What feature of oral bacteria makes them most resistant to both antimicrobial agents and host immune defenses?

Oral bacteria organized in biofilms (dental plaque) are substantially more resistant to antimicrobials than planktonic (free-floating) bacteria — up to 1,000-fold greater resistance. Mechanisms of biofilm resistance include: diffusion limitation of antimicrobials through the extracellular polymeric substance (EPS) matrix, metabolic heterogeneity (slow-growing persister cells are not killed by antibiotics that target active metabolic processes), altered gene expression within the biofilm, horizontal gene transfer promoting resistance, and physical barrier against phagocytosis. Dental caries and periodontitis are both fundamentally biofilm-associated infectious diseases that cannot be resolved by systemic antibiotics alone — mechanical disruption is essential.