Background: Thermophilic bacilli in both aerobic or facultative anaerobic forms have been
isolated for over a hundred years from different mesophilic or thermophilic environments as they are
potential source of bioactive secondary metabolites. But the taxonomic resolution in the Bacillus genus
at species or at strain level is very challenging for the insufficient divergence of the 16S rRNA
genes. One such recurring problem is among Bacillus anthracis, B. cereus and B. thuringiensis. The
disease-causing B. anthracis strains have their characteristic virulence factors coded in two wellknown
plasmids, namely pXO1 (toxin genes) and pXO2 (capsule genes).
Objective: The present study aimed at the molecular and genomic characterization of a recently reported
thermophilic and environmental isolate of B. anthracis, strain PFAB2.
Methods: We performed comparative genomics between the PFAB2 genome and different strains of
B. anthracis, along with closely related B. cereus strains.
Results: The pangenomic analysis suggests that the PFAB2 genome harbors no complete prophage
genes. Cluster analysis of Bray-Kurtis similarity resemblance matrix revealed that gene content of
PFAB2 is more closely related to other environmental strains of B. anthracis. The secretome analysis
and the in vitro and in vivo pathogenesis experiments corroborate the avirulent phenotype of this
strain. The most probable explanation for this phenotype is the apparent absence of plasmids harboring
genes for capsule biosynthesis and toxins secretion in the draft genome. Additional features of
PFAB2 are good spore-forming and germinating capabilities and rapid replication ability.
Conclusion: The high replication rate in a wide range of temperatures and culture media, the nonpathogenicity,
the good spore forming capability and its genomic similarity to the Ames strain together
make PFAB2 an interesting model strain for the study of the pathogenic evolution of B. anthracis.