Abstract
BackgroundChromium 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 Bacillus cereus SJ1, and to further study the underlying mechanisms at the molecular level using the obtained genome sequence.ResultsBacillus cereus 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, chrIA1, and two additional chrA genes encoding putative chromate transporters that likely confer chromate resistance were identified. Furthermore, we also found an azoreductase gene azoR and four nitroreductase genes nitR possibly involved in chromate reduction. Using reverse transcription PCR (RT-PCR) technology, it was shown that expression of adjacent genes chrA1 and chrI was induced in response to Cr(VI) but expression of the other two chromate transporter genes chrA2 and chrA3 was constitutive. In contrast, chromate reduction was constitutive in both phenotypic and gene expression analyses. The presence of a resolvase gene upstream of chrIA1, an arsenic resistance operon and a gene encoding Tn7-like transposition proteins ABBCCCD downstream of chrIA1 in B. cereus SJ1 implied the possibility of recent horizontal gene transfer.ConclusionOur results indicate that expression of the chromate transporter gene chrA1 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 chrIA1 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 B. cereus SJ1.
Highlights
Chromium is a toxic heavy metal, which primarily exists in two inorganic forms, Cr(VI) and Cr(III)
When examining induction of chromate resistance and reduction genes, several strains including Shewanella oneidensis MR-1 [20], Ochrobactrum tritici 5bvl1 [17] and Ralstonia metallidurans strain CH34 [21] have been shown to contain genes induced by chromate
Identification of Cr(VI)-reducing B. cereus SJ1 that is highly chromate resistant Strain SJ1 showing both high Cr(VI) resistance and reduction abilities was isolated from industrial wastewater of a metal plating factory
Summary
Chromium is a toxic heavy metal, which primarily exists in two inorganic forms, Cr(VI) and Cr(III). 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. Our main aim was to investigate chromate resistance and reduction by Bacillus cereus SJ1, and to further study the underlying mechanisms at the molecular level using the obtained genome sequence. Bacteria have developed different strategies of chromate resistance including chromate efflux and chromate reduction. Efflux of chromate, which is mediated by the chromate transporter protein ChrA, has been confirmed in Pseudomonas aeruginosa [15,16], Ochrobactrum tritici 5bvl1 [17] and Shewanella sp. When examining induction of chromate resistance and reduction genes, several strains including Shewanella oneidensis MR-1 [20], Ochrobactrum tritici 5bvl1 [17] and Ralstonia metallidurans strain CH34 [21] have been shown to contain genes induced by chromate
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