Bacteria in nature primarily live in surface-associated communities commonly known as biofilms. Because bacteria in biofilms, in many cases, display tolerance to host immune systems, antibiotics, and biocides, they are often difficult or impossible to eradicate. Biofilm formation, therefore, leads to various persistent
infections in humans and animals, and to a variety of complications in industry,
where solid–water interfaces occur. Knowledge about the molecular mechanisms
involved in biofilm formation is necessary for creating strategies to control biofilms. Recent studies have shown that extracellular DNA is an important component of the extracellular matrix of microbial biofilms. The present chapter is focussed on extracellular DNA as matrix component in biofilms formed by Pseudomonas aeruginosa as an example from the Gram-negative bacteria, and Streptococcus and Staphylococcus as examples from the Gram-positive bacteria. Besides the role of extracellular DNA in biofilm formation, the mechanisms involved in DNA release from P. aeruginosa, Streptococcus, and Staphylococcus are addressed.