Insight into the composition of the intercellular matrix of Streptococcus pneumoniae biofilms

Abstract

Biofilm matrices consist of a mixture of extracellular polymeric substances synthesized in large part by the biofilm-producing microorganisms themselves. These matrices are responsible for the cohesion and three-dimensional architecture of biofilms. The present study demonstrates the existence of a matrix composed of extracellular DNA, proteins and polysaccharides in the biofilm formed by Streptococcus pneumoniae. Extracellular DNA, visualized by fluorescent labeling, was an important component of the matrix. The existence of DNA–protein complexes associated with bacterial aggregates and other polymers was hypothesized based on the unexpected DNA binding activity of lysozyme LytC. The presence of intercellular DNA-LytC protein complexes in pneumococcal biofilms was demonstrated by confocal laser scanning microscopy. Evidence of extracel-lular polysaccharide different from the capsule was obtained by staining with Calcofluor dye and four types of lectin conjugated to Alexa fluorophores, and by incubation with glycoside hydrolases. The presence of residues of Glcp(1,4) and GlcNAc(1,4) in the pneumococcal biofilm was confirmed by GC-MS techniques.