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.