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Infect Immun, August 1998, p. 3825-3831, Vol. 66, No. 8
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Activation of the Complement Classical Pathway (C1q Binding) by Mesophilic Aeromonas hydrophila Outer Membrane Protein

Susana Merino,1 Maria Mercedes Nogueras,1 Alicia Aguilar,1 Xavier Rubires,1 Sebastian Albertí,1 Vicente Javier Benedí,2 and Juan M. Tomás1 *

Departamento de Microbiología, Facultad de Biología, Universidad de Barcelona, 08071 Barcelona,1 and Laboratorio de Microbiología, Departamento de Biología Ambiental, Universidad de las Islas Baleares, Palma de Mallorca,2 Spain

Received 2 March 1998/Accepted 27 May 1998

The mechanism of killing of Aeromonas hydrophila serum-sensitive strains in nonimmune serum by the complement classical pathway has been studied. The bacterial cell surface component that binds C1q more efficiently was identified as a major outer membrane protein of 39 kDa, presumably the porin II described by D. Jeanteur, N. Gletsu, F. Pattus, and J. T. Buckley (Mol. Microbiol. 6:3355-3363, 1992), of these microorganisms. We have demonstrated that the purified form of porin II binds C1q and activates the classical pathway in an antibody-independent manner, with the subsequent consumption of C4 and reduction of the serum total hemolytic activity. Activation of the classical pathway has been observed in human nonimmune serum and agammaglobulinemic serum (both depleted of factor D). Binding of C1q to other components of the bacterial outer membrane, in particular to rough lipopolysaccharide, could not be demonstrated. Activation of the classical pathway by this lipopolysaccharide was also much less efficient than activation by the outer membrane protein. The strains possessing O-antigen lipopolysaccharide bind less C1q than the serum-sensitive strains, because the outer membrane protein is less accessible, and are resistant to complement-mediated killing. Finally, a similar or identical outer membrane protein (presumably porin II) that binds C1q was shown to be present in strains from the most common mesophilic Aeromonas O serogroups.

* Corresponding author. Mailing address: Departamento de Microbiología, Facultad de Biología, Universidad de Barcelona, Diagonal 645, 08071 Barcelona, Spain. Phone: 34-93-4021486. Fax: 34-93-4110592. E-mail: juant{at}porthos.bio.ub.es.

Infect Immun, August 1998, p. 3825-3831, Vol. 66, No. 8
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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