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Background  Spontaneous  Bacillus anthracis  mutants resistant to infection by phage AP50c (AP50 R ) exhibit a mucoid colony phenotype and secrete an extracellular matrix.    Methods  Here we utilized a Roche/454-based whole genome sequencing approach to identify mutations that are candidates for conferring AP50c phage resistance, followed by genetic deletion and complementation studies to validate the whole genome sequence data and demonstrate that the implicated gene is necessary for AP50c phage infection.    Results  Using whole genome sequence data, we mapped the relevant mutations in six AP50 R  strains to  csaB . Eleven additional spontaneous mutants, isolated in two different genetic backgrounds, were screened by PCR followed by Sanger sequencing of the  csaB  gene. In each spontaneous mutant, we found either a non-synonymous substitution, a nonsense mutation, or a frame-shift mutation caused by single nucleotide polymorphisms or a 5 base pair insertion in  csaB . All together, 5 and 12 of the 17 spontaneous mutations are predicted to yield altered full length and truncated CsaB proteins respectively. As expected from these results, a targeted deletion or frame-shift mutations introduced into  csaB  in a different genetic background, in a strain not exposed to AP50c, resulted in a phage resistant phenotype. Also, substitution of a highly conserved histidine residue with an alanine residue (H270A) in CsaB resulted in phage resistance, suggesting that a functional CsaB is necessary for phage sensitivity. Conversely, introduction of the wild type allele of  csaB in cis  into the  csaB  deletion mutant by homologous recombination or supplying the wild type CsaB protein  in trans  from a plasmid restored phage sensitivity. The  csaB  mutants accumulated cell wall material and appeared to have a defective S-layer, whereas these phenotypes were reverted in the complemented strains.    Conclusions  Taken together, these data suggest an essential role for  csaB  in AP50c phage infection, most likely in phage adsorption. (The whole genome sequences generated from this study have been submitted to GenBank under SRA project ID: SRA023659.1 and sample IDs: AP50 R1: SRS113675.1, AP50 R2: SRS113676.1, AP50 R3: SRS113728.1, AP50 R4: SRS113733.1, AP50 R6: SRS113734.1, JB220 Parent: SRS150209.1, JB220 Mutant: SRS150211.1).

virology journal

Whole genome sequencing of phage resistant Bacillus anthracismutants reveals an essential role for cell surface anchoring protein CsaB in phage AP50c adsorption