Bordetellae

Whooping cough is an acute infectious disease, predominantly affecting children, characterized by a cyclical course and paroxysmal spasmodic cough.

The causative agent, Bordetella pertussis, was first discovered in 1900 in smears from a child's sputum and then isolated in pure culture in 1906 by J. Bordet and O. Gengou. The causative agent of a disease similar to whooping cough but milder, bordetella parapertussis, was isolated and studied in 1937 by G. Eldering and P. Kendrick and independently in 1937 by W. Bradford and B. Slavin. Bordetella bronchiseptica, the causative agent of a rare whooping cough-like disease in humans, was isolated in 1911 in dogs by N. Ferry and in humans by Brown in 1926. In 1984, a new species, Bordetella avium, was isolated; its pathogenicity for humans has not yet been established.

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Morphology of Bordetella

Bordetella belong to the class Betaproteobacteria, are gram-negative, and stain well with all aniline dyes. Sometimes bipolar staining is revealed due to volutin grains at the cell poles. The causative agent of whooping cough has the form of an ovoid rod (coccobacterium) measuring 0.2-0.5 x 1.0-1.2 μm. Parakoklyushnaya bacillus has the same shape, but is somewhat larger (0.6 x 2 μm). They are usually located singly, but can be located in pairs. They do not form spores; a capsule is found in young cultures and in bacteria isolated from a macroorganism. Bordetella are immobile, with the exception of B. bronchiseptica, which is a peritrichous bacteria. The G + C content in DNA is 61-70 mol %. They belong to hemophilic bacteria.

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Biochemical properties of Bordetella

Bordetella are strict aerobes, chemoorganotrophs. The optimum growth temperature is 35-36 °C. The causative agent of whooping cough in the smooth S-form (the so-called phase I), unlike the other two species of Bordetella, does not grow on MPB and MPA, since its reproduction is hampered by the accumulation of unsaturated fatty acids in the medium formed during growth, as well as colloidal sulfur and other metabolic products formed during growth. To neutralize them (or adsorb them), starch, albumin and charcoal or ion-exchange resins must be added to the medium for growing whooping cough bacteria. The microbe requires the presence of 3 amino acids in the growth medium - proline, cysteine and glutamic acid, the source of which are casein or bean hydrolysates. The traditional medium for growing the pertussis bacillus is the Bordet-Gengou medium (potato-glycerin agar with the addition of blood), on which it grows in the form of smooth, shiny, transparent dome-shaped colonies with a pearl or metallic mercury hue, about 1 mm in diameter, which grow on the 3rd-4th day. On another medium - casein-coal agar (CCA) - also on the 3rd-4th day, smooth convex colonies about 1 mm in diameter grow, having a grayish-cream color and viscous consistency. Colonies of parapertussis bacteria do not differ in appearance from pertussis, but are larger and are detected on the 2nd-3rd day, and colonies of B. bronchiseptica are detected already on the 1st-2nd day.

A characteristic feature of whooping cough bacteria is their tendency to rapidly change their cultural and serological properties when the composition of the nutrient medium, temperature and other growing conditions change. In the process of transition from the S-form (phase I) to the stable rough R-form (phase IV) through the intermediate phases II and III, smooth changes in antigenic properties are observed; pathogenic properties are lost.

Parapertussis bacteria and B. bronchiseptica, as well as phases II, III and IV pertussis bacteria grow on MPA and MPB. When grown on liquid medium, diffuse turbidity with a dense sediment at the bottom is observed; the cells can be somewhat larger and polymorphic, sometimes forming threads. In the R-form and intermediate forms, bacteria exhibit pronounced polymorphism.

On the Bordet-Gengou medium, all Bordetella form a slightly limited zone of hemolysis around the colonies, spreading diffusely into the medium.

Bordetella do not ferment carbohydrates, do not form indole, do not reduce nitrates to nitrites (with the exception of B. bronchiseptica). Parapertussis bacteria secrete tyrosinase, forming a pigment that colors the medium and culture brown.

Bordetella contain several antigen complexes. The somatic O-antigen is species-specific; the generic antigen is agglutinogen 7. The main agglutinogens in the causative agent of whooping cough are the 7th (generic), 1st (species) and the most frequently detected type-specific 2nd and 3rd. Depending on their combination, four serovariants are distinguished in Bordetella pertussis: 1,2,3; 1, 2.0; 1, 0, 3 and 1.0.0.

