Bacterial Cell's Architecture and Innate Immunity

PAMPS + TLR =

inflammation

gram stain

→ differentiates cells into gram-pos or gram-neg based on binding and retention of crystal violet dye by the peptidoglycan

peptidoglycan cell wall

→ unique to eubacteria due to the presence of N-acetylmuramic acid

→ functions to protect the cell from lysing in a hypotonic environment

→ it is recognized by innate immunity as a PAMP

gram-negatives

→ contains a thin cell wall sandwiched between two membranes

→ the outer membrane is unique due to lipopolysaccharide (LPS)

→ potent immunostimulatory molecule and important PAMP in innate immunity

gram-positives

→ single membrane surrounded by a thick peptidoglycan layer

→ LTA is similar to LPS in that its unique to this group and it’s a PAMP

→ cell surface proteins can be loosely associated or covalently bound to the peptidoglycan

anabolism

endergonic = energy in (takes energy)

→ free energy is required to increase the complexity of molecules (biosynthesis)

→ endergonic reaction

→ reduction of carbon compounds (reducing agents: NADPH)

→ enzymes (proteins) mediate these chemical reactions/couple these chemical reactions

other biosynthetic pathways

→ cell wall synthesis

→ LPS and LTA synthesis

→ RNA replication = synthesis

→ DNA replication = synthesis

→ protein translation = synthesis

gluconeogenesis

→ share many chemical reactions w/ glycolysis

→ REVERSIBLE REACTIONS

→ special enzymes are required for three ( PEP→pyruvate, fructose-6-P → fructose 1,6-bisphosphate, glucose → glucose-6-P)

synthesis of nucleosugars

→ nucleosugars = nucleotides covalently added to sugars

→ required for the synthesis of polysaccharides

→ “hydrolysis” of a phosphodiester bond of nucleosugars provides the “free energy” for forming a covalent bond between sugars

endergonic

takes in energy

exergonic

lets energy out

synthesis of polysaccharides

→ from glycogen to cell wall

→ requires nucleosugars (NNP-sugar)

→ the hydrolysis of the phosphodiester bonds provides the energy needed to generate glycosidic bond(s)

peptidoglycan synthesis (gram neg + gram pos)

→ synthesis of precursors occurs in the cytoplasm

→ di-saccharide pentapeptide is assembled on a lipid carrier on the cytoplasmic side, before its translocated across the membrane to the extracellular side

→ the N acetyl-muramic acie-pentapeptide-N acetly-glucosamine is attached to nascent (“new”) cell wall

→ polysaccharide strands are cross-linked via peptide chain moiety that involves the transpeptidation enzyme, Pbp (penicillin-binding protein

→ glycosidic and amino acid bonds must be broken to expand the cell wall as the growing bacterial cell expands. While the cell wall is turned over, the breakdown products are recycled

Lipoteichoic acid synthesis (gram pos only)

→ synthesis requires a lipid carrier which is di-glucosyl diacylglycerol

→ the precursors are made in the cytoplasm. An enzyme is involved in translocating this lipid carrier from the cytoplasmic side to the external

→ glycerol subunit in the LTA polymer comes from phosphatidylglycerol present in the cytoplasmic membrane

lipopolysaccharide synthesis (Gram Neg ONLY)

→ precursors for O-antigen and core lipid A are made in the cytoplasm

→ the o-antigen requires bactoprenol lipid carrier; the same lipid carrier involved in peptidoglycan synthesis. The “core” polysaccharide is assembled on lipid A, which serves as the lipid carrier. Synthesis occurs for both molecules on the cytoplasmic side of the inner membrane

→ both the o-antigen subunit-lipid carrier and core lipid A are translocated from the cytoplasm side to the periplasm in the inner membrane. On the periplasmic side, the o antigen is “polymerized” before it finally attached to the core lipid A to produce the “completed” LPS molecule

→ several enzymes are involved in the translocated of the LPS molecule from the inner to outer membrane

There are several ways the host becomes exposed to bacterial cell surface PAMPS like peptidoglycan, LPS, or LTA

→ activation of latent bacteriophages resulting in lysis of the bacterial cell

→ lysis of Gram-neg bacteria by complement-mediated, membrane attack complex

→ phagocytosis and processing by neutrophils at the site of infection

→ some bacteria lyse in response to antibiotics or stress (ex. nutrient limitations)

bacteria also “bleb” their membranes during normal growth and thus exposing the host to PAMP containing membranes during infection

→ blebbing of the outer membrane (OM) is influenced by cross-linking of lipoproteins to the cell wall and inter-cell wall strand cross-linking

→ mechanism(s) governing membrane vesicle formation in Gram-pos is currently unknown