Pseudomonas bacillus

The genus Pseudomonas belongs to the family Pseudomonadaceae (class Gammaproteobacteria, type Proteobacteria) and contains more than 20 species. Some of them are natural inhabitants of soil and water and therefore play a huge role in the circulation of substances in nature. Other species play a significant role in the pathology of humans (see also "Pathogens of glanders and melioidosis "), animals and plants.

Pseudomonas are gram-negative non-fermenting bacteria, which include representatives of the genus Pseudomonas, the typical species being Pseudomonas aeruginosa (blue pus bacillus), which is the causative agent of many purulent-inflammatory diseases, as well as some other species. Pseudomonas aeruginosa (blue pus bacillus)

The bacteria received their name for the characteristic blue-green coloration of the purulent discharge, which was first described by A. Lücke in 1862. However, the pathogen was isolated in pure culture by S. Gessard only in 1982. P. aeruginosa belongs to the Pseudomonadaceae family.

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Biochemical properties of Pseudomonas aeruginosa

Pseudomonas are gram-negative, motile, straight rods measuring 1-3 µm, located singly, in pairs, or in short chains. The mobility of Pseudomonas aeruginosa is ensured by the presence of one, rarely two polar flagella (mopotrichous or amphitrichous). They do not form spores and have type IV pili (fimbriae). Under certain conditions, they can produce capsule-like extracellular mucus of a polysaccharide nature. There are also so-called mucoid strains that produce an increased amount of mucus. Such bacteria are most often isolated from the sputum of patients with cystic fibrosis.

All pseudomonads are obligate aerobes that grow well on simple nutrient media. On a liquid nutrient medium, the bacteria form a characteristic grayish-silver film on the surface. On blood agar, hemolysis zones are observed around the colonies of Pseudomonas aeruginosa; to isolate a pure culture of Pseudomonas aeruginosa, selective or differential diagnostic nutrient media with the addition of antiseptics are used - malachite agar with the addition of brilliant green or CPC agar with acetamide. The optimal growth temperature is 37 °C, but Pseudomonas aeruginosa is capable of growing at 42 °C, which allows it to be distinguished from other pseudomonads. Colonies of Pseudomonas aeruginosa are smooth, round, dryish or slimy (in capsular strains). When cultivated on dense nutrient media, P. aeruginosa produces a peculiar sweetish smell of jasmine, strawberry soap or caramel. A characteristic biological feature of bacteria of the P. aeruginosa species is also their ability to synthesize water-soluble pigments that color the dressings of patients or nutrient media during their cultivation. Most often, they produce a phenazine pigment - pyocyanin of a blue-green color, but they can also form a green pigment fluorescein (pyoverdin), which fluoresces in UV rays, as well as red (pyorubin), black (pyomelanin) or yellow (oxyphenazine).

Pseudomonas aeruginosa does not ferment glucose and other carbohydrates, but it can oxidize them to obtain energy. For differential diagnostics, allowing to distinguish pseudomonads from other gram-negative rods, an OF test (glucose oxidation/fermentation test) is used on a special medium. For this, a pure culture of pseudomonads is inoculated into two test tubes, one of which is then incubated under aerobic conditions, and the other under anaerobic conditions. Pseudomonas can only oxidize lactose, therefore the color of the indicator changes only in the test tube kept under aerobic conditions. P. aeruginosa reduces nitrates to nitrites, and also has proteolytic activity: it liquefies gelatin, hydrolyzes casein. Pseudomonas aeruginosa has catalase and cytochrome oxidase.

Many strains of Pseudomonas aeruginosa produce bacteriocins called pyocins, which have bactericidal properties. Pyocynotyping of Pseudomonas aeruginosa strains is used for epidemiological marking and intraspecific identification of P. aeruginosa. For this purpose, the spectrum of pyocins secreted by the strain under study or its sensitivity to pyocins of other pseudomonads is determined.

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Antigenic properties of Pseudomonas aeruginosa

Pseudomonas aeruginosa has a complex antigen structure due to the presence of O- and H-antigens. The LPS of the cell wall is a type-specific thermostable O-antigen and is used for serotyping of P. aeruginosa strains. The thermolabile flagellar H-antigen is protective, and vaccines are based on it. Pili (fimbriae) antigens are also found on the surface of Pseudomonas aeruginosa cells. In addition, P. aeruginosa produces a number of extracellular products with antigenic properties: exotoxin A, protease, elastase, extracellular mucus.

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Pathogenicity factors of Pseudomonas aeruginosa

One of the main factors of pathogenicity of Pseudomonas aeruginosa and other pseudomonads is O-antigen - a lipopolysaccharide of the cell wall, the mechanism of action of which is the same as that of other gram-negative bacteria.

