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The existence and genetic organization of a functional type III secretion system in a mesophilic Aeromonas strain by initially using the Aeromonas hydrophila strain AH-3 is investigated. A defined insertion mutant in strain AH-3 shows reduced toxicity and virulence in comparison with the wild-type strain.

Research reveals that Aeromonas salmonicida strains hybridize with lateral flagellum gene DNA probe but cannot produce lateral flagella. Data indicate that the lateral flagellin gene A is inactivated in this specialized pathogen.

The expression of the type II secretion system (T3SS) regulon from Aeromonas hydrophila was analyzed by examining the activity of the aopN-aopD and aexT promoters via means of two different techniques, promoterless gfp fusions and real-time PCR. The results for both axsA and aopN mutants which are described as T3SS regulators in other species provided insight into their function as the major transcription regulator and valve of the T3SS, in Aeromonas hydrophila.

The first report of the presence of an S layer in pathogenic non-O:11 autoagglutinating Aeromonas isolates, which belong to serogroups O:14 and O:81, is presented. All S-layer-positive motile Aeromonas isolates of serogroups O:11, O:14 and O:81 (NIH system) investigated were autoagglutinating strains by precipitation after boiling, and so they mostly had the common phenotype SP- and PAB+ and it is concluded that precipitation after boiling is the proper marker to search for virulent and S-layer-positive motile Aeromonas strains from epizootic and clinical samples.

The symbiotic association between Hirudo medicinalis and Aeromonas veronii bv. sorbia and colonization by the latter require the presence of serum complement system. A. veronii mutants lacking the complement system cannot colonize H. medicinalis but aquires the ability to colonize if the complement system is restored.