Bacillus anthracis — Key Characteristics
- Large, box-car shaped, gram-positive rod in short or long chains.
- Non-swelling, oval spores formed when grown on culture media. (Not usually seen in clinical specimens.)
- Encapsulated rods may be seen in clinical specimens.
- Ground-glass appearance of colonies.
- Nonhemolytic on sheep blood agar.
- No growth on MacConkey.
- Catalase positive.
- Susceptible to penicillin.
- Lecithinase positive.
Identification pitfalls to avoid
Large, gram-positive bacilli that grow aerobically as nonhemolytic colonies on sheep blood agar should not be discarded as a contaminant, but should be worked up to rule out B. anthracis.
- B. megaterium (lecithinase negative, motile)
- B. thuringiensis (penicillin R, motile)
- Paenibacillus macerans (weak lecithinase, motile)