Practice question

Gram-negative bacteria possess a thin peptidoglycan layer confined to the periplasm between the inner cytoplasmic membrane and a distinctive outer membrane containing lipopolysaccharide and trimeric porins. The outer membrane provides substantial mechanical support, contributes to shape maintenance, and functions as a selective barrier against hydrophobic toxins, bile salts, and many antibiotics. When this membrane is disrupted by EDTA, which chelates magnesium and calcium ions that stabilize LPS contacts, or by mild detergents and lysozyme treatment, barrier and load-bearing capacity are lost. If peptidoglycan is simultaneously degraded, the rod-shaped cell collapses into an osmotically fragile, spherical structure that retains some membrane fragments and residual wall polymers. This intermediate is termed a spheroplast, in contrast to a protoplast produced from Gram-positive bacteria where enzymatic wall removal leaves only plasma membrane. Spheroplasts require isotonic media supplemented with sucrose or salts to prevent lysis but remain transcriptionally active. Their laboratory formation, studied extensively in Escherichia coli, illustrates recent evidence that outer membrane contributes to stiffness and growth integrity.