Prevalence of Genes Encoding the Type Three Secretion System and the Effectors AexT and AexU in the Aeromonas veronii Group

The prevalence of important virulence factors, the type III secretion system (T3SS) and two T3SS-dependent toxins, AexT and AexU, was evaluated in the Aeromonas veronii group (AVG). Members of the AVG have a broad host range, including vertebrates and invertebrates, and form a variety of associations, spanning from pathogenic to mutualistic. Our AVG strain collection consists of human, duck, eel, and leech isolates. These isolates were examined for the presence of the T3SS, AexT, and AexU through PCR analysis. Two loci of the T3SS, ascV and ascF-ascG, and aexT and aexU were PCR amplified and sequenced from these isolates. All 20 environmental and clinical isolates possessed the T3SS and both effectors, which indicates a much greater prevalence than reported by previous studies. Sequence analysis of the C-terminal domains of aexT and aexU revealed a much higher nucleotide substitution rate for aexU (19.7%) when compared to aexT (4%). The lack of sequence variability among aexT homologs suggests that it has a conserved function among the AVG. The increased variation of the aexU sequence suggests the presence of different alleles, which indicates that it may serve different functions.