Bordetella pertussis: Properties of the Bacteria

Bordetella pertussis

B. pertussis ( pertussis, Latin for intense cough) is the causativeagent of whooping cough, an infectious bacterial illness that affects the respiratory tract. Whooping cough is one of many diseases that can be prevented by vaccine.

Properties of the Bacteria

◗ Morphology

B. pertussis shows the following features:

·           B. pertussis are extremely small ovoid coccobacilli measuring0.2–0.5 3 1 mm.

·           Characteristically, they demonstrate pleomorphism in their morphology.

·           They are Gram negative, occurring in singles or in pairs, and are nonmotile and nonsporing.

·           Toluidine blue staining of the bacteria demonstrates characteristic bipolar metachromatic granules.

·           Freshly isolated strain of B. pertussis possesses a poorly defined capsule and also fimbriae.

·           In culture smears, the bacilli are arranged in loose clumps with clear spaces in between, giving a thumbprint appearance.

◗ Culture

B. pertussis is a strict aerobe with an optimum temperature forgrowth 35ºC. The bacteria are nutritionally fastidious. They do not grow on common laboratory media, such as blood agar and nutrient agar. Even on blood agar, the bacteria grow slowly and require 3–6 days to form pinpoint colonies.

The bacteria are usually grown on a rich medium supple-mented with charcoal, starch, blood, albumin, and growth fac-tors, such as nicotinamide. The latter is absolutely essential for the growth of the bacteria. Blood or albumin present in the medium is used apparently not to provide nutrition for the growth of bacteria but to neutralize toxic substances, such as fatty acids present in the agar.

Bordet–Gengou agar with 15–20% of blood is a common medium used for primary isolation of B. pertussis. This medium consists of glycerol, potato, agar, and 15–20% of blood. After 48–72 hours of incubation, B. pertussis produces small, smooth, opaque, grayish white refractile colonies resembling bisected pearls or mercury drops. A hazy zone of hemolysis is present around the colonies. Confluent growth of B. pertussis on the medium typically presents an aluminum paint appearance.

Charcoal agar with 10% blood has also been used for primary iso-lation of B. pertussis. The bacterium does not grow on MacConkey medium, but other Bordetella species grow on this medium.

◗ Biochemical reactions

B. pertussis shows the following biochemical reactions:

·           It is oxidase positive and catalase positive but biochemically inert.

·           The bacteria do not ferment carbohydrates.

·           The bacteria also do not produce indole, do not reduce nitrate, do not split urea, and do not utilize citrate.

◗ Other properties

Susceptibility to physical and chemical agents: B. pertussisis killed by heating at 55ºC for 30 minutes. It is also killed by drying and also by standard disinfectants. If left on culture plates, the bacteria die within few days, but they can survive in dry droplets for up to 5 days and for 3 days on cloth.