A molecular study on bacterial resistance to arsenic-toxicity in surface and underground waters of Latium (Italy).

Latium, a region in central Italy, is known for its extensive volcanic areas that make a significant contribution to the arsenic (As) contamination of freshwater environments, even though some degree of As water pollution may be caused by human activities. The information available on indigenous As-resistant prokaryotes in aquatic environments of Latium is, however, still limited. In this study, we describe new bacteria that are resistant to arsenic toxicity and were isolated from the surface waters of Lake Vico and the Sacco River, two groundwater systems in Latium, as well as from bottled natural mineral water from the same region. The 16S rRNA gene sequence analysis for the As-resistant strains in lake and river waters points to a prevalence of β- and γ-Proteobacteria, while α-Proteobacteria, Firmicutes and Bacteroidetes are represented to a lesser extent. By contrast, solely γ-Proteobacteria were isolated from groundwater samples. The presence of Actinobacteria was documented exclusively in bottled mineral water. In addition, we conducted a DNA sequence-based study on the gene codifying arsB, an As(III) efflux membrane protein pump related to arsenic resistance, for all the As-resistant bacterial isolates. A phylogenetic analysis was carried out on the newly sequenced 16S rRNA genes and arsB in the present study as well as on an additional 16S rRNA/arsB dataset we obtained previously from Lake Albano, from the Tiber and from a well in Bassano Romano located in Latium (Davolos and Pietrangeli, 2011). Overall, the phylogenetic diversity of As-resistant bacteria in underground water was very limited if compared with lentic and lotic waters. Lastly, our molecular data support the hypothesis that the horizontal gene transfer of ars in As-containing freshwater environments is not limited to closely-related genomes, but also occurs between bacteria that are distant from an evolutionary viewpoint, thereby indicating that such genetic events may be considered a source of microbial resistance to arsenic-toxicity.