They are small Gram negative coccobacilli that do not ferment carbohydrates. They are strict aerobes. They are parsitic in the respiratory tract of human beings, animals or birds. They produce toxins.The most significant type in humans is B.pertussis which causes pertussis or whooping cough or violent cough. It is a small coccobacilli, in cultures they are uniform in size and shape but later become longer and thread-like. It is nonmotile and nonsporing. It is capsulated and some possess fimbriae. They are aerobic, grow best at 35-360C, commonly used media is Bordet-Gengou Blood agar, slow growth ,after incubation small, dome shaped, smooth , opaque, greyish white, glistening ,resembling ‘mercury drops’
It is biochemically inactive,does not ferment sugars,oxidase and catalse positive. It is a delicate organism being killed readily by 550C, drying, disinfectants. Outside the body it survives in dried droplets for 5 days. It secretes pertussis toxin and excretes outside, also produces other toxins, LPS also contributes antigenicity.

Virulence Determinants-

  1. Heat labile toxin(HLT)- it is a cytoplasmic protein,inactivated by heating at 560C for 30 minutes. HLT in conjunction with endotoxin may be responsible for rhinorrhea,sneezing and mild cough in the catarrhal stage of pertussis.
  2. Tracheal Cytotoxin(TCT)- is a low molecular weight toxin,derived from the PG of cell wall. It is responsible for the destruction of the ciliated respiratory epithelial celss, resulting in the accumulation in the lungs by mucus, bacteria and inflammatory debris leading to severe cough.
  3. LPS Endotoxin- heat stable, adjuvant activity,pyrogenicity.
  4. Pertussis toxin(PT)- also known as lymphocytosis-promoting factor,pertussigen,histamine-sensitizing factor,islet-activating factor. PT has a mol.wt. of 117,000,made up of 6 polypeptide chains. It has got 2 units A & B. B binds to the cells and A cross the membrane and disturbs normal cell function by the elevation of cAMP.
  5. Haemolysin- it is responsible for the zone of hemolysis seen around colonies of B.pertussis on Bordet- Gengou medium.
  6. Filamentous haemagglutinin(FHA)- it is a protein that exists as filamentous rods,extending from the cell surface of the bacterium. It mediates the initial attachment of the bacterium to the ciliated epithelial cells of the upper respiratory tract.
  7. Agglutinogens(AGGs)- mammalian bordetellae possess 14 AGGs located on the fimbriae.

 Pathogenicity - B.pertussis is an obligate human parasite. Susceptible people become infected by inhaling respiratiory droplets from the people who serves as reservoirs. The organism colonise cilia lining the respiratory tract. It does not invade tissues or enter blood, but produce several substances such as endo/exotoxins that contribute to its virulence. These toxins destroy the ciliated epithelial cells, and thereby mucus accumulate in the respiratory pathway. After an incubation of 7-10 days the disease progresses through 3 stages- catarrhal stage, paroxysomal stage and convalescent stage.

The catarrhal stage is characterized by fever, sneezing, vomiting, mild dry persistent cough. A week or 2 later the paroxysomal stage begins as mucus and masses of bacteria fill the airway and immobilize the cilia. Strong, sticky, rope-like strings of mucus in the air way produce violent coughing. Failure to keep the airway open leads to cyanosis or bluing of the skin, because too little O2 gets to the blood, which accounts high death rate in infants. Straining to draw in air gives the characteristic loud whooping sound. Coughing and straining causes exhaustion, sometimes hemorrhage, rib fractures. Vomiting leads to dehydration, nutrient deficiency and electrolyte imbalance. After paroxysomal stage, 1-6 weeks convalescent stage occurs by milder coughing which continue for several months before subsiding. Secondary infections are common at this stage.

Laboratory Diagnosis - A calcium alginate swab on a flexible nichrome wire is passed gently along the floor of the nasal cavity until stopped by the posterior wall of the nasopharynx. The swab is left in place for 30 seconds to one minute to allow organisms to adsorb on to the swab. This pernasal swab is inoculated into BG medium with and  without methicillin and cephalexin which inhibits the commensals, incubated for at least 7 days. The colonies are identified by microscopy, biochemical characters and slide agglutination. The organism in nasopharyngeal secretions and on agar plates may be identified by direct fluorescent antibody test.The Bordetella  antibody can be detected in the patient serum by direct agglutination, indirect haemagglutination, complement fixation test and ELISA.

Treatment and prevention - It occurs in epidemic from periodically. Infection is transmitted by droplets and fomites contaminated with oropharyngeal secretions. The treatment includes the antitoxin given early in the disease, erythromycin, O2 therapy, rehydration, attention to nutrition and electrolyte balance. Specific vaccine has been found effective, which is administered in combination with diphtheria and tetanus(DPT). The organism is susceptible to several antibiotics, except Penicillin. Erythromycin, Ampicillin, Chloramphenicol are used.

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