The chapter "Molecular Basis of Bacterial Outer Membrane Permeability Revisited" by Hiroshi Nikaido provides an in-depth review of the molecular mechanisms underlying the permeability of the bacterial outer membrane (OM). The OM, a distinctive feature of Gram-negative bacteria, serves as a selective barrier to nutrient uptake and waste efflux. The chapter begins with an introduction to protein channels, focusing on classical porins, which are trimeric proteins that form non-specific diffusion channels. These porins are essential for the transport of small, hydrophilic molecules and are found in various bacterial species, including Gram-negative and some Gram-positive bacteria. The structure of classical porins is described in detail, highlighting their β-barrel architecture and the role of specific residues in channel selectivity. The chapter discusses the functional assays used to study porin activity, such as single-channel conductance measurements and planar bilayer studies, and emphasizes the importance of these techniques in understanding the molecular mechanisms of porin function. The regulation of porin expression and function is also covered, including environmental factors like osmotic stress, temperature, and pH that influence porin production and activity. The chapter explores the complex regulatory networks involving two-component systems and global regulators, which modulate porin expression in response to various environmental cues. Additionally, the chapter delves into the evolution of porins, noting that while external loops can undergo rapid structural changes in response to selective pressures, transmembrane strands tend to remain stable over long periods. The study of "slow porins," such as OprF in Pseudomonas aeruginosa, is discussed, highlighting their unique properties and the challenges in understanding their function. Overall, the chapter provides a comprehensive overview of the molecular basis of OM permeability, integrating recent advances in crystallography, computational modeling, and functional studies to offer insights into the complex mechanisms that govern nutrient and waste transport in Gram-negative bacteria.The chapter "Molecular Basis of Bacterial Outer Membrane Permeability Revisited" by Hiroshi Nikaido provides an in-depth review of the molecular mechanisms underlying the permeability of the bacterial outer membrane (OM). The OM, a distinctive feature of Gram-negative bacteria, serves as a selective barrier to nutrient uptake and waste efflux. The chapter begins with an introduction to protein channels, focusing on classical porins, which are trimeric proteins that form non-specific diffusion channels. These porins are essential for the transport of small, hydrophilic molecules and are found in various bacterial species, including Gram-negative and some Gram-positive bacteria. The structure of classical porins is described in detail, highlighting their β-barrel architecture and the role of specific residues in channel selectivity. The chapter discusses the functional assays used to study porin activity, such as single-channel conductance measurements and planar bilayer studies, and emphasizes the importance of these techniques in understanding the molecular mechanisms of porin function. The regulation of porin expression and function is also covered, including environmental factors like osmotic stress, temperature, and pH that influence porin production and activity. The chapter explores the complex regulatory networks involving two-component systems and global regulators, which modulate porin expression in response to various environmental cues. Additionally, the chapter delves into the evolution of porins, noting that while external loops can undergo rapid structural changes in response to selective pressures, transmembrane strands tend to remain stable over long periods. The study of "slow porins," such as OprF in Pseudomonas aeruginosa, is discussed, highlighting their unique properties and the challenges in understanding their function. Overall, the chapter provides a comprehensive overview of the molecular basis of OM permeability, integrating recent advances in crystallography, computational modeling, and functional studies to offer insights into the complex mechanisms that govern nutrient and waste transport in Gram-negative bacteria.