P. aeruginosa has a variety of pathogenicity factors that are involved in the development of pseudomonas infection. Among the most important of these are the following.

Adhesion and colonization factors are type IV pili (fimbriae) and extracellular mucus of P. aeruginosa.

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Toxins

LPS of the outer membrane of the cell wall of P. aeruginosa has endotoxin properties and is involved in the development of fever, oliguria, and leukopenia in patients.

Pseudomonas exotoxin A is a pitotoxin that causes profound disturbances in cellular metabolism by inhibiting protein synthesis in cells and tissues. Like diphtheria toxin, it is an ADP-ribosyltransferase that inhibits the elongation factor EF-2 and therefore causes disturbances in protein synthesis. It has also been proven that exotoxin A, along with protease, inhibits the synthesis of immunoglobulins and causes neutropenia. Exotoxin A is produced in an inactive form as a protoxin and is activated by various enzymes within the body. Exotoxin A has protective properties, i.e. antibodies to it protect the host cells from its damaging effects and prevent the development of bacteremia and Pseudomonas sepsis.

Exotoxin S (exotzyme S) is found only in highly virulent strains of Pseudomonas aeruginosa. The mechanism of its damaging effect on cells is still unclear, but it is known that infections caused by exoenzyme-3-producing strains of Pseudomonas aeruginosa often end fatally. Exotoxins A and S also disrupt the activity of phagocytes.

Leukocidin is also a cytotoxin with a pronounced toxic effect on human blood granulocytes.

Enterotokine and permeability factors play a certain role in the development of local tissue lesions in intestinal forms of Pseudomonas aeruginosa infection, causing disturbances in water-salt metabolism.

Enzymes of aggression

P. aeruginosa produces two types of hemolysins: thermolabile phospholipase C and thermostable glycolipid. Phospholipase C destroys phospholipids in surfactants on the alveolar surface of the lungs, causing the development of atelectasis (bronchiectasis) in respiratory tract pathology.

Neuraminidase also plays an important role in the pathogenesis of bronchopulmonary diseases of pseudomonas etiology and cystic fibrosis, as it is involved in the colonization of respiratory tract mucin.

Elastase, as well as other proteolytic enzymes of Pseudomonas aeruginosa and exotoxin A cause hemorrhages, tissue destruction and necrosis in the lesions of eye infections, pneumonia, and septicemia of Pseudomonas aeruginosa etiology.

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Pseudomonas resistance

P. aeruginosa is characterized by a fairly high resistance to antibiotics, which is explained by the poor permeability of the outer membrane of these bacteria due to an innate defect in porins, as well as the ability of bacteria to synthesize penicillinase.

P. aeruginosa remains viable in conditions of almost complete absence of food sources: it survives well in fresh, sea and even distilled water. It has also been proven that Pseudomonas aeruginosa cultures can survive and even multiply in solutions of disinfectants (for example, furacillin) intended for storing catheters and various medical instruments, washing wounds in burn and surgical hospitals.

At the same time, P. aeruginosa is sensitive to drying, the action of chlorine-containing disinfectants and is easily inactivated when exposed to high temperatures (boiling, autoclaving).

Epidemiology of diseases caused by Pseudomonas aeruginosa

Pseudomonas aeruginosa disease can develop as a result of autoinfection (endogenous infection) or exogenously. The source of infection is people (sick or carriers of bacteria), as well as various natural reservoirs of nature (soil and various fresh and salt water bodies). It has been established that about 5-10% of healthy people are carriers of various strains of P. aeruginosa (they normally colonize the intestine) and about 70% of patients in hospital. Pseudomonas are also found everywhere: in water supply and ventilation systems, on fruits and vegetables, houseplants, on the surface of soap, hand washers, towels, in breathing apparatus, etc. Therefore, Pseudomonas aeruginosa infection can be considered saproanthroponosis. The mechanisms and routes of infection in infections caused by Pseudomonas aeruginosa are contact, respiratory, blood, fecal-oral.

Pseudomonas aeruginosa infection can occur both in immunodeficient individuals with severe concomitant pathology (diabetes, burn disease, leukemia, cystic fibrosis, immunosuppression in oncological diseases and organ transplantation), and in the foyer of normal immunological reactivity of the body. It is known that the adhesive activity of P. aeruginosa increases with an increase in ambient temperature, so visiting a swimming pool, sauna, taking therapeutic baths can also provoke a Pseudomonas aeruginosa infection.

Pseudomonas aeruginosa is the causative agent of hospital-acquired (hospital) infections, i.e. diseases that occur in people undergoing hospital treatment. Infection with Pseudomonas aeruginosa in a clinic may be associated with medical procedures (catheterization of the bladder, endoscopic examination, wound washing, bandaging, treatment of burn surfaces with antiseptics, use of a ventilator, etc.), when infection occurs through dirty hands of personnel, instruments on the surface of which the microbe forms a biofilm, or with the use of contaminated solutions.

