Prophage spontaneous activation promotes DNA release enhancing biofilm formation in Streptococcus pneumoniae

©2010 Carrolo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Competing Interests: The authors have read the journal’s policy and have the following conflicts. J.M.C. has received research grants administered through his university and has consulted and received honoraria for speaking from Pfizer, Bial, GlaxoSmithKline and Novartis. M.R. has consulted for Wyeth Pharmaceuticals. The remaining authors have declared that no competing interests exist. This does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials.Streptococcus pneumoniae (pneumococcus) is able to form biofilms in vivo and previous studies propose that pneumococcal biofilms play a relevant role both in colonization and infection. Additionally, pneumococci recovered from human infections are characterized by a high prevalence of lysogenic bacteriophages (phages) residing quiescently in their host chromosome. We investigated a possible link between lysogeny and biofilm formation. Considering that extracellular DNA (eDNA) is a key factor in the biofilm matrix, we reasoned that prophage spontaneous activation with the consequent bacterial host lysis could provide a source of eDNA, enhancing pneumococcal biofilm development. Monitoring biofilm growth of lysogenic and non-lysogenic pneumococcal strains indicated that phage-infected bacteria are more proficient at forming biofilms, that is their biofilms are characterized by a higher biomass and cell viability. The presence of phage particles throughout the lysogenic strains biofilm development implicated prophage spontaneous induction in this effect. Analysis of lysogens deficient for phage lysin and the bacterial major autolysin revealed that the absence of either lytic activity impaired biofilm development and the addition of DNA restored the ability of mutant strains to form robust biofilms. These findings establish that limited phage-mediated host lysis of a fraction of the bacterial population, due to spontaneous phage induction, constitutes an important source of eDNA for the S. pneumoniae biofilm matrix and that this localized release of eDNA favors biofilm formation by the remaining bacterial population.M.C. and M.J.F. are supported by research grants from Fundação para a Ciência e Tecnologia, Portugal (SFRH/BD/35854/2007 and SFRH/BD/38543/2007 respectively). This work was partly supported by Fundação para a Ciência e Tecnologia, Portugal (PTDC/SAU-ESA/64888/2006 and PIC/IC/83065/2007), Fundação Calouste Gulbenkian and an unrestricted grant from GlaxoSmithKline. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscrip