Medusa head colony is seen in?

ANSWER: (B) Bacillus anthracisREF: Jawetz Microbiology, 24th Ed Ch. 12LAB DIAGNOSIS OF BACILLUS ANTHRACIS:Specimens to be examined are fluid or pus from a local lesion, blood, and sputum. Stained smears from the local lesion or of blood from dead animals often show chains of large grampositive rods. Anthrax can be identified in dried smears by immunofluorescence staining techniques.When grown on blood agar plates, the organisms produce nonhemolytic gray to white colonies with a rough texture and a ground-glass appearance. Comma-shaped outgrowths (Medusa head) may project from the colony.Gram stain shows large gram-positive rods. Carbohydrate fermentation is not useful. In semisolid medium, anthrax bacilli are always nonmotile, whereas related nonpathogenic organisms (eg, B cereus) exhibit motility by "swarming."Virulent anthrax cultures kill mice or guinea pigs upon intraperitoneal injection. Demonstration of capsule requires growth on bicarbonate-containing medium in 5-7% carbon dioxide. Lysis by a specific anthrax bacteriophage may be helpful in identifying the organism.An enzyme-linked immunoassay (ELISA) has been developed to measure antibodies against edema and lethal toxins, but the test has not been extensively studied. Acute and convalescent sera obtained 4 weeks apart should be tested. A positive result is a fourfold change or a single titer of greater than 1:32.