Two main kind of Aeromonas strains can be differenciated on the basis of their growth temperature: mesophilic strains (belonging to all the Aeromonas species besides A. salmonicida) and psychrophilic strains ( A. salmonicida). Strains from A. hydrophila or veronii with an S-layer (serotype O:11) are unable to activate complement, while all the other Aeromonas strains are able to do it, including A. salmonicida strains with an S- layer. This fact is related to the different accessibility of lipopolysaccharide (LPS) in the different Aeromonas strains showing S-layer. In all the cases the classical complement pathway (CPC) is the only one activated by these bacteria.
The place where is bound Clq (the first component of the CPC) is the major outer membrane porin (named porinII). The accessibility to this protein (porinII) is the crucial event in order to determine if one strain will be serum sensitive or resistant, or what is the same, the amount of Clq molecules that could bind to this protein will determine the biological effects of complement measured by their serum susceptibility or the opsonization level. The smooth (lipopolysaccharide with O-antigen) Aeromonas strains (either with or without S- layer) are usually serum resistant, but to different degree depending of the O-antigen LPS type. Strains lacking the O-antigen LPS (rough) are serum sensitive, and these strains lacking the porinII (mutants with defined insertions in this gene or clinical isolates resistant to some antibiotics) are resistant to the serum and showed a low level of opsonization (C3 binding).
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