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Pathogenicity factors of Bordetella

Fimbriae (agglutinogens), outer membrane protein pertactin (69 kDa) and filamentous hemagglutinin (surface protein) are responsible for the adhesion of the pathogen to the ciliary epithelium of the middle sections of the respiratory tract (trachea, bronchi). The capsule protects against phagocytosis. Hyaluronidase, lecithinase, plasmacoagulase, and adenylate cyclase are often present. Endotoxin (LPS) contains two lipids: A and X. The biological activity of LPS is determined by lipid X, lipid A has low pyrogenicity and is nontoxic. LPS has immunogenicity (whole-cell vaccine), but causes sensitization. There are three exotoxins. Pertussis toxin (117 kDa) is similar in structure and function to choleragen, exhibits ADP-ribosyltransferase activity (ribosylates transducin, a target cell membrane protein that is part of the system that inhibits cellular adenylate cyclase), is a strong immunogen, increases lymphocytosis and insulin production. Tracheal cytotoxin is a fragment of peptide glycan, is pyrogenic, arthritogenic, induces slow-wave sleep and stimulates the production of IL-1, in response to which nitric oxide (cytotoxic factor) is synthesized. It damages tracheal epithelial cells and causes ciliostasis. Heat-labile dermonecrotoxin is neurotropic, has vasoconstrictive activity, is homologous to cytotoxic necrotizing factor 1 (CNF1) of Escherichia coli. Its target is Rho proteins of cell membranes. Dermonecrotoxin is detected by an intradermal test on rabbits (Dold's test).

Immunity

After the disease, a stable lifelong immunity is formed; post-vaccination immunity lasts only 3-5 years.

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Epidemiology of whooping cough

The source of infection in whooping cough and parapertussis is a patient with a typical or latent form, especially in the period before the onset of spasmodic cough. In whooping cough-like disease caused by B. bronchiseptica, the source of infection can be domestic and wild animals, among which epizootics are sometimes observed (pigs, rabbits, dogs, cats, rats, guinea pigs, monkeys), and most often the respiratory tract is affected. The mechanism of infection is airborne. Bordetella have a specific tropism for the ciliary epithelium of the host's respiratory tract. People of all ages are susceptible to infection, but children from 1 to 10 years of age are most susceptible.

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Symptoms of whooping cough

The incubation period for whooping cough is from 3 to 14 days, more often 5-8 days. The pathogen, having entered the mucous membrane of the upper respiratory tract, multiplies in the cells of the ciliary epithelium and then spreads by the bronchogenic route to the lower sections (bronchioles, alveoli, small bronchi). Under the action of the exotoxin, the epithelium of the mucous membrane becomes necrotic, as a result of which the cough receptors are irritated and a constant flow of signals is created in the cough center of the medulla oblongata, in which a persistent focus of excitation is formed. This leads to the occurrence of spasmodic coughing attacks. Whooping cough is not accompanied by bacteremia. Secondary bacterial flora can lead to complications.

The following stages are distinguished during the course of the disease:

• catarrhal period, lasting about 2 weeks and accompanied by a dry cough; the patient's condition gradually worsens;
• convulsive (spasmodic), or spasmodic, period, lasting up to 4-6 weeks and characterized by attacks of uncontrollable “barking” cough occurring up to 20-30 times a day, and attacks can be provoked even by non-specific irritants (light, sound, smell, medical manipulations, examination, etc.);
• the resolution period, when coughing fits become less frequent and shorter in duration, necrotic areas of the mucous membrane of the upper respiratory tract are rejected, often in the form of “casts” from the trachea and bronchi; duration - 2-4 weeks.

Laboratory diagnostics of whooping cough

The main diagnostic methods are bacteriological and serological; for accelerated diagnostics, especially at an early stage of the disease, the immunofluorescence reaction can be used. To isolate a pure culture, mucus from the nasopharynx or sputum is used as material, which is sown on AUC or Bordet-Gengou medium. Sowing can also be done using the "cough plates" method. The grown culture is identified by a combination of cultural, biochemical and antigenic properties. Serological reactions - agglutination, complement fixation, passive hemagglutination - are mainly used for retrospective diagnostics of whooping cough or in cases where a pure culture has not been isolated. Antibodies to the pathogen appear no earlier than the 3rd week of the disease, the diagnosis is confirmed by an increase in the antibody titer in sera taken at 1-2-week intervals. In children of the first two years of life, serological reactions are often negative.

Treatment of whooping cough

Antibiotics (gentamicin, ampicillin) are used for treatment; they are effective in the catarrhal period of the disease and useless in the convulsive period.

Specific prevention of whooping cough

For routine prevention of the disease, children are vaccinated against whooping cough, using the adsorbed pertussis-diphtheria-tetanus vaccine (DPT), containing 20 billion killed pertussis bacteria in 1 ml. The separately produced killed pertussis vaccine, used in children's groups according to epidemiological indications, is based on the same component. This component is reactogenic (neurotoxic property), therefore, acellular vaccines containing from 2 to 5 components (pertussis toxoid, filamentous hemagglutinin, pertactin and 2 fimbrial agglutinogens) are currently being actively studied.