Validation of a partial rpoB gene sequence as a tool for phylogenetic identification of aeromonads isolated from environmental sources

A collection of 50 aeromonads isolated from environmental sources were studied, together with all known Aeromonas nomenspecies, by phenotypic, amplified 16S rDNA restriction analysis (16S rDNA RFLP) and by partial sequence alignment of both 16S rDNA and rpoB genes. Although most of the type strain showed a unique phenotypic pattern, a database constructed on type strain phenotype allowed the identification of only 24% of the isolates. Analysis of 16S rDNA RFLP and the rpoB sequence were almost concordant in identifying environmental isolates at the species level, except for strains belonging to Aeromonas caviae spp., which were not differentiated from Aeromonas aquariorum, nor Aeromonas hydrophila susbsp. dhakensis by 16S rDNA RFLP. In addition, rpoB gene analysis clustered separately a group of isolates found in snails within the A. hydrophila species. In contrast to 16S rDNA RFLP and rpoB, the partial 16S rDNA sequence analysis was weak in resolving species identity. Part of these results, phenotypic and phylogenetic data, showed that Aeromonas molluscorum and Aeromonas sharmana are distant from all other Aeromonas species and that the type species of A. hydrophila subsp. anaerogenes is similar to A. caviae and should be considered synonymous.