functional categories of most differentially expressed genes. Most significant (B* > 0) differentially expressed genes of S. mutans, grouped in functional categories, in biofilms formed on hydroxyapatite (A), titanium (B) and composite (C) vs. polystyrene surfaces. Gene annotations are based on information provided by TIGR. *Bayesian test value, i.e. the probability for a gene to be really differentially expressed.