| A well-characterized set of 94 Aeromonas strains from a variety of geographical, clinical and environmental sources representing the entire genus Aeromonas was examined using DNA sequence from four loci. In the genus Aeromonas, the species boundaries defined by phenotypic tests and DNA/DNA hybridization were confirmed using sequence data from three gene loci: recA, chiA, and gyrB. 16S rDNA sequence was only confirmatory at the genus level. An DNA sequence data was generated using PCR initiated with conserved primers developed for this study. For 16S rDNA, 42/585 (7%), of the nucleotide bases were variable in 93/94 strains representing 17 genomospecies of Aeromonas . For recA, 247/1176 (21%) were variable, for chiA 219/500 (44%) were variable, and for gyrB, 215/650 (33%) were variable. Analysis of aligned sequences using phylogenetic trees revealed that most strains clustered with their conspecifics (same species). Unknown strains received as Aeromonas species could be taxonomically identified to the species level by sequence data. GenBank sequence from the use of outlier groups indicated that Aeromonas is closer to Pseudomonas than to Vibrio. Three independent lines of evidence supported recombination between species of Aeromonas . First, in tree analysis, several strains (n = 9) clustered with their conspecifics for 2 out of the 3 loci and the loci for which the strain didn't cluster with its conspecific was considered to have undergone recombination. Second, sequence data from the chiA locus for three strains consistently produced sequences that were interpreted to be at least two distinct loci, despite the amplification of a single band from the initial PCP. Third, PCR of the hyl4 hemolysin locus revealed multiple (2–5) bands in most strains, consistent with multiple hlyA loci. The presence of recombination implied that no single test or DNA sequence from a single locus could be used to unambiguously classify a strain to the species level. Nonetheless, our results greatly expand the amount of sequencing information available for all of the species, demonstrate the utility of DNA sequence for classifying strains of Aeromonas, present evidence of recombination within Aeromonas, and provide insights into the molecular evolution of the genus. |
