L8: Bacterial Cell Envelope Structure

gram positive (monoderm)

bacteria with cytoplasmic or inner membrane (IM) and a thick layer of peptidoglycan (PG). Stain purple

gram negative (diderm)

bacteria with an IM, thin layer of PG, and an outer membrane (OM). Stain pink

porins

allow passage of hydrophilic molecules across membrane

gram stain

differential stain that distinguishes between two types of bacteria in the same specimen

crystal violet

purple stain that binds to negatively charged cell surface structures, visible on gram positive bacteria

Safranin

pink stain visible on gram negative bacteria

lipopolysaccharide (LPS)

large glycolipid found in outer membrane of gram negative bacteria

O-antigen (O specific polysaccharide)

hexoses in branched repeating units found in LPS. Bind to host tissues and inhibit phagocytosis. Hydrophilic barrier that protects against hydrophobic antibiotics and bile salts

Lipid A (endotoxin)

anchors LPS in outer leaflet of OM. Tightly packed fatty acids help exclude hydrophilic compounds, including antibiotics. essential for bacterial viability

peptidoglycan (PG) sacculus

large, covalently linked macromolecule that determines shape of the cell. Glycan strands run around circumference of cell, strands connected to each other by peptide crosslinks

MraY

attaches NAM to bactoprenol in cytoplasm, forming Lipid II

bactoprenol

hydrophobic carrier of growing peptidoglycan chain. in cytoplasmic membrane

MurG

attaches NAG to Lipid I (NAM-bactoprenol complex), forming Lipid II

MurJ

Flips Lipid II from being cytoplasm-facing to periplasm-facing

transglycoslyation

process that adds Lipid II monomer to the growing strand of glycan. Often mediated by high-MW PBPs, ex. PBP1, or SEDS (RodA, FtsW)

transpeptidation

process that creates peptide cross bridges between peptide chains attached to NAM molecules in adjacent glycan strands. Often mediated by high-MW PBPs such as PBP1, 2, and 3

penicillin-binding proteins (PBPs)

proteins that modify peptides of PG, bind to radiolabeled penicillin

high-MW PBPs

PBPs that perform either transpeptidation only (PBP2, PBP3), or both transpeptidation and transglycoslyation (PBP1). Usually at least one is essential for viability

low-MW PBPs

PBPs that make alternative types of peptide crosslinks, such as removal of amino acids from peptide stems, breaking of peptide crosslinks in mature PG

shape, elongation, division, and sporulation proteins (SEDS)

proteins that have transglycosylation activity (RodA, FtsW). RodA and FtsW are essential for viability.

beta-lactam antibiotic

antibiotic molecule containing beta-lactam ring. Blocks transpeptidase reaction. Beta-lactam ring mimics D-ala-D-ala residues at the end of PG peptides, transpeptidase attacks penicillin instead of the correct substrate. Beta-lactam ring breaks during attack by PBP, forming an irreversible adduct and inactivating PBP

beta-lactamases

enzymes that mediate resistance to beta lactams. Break the beta-lactam ring before PBP can react with it