Pathogenic Bordetellae: Causative agents of whooping cough

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The History of Discovery


The causative agent of whooping cough Bordetella pertussis was discovered and isolated from patients in pure culture by J. Bordet and O. Gengou in 1906.


Classification


Bordetellae pertain to the family Alcaligenaceae and genus Bordetella. The main pathogen is Bordetella pertussis, causative agent of whooping cough. Bordetella parapertussis cause similar milder disease. Bordetella bronchiseptica rarely produce human respiratory diseases (opportunistic pathogen).


Structure and Properties of Bordetellae


Morphology

The bacteria are small 0.5-1.0 μm gram-negative oval-shaped non-motile rods, except Bordetella bronchiseptica, which possesses polar flagella. They are non-sporeforming, Bordetella pertussis produce capsule. Bacteria stain poorly with aniline dyes, the ends of bacterial body stain more intensively.

Cultivation

  • В. parapertussis and B. bronchiseptica can grow on basic nutrient media. Isolation of В. pertussis requires enriched media. As additional growth factors amino acids cystein and methionin are applied.
  • Bordet-Gengou medium (potato-blood-glycerol agar) with penicillin or caseine-charcoal agar can be used. The plates are incubated at 35-37 °C for 3-7 days in a moist environment.
  • The colonies are small, convex, and glistening, resembling globules of mercury. They can dissociate into S- or R-forms. B. bronchiseptica synthesizes brownish pigment.

Biochemical properties

  • Bordetellae are obligate aerobes.
  • В. pertussis shows minimal biochemical activity. The bacteria metabolize glucose with acid production. They lack proteolytic activity and urease, but produce catalase.
  • В. parapertussis and B. bronchiseptica are more active, producing urease, nitrate reductase, etc. Some strains express hemolytic activity

Antigenic structure

  • The causative agents of whooping cough share a common thermostable somatic O-antigen and superficial capsular antigens.
  • At least 14 somatic antigenic variations have been identified in various Bordetella strains. Factor 7 is generic and common to all Bordetella bacteria; factor 1 is essential for B. pertussis, factor 14 – for B. parapertussis, and factor 12 for B. bronchiseptica.

Virulence factors

  • В. pertussis produces various toxic and aggressive substances.
  • Pili play a role in adherence of the bacteria to the ciliated epithelial cells of the upper respiratory tract.
  • Capsule of В. pertussis protects against phagocytosis and takes part in adhesion.
  • Most of virulence factors are governed by genetic bvg regulon (bordetella virulence gene).
  • Filamentous hemagglutinin mediates adhesion to ciliated epithelial cells.
  • Pertussis toxin (exotoxin) is the main virulence substance. It has typical A and B subunit structure and renders ADP-ribosylating activity, influencing cellular metabolism.
  • It also stimulates lymphocytosis, sensitization to histamine, and enhances insulin secretion.
  • Adenylate cyclase toxin, dermonecrotic toxin and hemolysin are also regulated by bvg genes.
  • The tracheal cytotoxin inhibits DNA synthesis in ciliated cells.
  • The lipopolysaccharide of the cell wall may cause the damage of epithelial cells of upper respiratory tract.

 

Resistance

B. pertussis is sensitive to environmental factors. Nevertheless, it can withstand exposure to sunlight for about one hour. The bacterium is inactivated by heating at temperature of 56°С for 10-15 minutes. It is rapidly destroyed in solutions of conventional disinfectants (e.g., phenols or chlorines).


Pathogenesis and Clinical Findings in Whooping Cough


  • Whooping cough, caused by В. pertussis, is a severe infectious disease of childhood.
  • This ailment affects only humans (anthroponosis), being transmitted by air droplet route.
  • The possible sources of infection are patients in the early catarrhal stage of disease and carriers. Communicability is high, ranging from 30% to 90%.
  • Bacteria attach to and propagate on the epithelial surface of the trachea and bronchi. The blood is not invaded. The bacteria liberate the toxins and substances that irritate epithelial cells, causing intensive coughing.
  • After an incubation period of about 2 weeks, the “catarrhal stage” develops, with mild coughing and sneezing.
  • During the “paroxysmal” stage, the cough becomes explosive. The stage lasts for another 4 or 6 weeks. Necrosis of parts of the epithelium and polymorphonuclear infiltration produces peribronchial inflammation and interstitial pneumonia.
  • Blood cell count reveals marked lymphocytosis. Secondary infection by staphylococci or H. influenzae may easily cause bacterial pneumonia. Obstruction of the smaller bronchioles and diminished oxygenation of the blood can cause convulsions in infants with whooping cough.
  • The disease course is protracted and may last for 2-3 months in total; convalescence develops slowly. Whooping cough confers stable long-term immunity, but rare recurrent diseases in adults may be severe.
  • Bordetella parapertussis produce a disease similar to whooping cough. The infection is often subclinical. These bacteria usually have a silent copy of the pertussis toxin gene.
  • Bordetella bronchiseptica may cause so-called “kennel cough” in dogs. In rare cases it may be responsible for human respiratory infections especially in immunocompromised persons.

Laboratory Diagnosis of Whooping Cough


  • A saline nasal wash is the most preferable specimen. Cough droplets obtained with “cough plate” method during patient’s paroxysm or nasopharyngeal swabs are of lesser clinical relevance.
  • For rapid diagnosis of bacteria in the specimen immunofluorescent test is used.
  • For culture isolation the specimens are inoculated into Bordet-Gengou medium or casein-charcoal medium supplemented with antibiotics, inhibiting concomitant microflora.
  • After incubation on Bordet-Gengou medium the pure culture of bacteria is further identified by its morphological, cultural, biochemical tests. Antigenic properties are determined by slide agglutination test with specific antibodies.
  • Serological diagnosis is employed at the end of the second week of the disease. Antibodies against filamentous hemagglutinin and pertussis toxin are determined by ELISA.
  • PCR is the most sensitive method of pertussis diagnosis. Primers for both В. pertussis and В. parapertussis should be included.
  • If available, PCR test should replace both cultural method and serological testing.

Treatment and Prophylaxis of Whooping Cough


  • Treatment with antibiotics from macrolide or azalide groups during the catarrhal stage of the disease fosters the elimination of pathogens and may have prophylactic effect.
  • Treatment in paroxysmal stage demonstrates only low impact on the clinical course of the disease.
  • For specific prophylaxis effective inactivated pertussis vaccine is used. Administration of acellular vaccines based on pertussis toxoid or filamentous hemagglutinin is preferable in comparison with the whole cell vaccines because of greatly decreased side effects. Infants should obtain three doses of pertussis vaccine during the first year of life followed by repeat boosters for a total of five.
  • Pertussis vaccine is usually administered in combination with toxoids of diphtheria and tetanus (DPT vaccine).