Protective role of the capsule and impact of serotype 4 switching on Streptococcus mitis

Håkon V Rukke, Raja Sab Kalluru, Urska Repnik, Alice Gerlini, Ricardo J José, Jimstan Periselneris, Helina Marshall, Gareth Griffiths, Marco Rinaldo Oggioni, Jeremy S Brown, Fernanda C Petersen

Research output: Contribution to journalArticlepeer-review

19 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)3790-3801
Number of pages12
JournalInfection and Immunity
Issue number9
Early online date23 Jun 2014
Publication statusPublished - Sep 2014


  • Animals
  • Bacteremia
  • Bacterial Capsules
  • Cell Line
  • Disease Models, Animal
  • Female
  • Horses
  • Humans
  • Lung
  • Macrophages
  • Mice
  • Mice, Inbred BALB C
  • Phagocytosis
  • Respiratory Tract Infections
  • Serotyping
  • Streptococcal Infections
  • Streptococcus mitis
  • Streptococcus pneumoniae
  • Virulence
  • Journal Article
  • Research Support, Non-U.S. Gov't


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