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Infection and Immunity, November 2007, p. 5405-5414, Vol. 75, No. 11
0019-9567/07/$08.00+0     doi:10.1128/IAI.00717-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Surface Protein Srr-1 of Streptococcus agalactiae Binds Human Keratin 4 and Promotes Adherence to Epithelial HEp-2 Cells{triangledown}

Ulrike Samen,1* Bernhard J. Eikmanns,1 Dieter J. Reinscheid,1,2 and Frédéric Borges1

Institute of Microbiology and Biotechnology, University of Ulm, 89069 Ulm, Germany,1 University of Applied Sciences Bonn-Rhein-Sieg, 53359 Rheinbach, Germany2

Received 28 May 2007/ Returned for modification 26 June 2007/ Accepted 10 August 2007

Streptococcus agalactiae is frequently the cause of bacterial sepsis and meningitis in neonates. In addition, it is a commensal bacterium that colonizes the mammalian gastrointestinal tract. During its commensal and pathogenic lifestyles, S. agalactiae colonizes and invades a number of host compartments, thereby interacting with different host proteins. In the present study, the serine-rich repeat protein Srr-1 from S. agalactiae was functionally investigated. Immunofluorescence microscopy showed that Srr-1 was localized on the surface of streptococcal cells. The Srr-1 protein was shown to interact with a 62-kDa protein in human saliva, which was identified by matrix-assisted laser desorption ionization-time-of-flight analysis as human keratin 4 (K4). Immunoblot and enzyme-linked immunosorbent assay experiments allowed us to narrow down the K4 binding domain in Srr-1 to a region of 157 amino acids (aa). Furthermore, the Srr-1 binding domain of K4 was identified in the C-terminal 255 aa of human K4. Deletion of the srr-1 gene in the genome of S. agalactiae revealed that this gene plays a role in bacterial binding to human K4 and that it is involved in adherence to epithelial HEp-2 cells. Binding to immobilized K4 and adherence to HEp-2 cells were restored by introducing the srr-1 gene on a shuttle plasmid into the srr-1 mutant. Furthermore, incubation of HEp-2 cells with the K4 binding domain of Srr-1 blocked S. agalactiae adherence to epithelial cells in a dose-dependent fashion. This is the first report describing the interaction of a bacterial protein with human K4.

* Corresponding author. Mailing address: Division of Gene Therapy, University of Ulm, 89081 Ulm, Germany. Phone: 49-731-50033617. Fax: 49-731-50046102. E-mail: u.samen{at}gmx.de

{triangledown} Published ahead of print on 20 August 2007.

Editor: J. B. Bliska

Infection and Immunity, November 2007, p. 5405-5414, Vol. 75, No. 11
0019-9567/07/$08.00+0     doi:10.1128/IAI.00717-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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