Characterization and genomic analysis of chromate resistant and reducing Bacillus cereus strain SJ1

<p>Abstract</p> <p>Background</p> <p>Chromium is a toxic heavy metal, which primarily exists in two inorganic forms, Cr(VI) and Cr(III). Chromate [Cr(VI)] is carcinogenic, mutational, and teratogenic due to its strong oxidizing nature. Biotransformation of Cr(VI) to less-toxic Cr(III) by chromate-resistant and reducing bacteria has offered an ecological and economical option for chromate detoxification and bioremediation. However, knowledge of the genetic determinants for chromate resistance and reduction has been limited so far. Our main aim was to investigate chromate resistance and reduction by <it>Bacillus cereus </it>SJ1, and to further study the underlying mechanisms at the molecular level using the obtained genome sequence.</p> <p>Results</p> <p><it>Bacillus cereus </it>SJ1 isolated from chromium-contaminated wastewater of a metal electroplating factory displayed high Cr(VI) resistance with a minimal inhibitory concentration (MIC) of 30 mM when induced with Cr(VI). A complete bacterial reduction of 1 mM Cr(VI) was achieved within 57 h. By genome sequence analysis, a putative chromate transport operon, <it>chrIA</it>1, and two additional <it>chrA </it>genes encoding putative chromate transporters that likely confer chromate resistance were identified. Furthermore, we also found an azoreductase gene <it>azoR </it>and four nitroreductase genes <it>nitR </it>possibly involved in chromate reduction. Using reverse transcription PCR (RT-PCR) technology, it was shown that expression of adjacent genes <it>chrA</it>1 and <it>chrI </it>was induced in response to Cr(VI) but expression of the other two chromate transporter genes <it>chrA</it>2 and <it>chrA</it>3 was constitutive. In contrast, chromate reduction was constitutive in both phenotypic and gene expression analyses. The presence of a resolvase gene upstream of <it>chrIA</it>1, an arsenic resistance operon and a gene encoding Tn7-like transposition proteins ABBCCCD downstream of <it>chrIA</it>1 in <it>B. cereus </it>SJ1 implied the possibility of recent horizontal gene transfer.</p> <p>Conclusion</p> <p>Our results indicate that expression of the chromate transporter gene <it>chrA</it>1 was inducible by Cr(VI) and most likely regulated by the putative transcriptional regulator ChrI. The bacterial Cr(VI)-resistant level was also inducible. The presence of an adjacent arsenic resistance gene cluster nearby the <it>chrIA</it>1 suggested that strong selective pressure by chromium and arsenic could cause bacterial horizontal gene transfer. Such events may favor the survival and increase the resistance level of <it>B. cereus </it>SJ1.</p