Fitness of the pMV158 replicon in Streptococcus pneumoniae

► Promiscuous plasmid pMV158 does not burden the pneumococcal host. ► Plasmid pMV158 and its deleted-derivative pLS1 are stably inherited in pneumococcus. ► Plasmid pLS1 imposes a small but significant burden to Streptococcus pneumoniae. ► pLS1-carrying pneumococcal cells are outcompeted by plasmid-free cells.