 10 ecovars (hominis, bovis, ovis, etc.).

More than 50 types of antigens have been found in staphylococci, antibodies are formed in the body to each of them, many of the antigens have allergenic properties. By specificity, antigens are divided into generic (common to the entire genus Staphylococcus); cross-reacting - antigens common with isoantigens of human erythrocytes, skin and kidneys (they are associated with autoimmune diseases); species and type-specific antigens. According to type-specific antigens detected in the agglutination reaction, staphylococci are divided into more than 30 serovariants. However, the serological method of typing staphylococci has not yet received widespread use. Protein A, which is formed by S. aureus, is considered species-specific. This protein is located superficially, it is covalently linked to peptideglycan, its m.m. is about 42 kD. Protein A is especially actively synthesized in the logarithmic growth phase at a temperature of 41 °C, is thermolabile, and is not destroyed by trypsin; its unique property is the ability to bind to the Fc fragment of immunoglobulins IgG (IgG1, IgG2, IgG4), and to a lesser extent to IgM and IgA. Several regions capable of binding to a region of the immunoglobulin polypeptide chain located on the border of the CH2 and CH3 domains have been identified on the surface of protein A. This property has found wide application in the coagglutination reaction: staphylococci loaded with specific antibodies, which have free active centers, give a rapid agglutination reaction when interacting with an antigen.

The interaction of protein A with immunoglobulins leads to dysfunction of the complement and phagocyte systems in the patient's body. It has antigenic properties, is a strong allergen and induces the proliferation of T- and B-lymphocytes. Its role in the pathogenesis of staphylococcal diseases has not yet been fully elucidated.

S. aureus strains vary in their sensitivity to staphylococcal phages. To type S. aureus, an international set of 23 temperate phages is used, which are divided into four groups:

• Group 1 - phages 29.52, 52A, 79, 80;
• Group 2 - phages 3A, 3C, 55, 71;
• Group 3 - phages 6, 42E, 47, 53, 54, 75, 77, 83A, 84, 85;
• Group 4 - phages 94, 95, 96;
• outside the groups - phage 81.

The relationship of staphylococci to phages is peculiar: the same strain can be lysed either by one phage or by several simultaneously. But since their sensitivity to phages is a relatively stable feature, phage typing of staphylococci is of great epidemiological importance. The disadvantage of this method is that no more than 65-70% of S. aureus can be typed. In recent years, sets of specific phages have been obtained for typing S. epidermidis.

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Laboratory diagnostics of staphylococcal infections

The main method is bacteriological; serological reactions have been developed and implemented. If necessary (in case of intoxication), a biological test is used. The material for bacteriological examination is blood, pus, mucus from the pharynx, nose, wound discharge, sputum (in case of staphylococcal pneumonia), feces (in case of staphylococcal colitis), in case of food intoxication - vomit, feces, gastric lavage, suspicious products. The material is inoculated on blood agar (hemolysis), on milk-salt (milk-yolk-salt) agar (the growth of foreign bacteria is inhibited due to NaCl, pigment and lecithinase are better detected). The isolated culture is identified by species characteristics, the presence of the main characteristics and factors of pathogenicity (golden pigment, mannitol fermentation, hemolysis, plasmacoagulase) is determined, sensitivity to antibiotics is necessarily checked, and phage typing is performed if necessary. Among the serological reactions for the diagnosis of purulent-septic diseases, RPGA and IFM are used, in particular for determining antibodies to teichoic acid or species-specific antigens.

Three methods are used to determine the enterotoxigenicity of staphylococci:

• serological - using specific antitoxic serums in a gel precipitation reaction, enterotoxin is detected and its type is determined;
• biological - intravenous administration of filtrate of broth culture of staphylococcus to cats at a dose of 2-3 ml per 1 kg of weight. Toxins cause vomiting and diarrhea in cats;
• indirect bacteriological method - isolation of a pure culture of staphylococcus from a suspicious product and determination of its pathogenicity factors (the formation of enterotoxin correlates with the presence of other pathogenicity factors, in particular RNase).

The simplest and most sensitive method for detecting enterotoxin is the serological method.

Treatment of staph infections

For the treatment of staphylococcal diseases, beta-lactam antibiotics are mainly used, to which sensitivity should first be determined. In severe and chronic staphylococcal infections, a positive effect is achieved by specific therapy - the use of autovaccine, anatoxin, antistaphylococcal immunoglobulin (human), antistaphylococcal plasma.

Specific prevention of staphylococcal infections

To create artificial immunity against staphylococcal infection, staphylococcal anatoxin (liquid and tablet) is used, but it creates antitoxic immunity only against staphylococci lysed mainly by group I phages. The use of vaccines from killed staphylococci or their antigens, although it leads to the appearance of antimicrobial antibodies, but only against those serovarcanths from which the vaccine is made. The problem of finding a highly immunogenic vaccine effective against many types of pathogenic staphylococci is one of the most important problems of modern microbiology.