Pseudomonas aeruginosa usually enter the human body through damaged tissues. When attached, they colonize the wound or burn surface, mucous membranes or human skin and multiply. In the absence of immune mechanisms against Pseudomonas aeruginosa infection in humans, the local process (infection of the urinary tract, skin, respiratory tract) can become widespread (generalized). Bacteremia leads to the spread of the pathogen and the development of sepsis, which often causes the formation of secondary purulent foci of infection. When exposed to pathogenic factors (exotoxins, aggressive enzymes), the functioning of organs and systems is disrupted and severe complications can develop - disseminated intravascular coagulation syndrome, shock, and respiratory distress syndrome.

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Symptoms of diseases caused by Pseudomonas aeruginosa

Pseudomonas aeruginosa causes purulent-inflammatory diseases of various localizations: wound infections, burn disease, meningitis, urinary tract infections, skin infections, eye diseases, necrotic pneumonia, sepsis, etc. Mortality from Pseudomonas aeruginosa sepsis reaches 50%.

Immunity

Antitoxic and antibacterial antibodies are found in the blood serum of healthy people, as well as those who have recovered from Pseudomonas aeruginosa infections, but their role in protecting against recurrent diseases has been little studied.

Laboratory diagnostics of diseases caused by Pseudomonas aeruginosa

The main diagnostic method is bacteriological examination. The materials for examination are blood (in septicemia), cerebrospinal fluid (in meningitis), pus and wound discharge (in infected wounds and burns), urine (in urinary tract infections), sputum (in respiratory tract infections), etc. Bacterioscopy of smears from the material under examination is of little information. When identifying P. aeruginosa, the nature of their growth on CPC agar, pigment formation, the presence of a characteristic specific odor of the culture, a positive pyrochrome oxidase test, detection of thermophilicity (growth at 42 °C), the ability to oxidize glucose in the OF test are taken into account. For intraspecific identification of bacteria, serotyping, pyopinotyping, and phage typing are carried out.

The serological method of research is aimed at detecting specific antibodies to Pseudomonas aeruginosa antigens (usually exotoxin A and LPS) using the complete immunofluorescence assay, the opsonophagocytic reaction, and some other tests.

Treatment of diseases caused by Pseudomonas aeruginosa

Antibiotics are used to treat pseudomonas infection, and it is recommended to combine drugs from different groups. Antimicrobial therapy is prescribed only after determining the antibiogram. In emergency cases, antibiotics are used empirically.

For the treatment of severe forms of Pseudomonas aeruginosa infection, hyperimmune plasma obtained from the blood of volunteers immunized with a polyvalent corpuscular Pseudomonas aeruginosa vaccine is also used.

For local treatment of skin infections (trophic ulcers, ecthyma, burn wounds) caused by P. aeruginosa, anti-pseudomonas heterologous immunoglobulin is used, obtained from the blood serum of rams hyperimmunized with a suspension of cultures of Pseudomonas aeruginosa of 7 different immunotypes, killed with formalin.

In addition, for the treatment of purulent skin infections, abscesses, thermal burns complicated by pseudomonas infection, cystitis, mastitis and other diseases of pseudomonas etiology (except sepsis), pseudomonas bacteriophage (bacteriophage pyocyansus) or polyvalent liquid pyobacteriophage can be used.

Prevention of diseases caused by Pseudomonas aeruginosa

Effective sterilization, disinfection and antisepsis, as well as compliance with aseptic rules are the main measures of non-specific prevention of pseudomonas infection in a hospital. The plan of preventive measures must necessarily include control over the contamination of the external environment (air, various objects, instruments and equipment), compliance with personal hygiene rules.

For the purpose of non-specific prevention of purulent-inflammatory diseases, patients with weakened anti-infective immunity are recommended to be prescribed immunomodulators.

Vaccines are used to create active immunity against Pseudomonas aeruginosa infection. Currently, vaccines have been developed from Pseudomonas aeruginosa LPS, polysaccharide subcorpuscular (chemical) vaccines, ribosomal vaccines, preparations from P. aeruginosa flagellar antigens and extracellular mucus components, as well as anatoxins from extracellular proteases and exotoxin A. In Russia, a polyvalent corpuscular Pseudomonas vaccine (from 7 strains of P. aeruginosa) and a staphyloproteus-Pseudomonas vaccine are used.

Active immunization against infections caused by P. aeruginosa is indicated for patients from risk groups (patients with cystic fibrosis, diabetes, and immunodeficient individuals). However, due to the fact that the immune response to vaccines in people with immunodeficiencies is late and not always complete, great importance is attached to combining methods of active and passive immunization.