Characterization of virulence and antimicrobial resistance genes of Aeromonas media strain SD/21-15 from marine sediments in comparison with other Aeromonas spp

Aeromonas media is a Gram-negative bacterium ubiquitously found in aquatic environments. It is a foodborne pathogen associated with diarrhea in humans and skin ulceration in fish. In this study, we used whole genome sequencing to profile all antimicrobial resistance (AMR) and virulence genes found in A. media strain SD/21-15 isolated from marine sediments in Denmark. To gain a better understanding of virulence and AMR genes found in several A. media strains, we included 24 whole genomes retrieved from the public databanks whose isolates originate from different host species and environmental samples from Asia, Europe, and North America. We also compared the virulence genes of strain SD/21-15 with A. hydrophila, A. veronii, and A. salmonicida reference strains. We detected Msh pili, tap IV pili, and lateral flagella genes responsible for expression of motility and adherence proteins in all isolates. We also found hylA, hylIII, and TSH hemolysin genes in all isolates responsible for virulence in all isolates while the aerA gene was not detected in all A. media isolates but was present in A. hydrophila, A. veronii, and A. salmonicida reference strains. In addition, we detected LuxS and mshA-Q responsible for quorum sensing and biofilm formation as well as the ferric uptake regulator (Fur), heme and siderophore genes responsible for iron acquisition in all A. media isolates. As for the secretory systems, we found all genes that form the T2SS in all isolates while only the vgrG1, vrgG3, hcp, and ats genes that form parts of the T6SS were detected in some isolates. Presence of bla MOX-9 and bla OXA-427 β-lactamases as well as crp and mcr genes in all isolates is suggestive that these genes were intrinsically encoded in the genomes of all A. media isolates. Finally, the presence of various transposases, integrases, recombinases, virulence, and AMR genes in the plasmids examined in this study is suggestive that A. media has the potential to transfer virulence and AMR genes to other bacteria. Overall, we anticipate these data will pave way for further studies on virulence mechanisms and the role of A. media in the spread of AMR genes.

Nucleotide sequences and characterization of haemolysin genes from Aeromonas hydrophila and Aeromonas sobria

Extracellular haemolysin is thought to be one of the important virulence factors in Aeromonas infection. Two extracellular haemolysin genes (AHH3 and AHH4) from Aeromonas hydrophila strain 28SA, one (AHH5) from A. hydrophila strain AH-1 and one (ASA1) from Aeromonas sobria strain 33 were cloned into cosmid and plasmid vector DNA in Escherichia coli. The nucleotide sequences of the open reading frames of AHH3 and AHH4 are both 1476 basepairs (bp), whereas AHH5 and ASA1 are 1455 and 1467 bp in length, respectively. The deduced amino acid sequences of AHH3, AHH4, AHH5 and the previously reported aerolysin from A. hydrophila showed a significant degree of sequence homology of over 90% each. The amino acid identity of the ASA1 haemolysin and those from A. hydrophila and Aeromonas trota aerolysins ranged from 58-68%. From DNA hybridization analysis using our cloned haemolysin genes as probes, we found that the AHH5 and ASA1 DNA probes hybridized with about 31 and 75% strains of motile Aeromonas species, respectively. The activity of haemolysins of cloned genes were different in medium agar containing various erythrocytes.

Characterization of Aeromonas sobria hemolysin by use of monoclonal antibodies against Aeromonas hydrophila hemolysins

Aeromonas sobria produces hemolysin in a form activable with trypsin under defined cultural conditions. In immunoblotting analyses with the culture supernatant of A. sobria, the monoclonal antibody reacting specifically to Aeromonas hydrophila CA-11 hemolysin bound to the 53,000- and 49,000-dalton bands before and after trypsinization, respectively. The monoclonal antibody reacting to A. hydrophila AH-1 hemolysin did not bind either band. A. sobria hemolysin is, therefore, related antigenically to CA-11 hemolysin, while the molecular weights before and after activation differ from those of A. hydrophila hemolysins, being 54,000 and 51,000, respectively. The hemolytic and enterotoxigenic activities of A. sobria hemolysin were both neutralized by the monoclonal antibody against CA-11 hemolysin. It seems, therefore, that the same site on A. sobria hemolysin is responsible for both biological activities.

Development of a highly sensitive bead-ELISA to detect bacterial protein toxins

A highly sensitive sandwich enzyme-linked immunosorbent assay to detect bacterial toxins was developed. Fab' of anti-toxin IgG was conjugated with horseradish peroxidase by the maleimide method and tetramethylbenzidine was used as substrate. As the solid phase, a 6.5 mm diameter polystyrene bead was used and this was coated with the anti-toxin IgG. The entire assay could be completed within 3.5 hr. The sensitivity of this bead-ELISA was found to be quite high with various bacterial toxins: less than 20 pg/ml for thermostable direct hemolysin of Vibrio parahaemolyticus, less than 60 pg/ml for Shiga toxin, less than 20 pg/ml for VT2 (Shiga-like toxin II) of Escherichia coli, less than 200 pg/ml for heat-labile enterotoxin of E. coli, and less than 6 pg/ml for cholera enterotoxin.