Unclassified Bacillus species resembling Bacillus anthracis: Potential for misdiagnosis during anthrax alert

During the course of surveillance for Bacillus anthracis, two unidentified Bacillus strains that shared many of the primary identification characteristics of B. anthracis were isolated. Both isolates produced flat, grey, nonhemolytic colonies; were encapsulated; penicillin susceptible; and were judged nonmotile when tested in motility test medium. While both isolates were negative for phage-encoded virulence traits, isolate no. 1, recovered from a nasal culture, did share chromosomal rpoB gene sequences with B. anthracis. Careful evaluations of wet preparations, however, showed that both isolates were sluggishly motile. This feature, coupled with growth of the two isolates on media containing phenylethyl alcohol, allowed early discrimination from B. anthracis while awaiting gamma phage susceptibility test results. The test showed that, in contrast to B. anthracis, both isolates were resistant to phage lysis. Neither isolate could be definitively identified to the species level by reference laboratories.