Gram-positive Mercury Resistance Genes in Gram-negative Oral Bacteria

Objective: The merA gene codes for mercuric reductase, which confers narrow spectrum mercury resistance. The merA genes are widespread in Gram-negative and Gram-positive genomes and are often linked to antibiotic resistance genes. Previously, it has been shown that Gram-positive antibiotic resistance genes can be found naturally in Gram-negative bacteria or conjugally transferred under laboratory conditions. We screened Gram-negative bacteria isolated from oral specimens collected from healthy children in the Casa Pia Schools, Lisbon, Portugal, for their ability to grow on 100-200 mM mercury and look for Gram-positive merA genes. Methods: Those that were positive were screened using DNA-DNA hybridization, polymerase chain reaction (PCR), hybridization of the PCR product and selective sequencing of the PCR product to confirm the presence of the Gram-positive merA genes. Results: Eight Gram-negative genera, Citrobacter, Neisseria, Acinetobacter, Enterobacter, Escherichia, Klebsiella, Pseudomonas, and Serratia were positive for Gram-positive merA genes. We have also been able to move the merA genes into Enterococcus faecalis recipient. We sequenced 1,000 bp of the merA gene from A. junii and an E. coli. These showed between 95% -96% homology with Staphylococcus aureus merA gene. With the other genes smaller number of base pairs have been sequenced but they showed between 89-97% homology with Bacillus merA gene. Conclusion: Gram-negative bacteria from 8 genera have been shown to carry conserved Gram-positive merA genes. This work was supported by NIDCR Grant U01-DE-1189 and Contract N01-DE-72623.