Analysing diversity among β-lactamase encoding genes in aquatic environments
The most common mechanism of resistance to β-lactam antibiotics is the production of β-lactamases. These enzymes are encoded by genes that evolve rapidly, thus constituting a group characterized by high levels of molecular diversity. Most of the genetic determinants of resistance to β-lactam antibiotics characterized until now were obtained from clinical isolates. This study was designed in order to exploit the presence of β-lactamase gene sequences in an aquatic environment, and to get information on the distinctive features of those sequences when compared to others available on databases. DNA sequences potentially encoding proteins of three different families of clinically relevant β-lactamases were assessed: TEM, IMP and OXA-2 derivatives. The presence of bla sequences in DNA extracted from water samples from the lagoon Ria de Aveiro was checked by PCR and hybridization. Sequences representing the three families of β-lactamases studied were detected. The molecular diversity of the amplicons was assessed by cloning and sequence analysis, and denaturing gradient gel electrophoresis (PCR-DGGE) separation. Most of the retrieved sequences (particularly sequences representing bla TEM and bla OXA-2) were identical or very similar to β-lactamase gene sequences previously characterized from clinical isolates. Phylogenetic analysis suggests that this aquatic ecosystem is a reservoir of molecular diverse putative bla sequences. The patterns of molecular diversity found within the β-lactamase gene families studied do not correspond to those reported in studies focussing on clinical isolates.