Francisella tularensis: Properties of the bacteria

Francisella tularensis

F. tularensis causes tularemia, also known as glandular fever,rabbit fever, tick fever, and deerfly fever—an acute, febrile, gran-ulomatous, infectious disease in humans and other mammals.

Properties of the bacteria

◗ Morphology

F. tularensis shows the following features:

·           F. tularensis is a minute (0.230.2 to 0.7mm) and poorlystained Gram-negative intracellular coccobacillus.

·           The bacterium is capsulated and nonmotile.

·           Like Mycoplasma organisms, this organism is filterable and multiplies, i.e., by filament formation and budding, besides binary fission.

◗ Culture

Like Bordetella, the bacteria are strictly aerobic and fastidious, hence cannot be cultured on common laboratory media. The organism requires enriched media, such as Francis’ blood dextrose cystine agar and a longer period of incubation of 3–5 days. F. tularensisproduces minute transparent colonies on these media after incubation of 3–5 days. However, F. tularensis can grow on the chocolate agar if supplemented with cysteine and occasionally will grow on blood agar media.

◗ Other properties

Susceptibility to physical and chemical agents:  F. tularen-sis is killed by heating at 558C for 30 minutes. It is also killed by drying and by standard disinfectants. Decontamination of inanimate objects by using 10% bleach followed by using 70% alcohol solution is effective.

F. tularensis has two biovars: Jellison type A (F. tularen-sis biovar tularensis) and Jellison type B (F. tularensis biovar palaearctica). These are serologically similar, but differ primar-ily in their fermentation reactions, virulence, and geographical distribution.

Jellison type A is more virulent and causes severe disease in humans. As few as 10 bacteria injected subcutaneously and 25 bacteria on inhalation can cause disease. This biovar is gen-erally found in North American rabbits and ticks. Jellison type B causes a milder form of disease in humans and is found pri-marily in Asian and European rodents.