Some strains of P aeruginosa produce large amounts of extracellular polysaccharide. These mucoid strains usually are isolated only from patients with cystic fibrosis. The role of these polysaccharides in the pathogenesis of P aeruginosa chronic lung infections is unknown, but they may impede phagocytosis and impair diffusion of antibiotics and thus facilitate colonization and persistence. Interestingly, mucoid strains are frequently deficient in production of elastase, toxin A, and flagella, and their LPS lacks long polysaccharide side chains.