Systematic Veterinary Bacteriology and Mycology

CULTURAL CHARACTERISTICS

• Grows readily under aerobic and as facultative anaerobic at an optimum temperature of 35-37°C.
• On agar plates, irregular, round, raised, dull, opaque, grayish white, 2-3mm in diameter frosted glass appearance colonies are produced.
• Under the low power microscope, the slightly serrated edge of the colony is composed of long, interlacing chains of bacilli, resembling locks of matted hair. This is referred to as medusa head or judge's wig or woman’s curling hair type of growth.
• B. licheniformis produces characteristic hair-like outgrowths from streaks of the organisms on agar media. Colonies become brown with age.
• The name of this species derives from the similarity of its colonies to lichen.
• In medium containing iron salts, virulent B.anthracis produces pink or purple coloured pigmented colonies.
• Virulent capsulated strains form rough colonies, while avirulent attenuated strains form smooth colonies.
• In gelatin stab, fine filaments of growth develop laterally along the line of inoculum.
• The growth nearer to the surface of the medium is the longest and then progressively shorter where there is less oxygen, resembling inverted fir tree appearance.
• On haemolysis, the B.anthracis produces slight haemolysis with capsule production, compared with anthracoid organisms.
• B. cereus produces a wide zone of complete haemolysis around the colonies.
• A selective medium (PLET) consisting of polymyxin, lysozyme, EDTA and thallous acetate added to heart infusion agar is useful for isolating B.anthracis from mixtures containing other spore-bearing bacilli.