Laboratory differentiation — Which test(s) can be used to distinguish Bacillus anthracis (anthrax bacillus) from anthracoid, non-anthrax Bacillus species?

Introduction / Context:
Accurate differentiation of Bacillus anthracis from other nonpathogenic Bacillus species (often called anthracoid bacilli) is vital for public health and biodefense. A combination of phenotypic and phage susceptibility tests provides rapid, practical discrimination in routine laboratories.

Given Data / Assumptions:

• B. anthracis shows distinctive colony morphology and capsule formation under appropriate conditions.
• Gamma phage lysis is a classic confirmatory trait for B. anthracis.
• Other Bacillus species often differ in motility, hemolysis, and phage susceptibility.

Concept / Approach:
Three helpful criteria are: (1) "Medusa head" colonies with irregular, filamentous margins; (2) demonstrable capsule (poly-D-glutamate) in animal tissues or on bicarbonate/serum media; and (3) susceptibility to specific bacteriophage (gamma phage). While no single feature should be used in isolation, the combination strongly supports B. anthracis versus anthracoid species such as B. cereus or B. subtilis.

Step-by-Step Solution:

Verification / Alternative check:
PCR assays targeting anthrax toxin genes (pagA, lef, cya) and capsule genes (cap) provide molecular confirmation and are used in reference labs.

Why Other Options Are Wrong:

• Each individual option is supportive but strongest when combined; choosing a single option ignores the comprehensive approach.

Common Pitfalls:
Confusing B. cereus (often hemolytic and motile) with B. anthracis; relying solely on Gram stain without functional testing.

Final Answer:
all of these