ABSTRACT
The chromate ion transporter (CHR) superfamily comprises transporters that confer chromate resistance by extruding toxic chromate ions from cytoplasm. Burkholderia xenovorans strain LB400 has been reported to encode six CHR homologues in its multireplicon genome. We found that strain LB400 displays chromate-inducible resistance to chromate. Susceptibility tests of Escherichia coli strains transformed with cloned B. xenovorans chr genes indicated that the six genes confer chromate resistance, although under different growth conditions, and suggested that expression of chr genes is regulated by sulfate. Expression of chr genes was measured by quantitative reverse transcription-PCR (RT-qPCR) from total RNA of B. xenovorans LB400 grown under different concentrations of sulfate and exposed or not to chromate. The chr homologues displayed distinct expression levels, but showed no significant differences in transcription under the various sulfate concentrations tested, indicating that sulfate does not regulate chr gene expression in B. xenovorans. The chrA2 gene, encoded in the megaplasmid, was the only chr gene whose expression was induced by chromate and it was shown to constitute the chromate-responsive chrBACF operon. These data suggest that this determinant is mainly responsible for the B. xenovorans LB400 chromate resistance phenotype.
Subject(s)
Burkholderia/enzymology , Burkholderia/metabolism , Chromates/metabolism , Gene Expression , Ions/metabolism , Membrane Transport Proteins/metabolism , Burkholderia/genetics , Escherichia coli/enzymology , Escherichia coli/genetics , Escherichia coli/metabolism , Gene Expression Profiling , Membrane Transport Proteins/genetics , Real-Time Polymerase Chain Reaction , Recombinant Proteins/genetics , Recombinant Proteins/metabolism , Sulfates/metabolismABSTRACT
The presence of chromate-resistance genes in enterobacteria was evaluated in a collection of 109 antibiotic-resistant nosocomial isolates from nine major cities in México. Results were compared with the presence of mercury-resistance genes. Susceptibility tests showed that 21% of the isolates were resistant to chromate (Cr(R)), whereas 36% were resistant to mercury (Hg(R)). Cr(R) levels were high in Klebsiella pneumoniae (61%), low in Enterobacter cloacae (12%) and Escherichia coli (4%), and null in Salmonella sp. isolates. Colony hybridization demonstrated that the majority of metal-resistant isolates hybridized with chrA gene (87% of Cr(R) isolates), encoding a CHR transporter homologue, and merA gene (74% of Hg(R) isolates), encoding MerA mercuric reductase, suggesting that most isolates expressed these widespread metal-resistance systems. Southern blot hybridization of Cr(R) isolates showed that plasmids of 80, 85, and 95 kb from K. pneumoniae isolates, and of 100 kb from an E. cloacae isolate, contained chrA-related sequences. These plasmids belonged to IncN or IncP incompatibility groups, and conferred Cr(R), as well as multiple antibiotic resistance, when transferred by conjugation to an E. coli standard strain. These data indicated that Cr(R) genes may be distributed among clinical enterobacteria via conjugative plasmids, probably by coselection with antibiotic-resistant genes.
