Amplification products were visualized following electrophoresis in agarose gels. Inc-group-specific PCR fragments were purified with the Wizard SV and PCR Clean-up System (Promega) and sequenced at the Department of Genetics, CINVESTAV, Irapuato, México. For colony assays, bacteria were inoculated on LB agar plates, and after overnight

growth, colonies were lysed with 10% sodium dodecyl sulfate, debris were removed, and DNA was alkali-denatured. DNA was then transferred to nitrocellulose membranes (Hybond-N+; Amersham) and fixed by UV-light exposure. DNA for Southern blot assays was isolated by the alkaline lysis procedure described above, separated by agarose gel electrophoresis, and transferred to nitrocellulose membranes by capillarity. The coding Afatinib clinical trial KU-57788 mw region of the chrA gene was utilized as a probe for chromate-resistance (CrR) genes; a 1.25-kb fragment was PCR-amplified from the pEPL1 plasmid (7.7 kb), which contains a BamHI-PstI 3.8-kb fragment bearing the pUM505 chrA gene cloned in the pUCP20 vector

(Ramírez-Díaz et al., 2011). PCR was conducted employing forward oligonucleotide 1D (5′-GAGCGTTGCGAATGAAGAGTCG-3′) and reverse oligonucleotide 1R (5′-GGAAGCATGAAACCGAGTCCC-3′). As a probe for mercury-resistance (HgR) genes, a 1.18-kb fragment comprising most of the merA gene was amplified from pUM505 using forward oligonucleotide MerA-2D (5′-CATATCGCCATCATTGGCAGC-3′) and reverse oligonucleotide MerA-2R (5′-CCTCGATGACCAGCTTGATGAAG-3′). PCRs were carried out with Accuprime Super Mix II (Invitrogen) with an initial denaturation for 5 min at 95 °C succeeded by 30 cycles as follows: a denaturation step at 95 °C for 1 min; an annealing step at 60 °C for 45 s, and an elongation step at 72 °C for 1 min, with a final extension at 72 °C for 10 min. PCR products were purified as described previously and labeled with the Gene Images AlkPhos Direct Labeling kit (Amersham). Conditions for labeling, hybridization, and signal detection were as recommended by the provider at high stringency (63 °C). To investigate the presence of CrR genes in nosocomial bacteria, MG-132 a collection of 109 antibiotic-resistant

enterobacterial isolates from Mexican hospitals was utilized. This bacterial group was previously characterized by its resistance to multiple antibiotics, including beta-lactams, third-generation cephalosporins, and carbapenems (Miranda et al., 2004; Silva-Sánchez et al., 2011). MIC distribution curves demonstrated different levels of chromate susceptibility for each bacterial species (Fig. 1). A clear bimodal distribution of E. coli and K. pneumoniae allowed us to separate CrS from CrR isolates (Fig. 1a and c); for E. cloacae, where a single susceptibility group was found, an arbitrary separation was employed (Fig. 1b). Thus, for E. coli and E. cloacae, species exhibiting a low level of CrR isolates separated from the CrS predominant group, a cutoff value of ≥ 1.