Bacterial cells are between 0.3 and 5 μmm in size. They have three basic forms: cocci, straight rods, and curved or spiral rods. The nucleoid consists of a very thin, long, circular DNA molecule which is double stranded and not surrounded by a membrane. Among the nonessential genetic structures are the plasmids. The cytoplasmic membrane harbors numerous proteins such as permeases, cell wall synthesis enzymes, sensor proteins, secretion system proteins, and, in aerobic bacteria, respiratory chain enzymes. The membrane is surrounded by the cell wall, the most important element of which is the supporting murein skeleton. The cell wall of Gram-negative bacteria features a porous outer membrane into the outer surface of which the lipopolysaccharide responsible for the pathogenesis of Gram-negative infections is integrated. The cell wall of Gram-positive bacteria does not possess such an outer membrane. Its murein layer is thicker and contains teichoic acids and wall-associated proteins that contribute to the pathogenic process in Gram-positive infections. Many bacteria have capsules made of polysaccharides that protect them from phagocytosis. Attachment pili facilitate adhesion to host cells. Motile bacteria possess flagella. Some bacteria produce spores, dormant forms that are highly resistant to chemical and physical stress.