title: Protective role of the capsule and impact of serotype 4 switching on Streptococcus mitis.
abstract: The polysaccharide capsule surrounding Streptococcus pneumoniae is essential for virulence. Recently, Streptococcus mitis, a human commensal and a close relative of S. pneumoniae, was also shown to have a capsule. In this study, the S. mitis type strain switched capsule by acquisition of the serotype 4 capsule locus of S. pneumoniae TIGR4, following induction of competence for natural transformation. Comparison of the wild type with the capsule-switching mutant and with a capsule deletion mutant showed that the capsule protected S. mitis against phagocytosis by RAW 264.7 macrophages. This effect was enhanced in the S. mitis strain expressing the S. pneumoniae capsule, which showed, in addition, increased resistance against early clearance in a mouse model of lung infection. Expression of both capsules also favored survival in human blood, and the effect was again more pronounced for the capsule-switching mutant. S. mitis survival in horse blood or in a mouse model of bacteremia was not significantly different between the wild type and the mutant strains. In all models, S. pneumoniae TIGR4 showed higher rates of survival than the S. mitis type strain or the capsule-switching mutant, except in the lung model, in which significant differences between S. pneumoniae TIGR4 and the capsule-switching mutant were not observed. Thus, we identified conditions that showed a protective function for the capsule in S. mitis. Under such conditions, S. mitis resistance to clearance could be enhanced by capsule switching to serotype 4, but it was enhanced to levels lower than those for the virulent strain S. pneumoniae TIGR4.
authors: Department of Oral Biology, Faculty of Dentistry, University of Oslo, Oslo, Norway.
authors: Håkon V
authors: HV
authors: Rukke
authors: Department of Molecular Biosciences, Faculty of Mathematics and Natural Sciences, University of Oslo, Oslo, Norway.
authors: Raja Sab
authors: RS
authors: Kalluru
authors: Department of Molecular Biosciences, Faculty of Mathematics and Natural Sciences, University of Oslo, Oslo, Norway.
authors: Urska
authors: U
authors: Repnik
authors: Laboratorio Microbiologia Molecolare e Biotecnologia, Dipartimento Biologia Molecolare, Università di Siena, Siena, Italy UOC Batteriologia, Azienda Ospedaliera Universitaria Senese, Siena, Italy.
authors: Alice
authors: A
authors: Gerlini
authors: Centre for Inflammation and Tissue Research, Department of Medicine, Royal Free and University College Medical School, Rayne Institute, London, United Kingdom.
authors: Ricardo J
authors: RJ
authors: José
authors: Centre for Inflammation and Tissue Research, Department of Medicine, Royal Free and University College Medical School, Rayne Institute, London, United Kingdom.
authors: Jimstan
authors: J
authors: Periselneris
authors: Centre for Inflammation and Tissue Research, Department of Medicine, Royal Free and University College Medical School, Rayne Institute, London, United Kingdom.
authors: Helina
authors: H
authors: Marshall
authors: Department of Molecular Biosciences, Faculty of Mathematics and Natural Sciences, University of Oslo, Oslo, Norway.
authors: Gareth
authors: G
authors: Griffiths
authors: Laboratorio Microbiologia Molecolare e Biotecnologia, Dipartimento Biologia Molecolare, Università di Siena, Siena, Italy UOC Batteriologia, Azienda Ospedaliera Universitaria Senese, Siena, Italy.
authors: Marco Rinaldo
authors: MR
authors: Oggioni
authors: Centre for Inflammation and Tissue Research, Department of Medicine, Royal Free and University College Medical School, Rayne Institute, London, United Kingdom.
authors: Jeremy S
authors: JS
authors: Brown
authors: Department of Oral Biology, Faculty of Dentistry, University of Oslo, Oslo, Norway f.c.petersen@odont.uio.no.
authors: Fernanda C
authors: FC
authors: Petersen
mesh_terms: D000818:Animals; D016470:Bacteremia / Q000276:immunology / Q000382:microbiology; D016667:Bacterial Capsules / Q000276:immunology*; D002460:Cell Line; D004195:Disease Models, Animal; D005260:Female; D006736:Horses / Q000276:immunology / Q000382:microbiology; D006801:Humans; D008168:Lung / Q000276:immunology / Q000382:microbiology; D008264:Macrophages / Q000276:immunology / Q000382:microbiology; D051379:Mice; D008807:Mice, Inbred BALB C; D010587:Phagocytosis / Q000276:immunology; D012141:Respiratory Tract Infections / Q000276:immunology / Q000382:microbiology; D012703:Serotyping; D013290:Streptococcal Infections / Q000276:immunology* / Q000382:microbiology; D034361:Streptococcus mitis / Q000276:immunology*; D013296:Streptococcus pneumoniae / Q000276:immunology; D014774:Virulence / Q000276:immunology