Demonstration of a capsule plasmid in Bacillus anthracis

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Abstract
Virulent and certain avirulent strains of B. anthracis harbor a plasmid, designated pXO2, which is involved in the synthesis of capsules. Two classes of rough, noncapsulated (Cap-) variants were isolated from the capsule-producing (Cap+) Pasteur vaccine strains ATCC 6602 and ATCC 4229. One class was cured of pXO2; the other still carried it. Reversion to Cap+ was demonstrable only in rough variants which had retained pXO2. Proof that pXO2 is involved in capsule synthesis came from experiments in which the plasmid was transferred by CP-51-mediated transduction and by a mating system in which plasmid transfer is mediated by a B. thuringiensis fertility plasmid, pXO12. Cells of B. cereus and a previously noncapsulated (pXO2-) strain of B. anthracis produced capsules after the acquisition of pXO2.