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#bacterial structure

7 public questions tagged with this topic.

Microbial Physiology

What is the function of Teichoic acids in Gram-positive bacteria?

Wall teichoic acids covalently attached to peptidoglycan via phosphodiester to N-acetylmuramic acid and lipoteichoic acids anchored to membrane diacylglycerol through glycolipid are hallmark anionic glycopolymers of Gram-positive envelope, comprising up to 60 percent of wall dry weight. Their polyanionic phosphate backbone creates high density of negative charge at surface, enabling binding of divalent cations like magnesium and calcium, concentrating them near membrane for activity of enzymes and protection against cationic antimicrobial peptides. By controlling local ionic environment, teichoic acids modulate activity of autolysins, preventing uncontrolled wall degradation, and regulate insertion of penicillin-binding proteins and cell division machinery through spatial cues. They also function as phage receptors, contribute to biofilm formation via electrostatic interactions, and influence host immune detection via TLR2. In contrast, porin function belongs to Gram-negatives, endotoxin activity derives from lipopolysaccharide lipid A, and peptidoglycan degradation is carried out by lysozymes and lytic transglycosylases. Hence providing structural support via cation homeostasis and ion transport regulation captures multifaceted roles.

Ref: Brown et al., Ann Rev Microbiol 2013, Teichoic Acids; Percy & Grundling, Ann Rev Microbiol 2014, LTA Functions.

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Microbial Physiology

Which bacterial structure prevents plasmolysis in hypertonic conditions?

Hypertonic surroundings draw water out of bacterial cytoplasm through semipermeable inner membrane, generating inward osmotic pressure that tends to detach inner membrane from wall and shrink cytoplasm, a process termed plasmolysis. Survival depends on mechanical resistance supplied by peptidoglycan, a covalently crosslinked heteropolymer forming a single macromolecule sacculus surrounding entire cell, often described as exoskeleton. Its glycan strands crosslinked via peptide bonds possess high tensile strength, allowing it to withstand internal turgor of 2 to5 atmospheres in Gram-negatives and up to 20 atmospheres in Gram-positives, plus external osmotic shifts. Capsular polysaccharide, while important for immune evasion and desiccation resistance, is gel-like, highly hydrated, and lacks load-bearing stiffness. Outer membrane of Gram-negatives contributes some rigidity but remains fluid and present only in one group. Cytoplasmic proteins alone cannot counteract membrane deformation. Experimental evidence shows lysozyme digestion of wall in isotonic sucrose yields osmotically fragile protoplasts that burst upon dilution. Bacteria also accumulate compatible solutes like glycine betaine, trehalose, and ectoine to balance osmolarity, but primary immediate defense against plasmolysis remains peptidoglycan.

Ref: Rojas et al., PNAS 2014, Stiffness of Bacterial Cell Wall; Alberts et al., Molecular Biology Cell, Cell Wall Mechanics.

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Microbial Physiology

The MreB cytoskeletal protein plays a role in:

MreB is a widely distributed bacterial homolog of eukaryotic actin conserved across rod-shaped bacteria including Escherichia coli, Bacillus subtilis, and Caulobacter crescentus, essential for maintaining cylindrical morphology and coordinating growth. Unlike eukaryotic actin that forms long helical filaments, MreB assembles into short, antiparallel double protofilaments associated with inner face of cytoplasmic membrane in an ATP-dependent manner, often guided by membrane regions enriched in specific phospholipids. Its primary morphogenetic function is to act as a dynamic scaffold organizing cell wall synthetic complexes, recruiting penicillin-binding protein PBP2, rod shape-determining proteins RodA and RodZ, and class A bifunctional transglycosylases, directing their processive circumferential motion around cell long axis tracked by single-molecule imaging. This coordinated movement ensures new peptidoglycan glycan strands are inserted in ordered hoops rather than randomly, preserving width and straightness during elongation. Pharmacological depolymerization with compound A22 or genetic depletion converts rods into spheres, reduces growth rate, and disorganizes wall insertion. FtsZ instead assembles Z-ring for division, illustrating division of labor between elongation and cytokinesis.

Ref: Burkman et al., Ann Rev Microbiol 2018, MreB and Rod Shape; Errington, Nature Rev Microbiol 2015, Bacterial Cytoskeleton.

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Microbial Physiology

What happens to Gram-negative bacteria when the outer membrane is removed?

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.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 3: Bacterial Envelope; Nature 2018, Outer membrane as load-bearing element.

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