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#bacterial cell wall

13 public questions tagged with this topic.

Microbial Physiology

Which type of bond in peptidoglycan is hydrolyzed by lysozyme?

Peptidoglycan glycan backbone consists of repeating beta-linked disaccharide where N-acetylglucosamine and N-acetylmuramic acid are joined via beta-1,4 glycosidic bonds between C1 of one sugar and C4 of other, creating long linear strands that run roughly perpendicular to cell long axis. This beta-1,4 linkage creates specific geometry recognized by lysozyme, a muramidase ubiquitous in innate immunity found in tears, saliva, egg white, and macrophage granules. Lysozyme's active site cleft accommodates hexasaccharide, distorts MurNAc residue into half-chair conformation, and catalyzes hydrolysis via glutamate 35 donating proton and aspartate 52 stabilizing oxocarbenium intermediate, cleaving between MurNAc and GlcNAc. Cleavage weakens sacculus, leading to osmotic lysis when combined with turgor. Beta-1,3 linkages characterize pseudomurein and cellulose-like polymers but resist lysozyme, while alpha-1,4 is found in amylose and alpha-1,6 in glycogen branching. Specificity explains why pseudomurein-containing archaea and peptidoglycan O-acetylated pathogens resist lysozyme, and why synthetic beta-1,4 fragments activate NOD2 innate signaling. Mechanistic studies show lysozyme distorts D ring into boat conformation, lowering activation energy for glycosidic cleavage, and this precise stereochemical requirement underlies why modifications like N-deacetylation, O-acetylation at C6 of MurNAc found in pathogenic Staphylococcus aureus and Neisseria gonorrhoeae confer lysozyme resistance, promoting survival within neutrophils and facilitating colonization of mucosal surfaces.

Ref: Vollmer et al., FEMS Microbiol Rev 2008, Lysozyme Specificity; Callewaert & Michiels, J Biosci 2010, Muramidases.

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Microbial Physiology

Which enzyme is involved in the cross-linking of peptidoglycan?

After glycan polymerization by transglycosylases elongating alternating N-acetylglucosamine and N-acetylmuramic acid strands, final strength requires crosslinking of stem peptides attached to MurNAc. Stem typically comprises L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and terminal D-alanyl-D-alanine dipeptide. DD-transpeptidases, members of penicillin-binding protein family including PBP1a, PBP1b, PBP2a, and PBP3, perform nucleophilic attack where serine in active site forms bond with penultimate D-alanine, releasing terminal D-alanine, then transfers acyl-intermediate to amino group of adjacent diamino acid, creating 4-3 crosslink. Some bacteria also have LD-transpeptidases forming 3-3 linkages. Lysozyme hydrolyzes glycan backbone rather than crosslinks, ATP synthase generates ATP from proton motive force, DNA gyrase introduces negative supercoiling. Inhibition of transpeptidase by beta-lactams that mimic D-Ala-D-Ala substrate leaves nascent peptidoglycan poorly crosslinked, compromising mechanical strength so that turgor pressure causes lysis. This step is crucial for shape determination and antibiotic susceptibility, explaining why PBP mutations confer resistance. Recent cryo-EM structures capture PBP2 in active conformation with nascent peptidoglycan strand threaded through donor site, revealing how transpeptidase orients peptide for crosslinking, and how beta-lactams occupy same pocket mimicking acyl-D-Ala-D-Ala, explaining structure-activity relationships used to design carbapenems and cephalosporins that evade certain beta-lactamases.

Ref: Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan Crosslinking; Lovering et al., Ann Rev Biochem 2012, PBPs.

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Microbial Physiology

What is the role of bactoprenol in bacterial cell wall synthesis?

Peptidoglycan precursor synthesis involves cytoplasmic steps generating lipid II, composed of N-acetylmuramic acid-pentapeptide linked to undecaprenyl pyrophosphate, a 55-carbon polyisoprenoid lipid carrier historically named bactoprenol. Bactoprenol is embedded in inner membrane with long hydrophobic tail anchoring within bilayer while pyrophosphate-linked disaccharide protrudes. Its function is to shuttle hydrophilic precursor across hydrophobic membrane barrier from cytoplasmic side where it is made to periplasmic side where polymerization occurs. After transfer of precursor to growing glycan chain by transglycosylases, undecaprenyl pyrophosphate remains and must be dephosphorylated to monophosphate by phosphatase UppP to regenerate carrier for next cycle, a step targeted by bacitracin antibiotic which sequesters pyrophosphate form. Without this recycling, synthesis stalls rapidly because cellular pool of undecaprenyl phosphate is limited to about 10^5 molecules per cell. Bactoprenol does not act as porin, provide energy for crosslinking, or degrade peptidoglycan; its role is purely logistical carrier function essential for cell elongation and septation, coupling cytoplasmic and extracytoplasmic phases of wall assembly.

Ref: Barreteau et al., FEMS Microbiol Rev 2008, Bacterial Cell Wall Recycling; Manat et al., Ann Rev Microbiol 2014, Lipid II Cycle.

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Microbial Physiology

What is the primary lipid structure in Gram-positive bacteria?

Gram-positive bacteria lack lipopolysaccharide and outer membrane, so their cytoplasmic membrane directly faces thick peptidoglycan. Primary lipid structure remains conventional phospholipid bilayer built from glycerol-3-phosphate esterified to straight-chain or branched fatty acids, typically phosphatidylglycerol, cardiolipin, and lysylphosphatidylglycerol that modulates surface charge to resist cationic antimicrobial peptides. These phospholipids provide fluid matrix housing respiratory complexes, transporters, and lipid II flipping machinery essential for wall synthesis. Glycerol diethers with phytanyl chains are hallmark of archaeal membranes, not Gram-positives. Lipopolysaccharides define Gram-negative outer leaflet, while hopanoids, pentacyclic triterpenoids similar to eukaryotic cholesterol, are produced by some bacteria for membrane ordering but are not primary bulk lipids in typical Gram-positives like Staphylococcus aureus or Bacillus subtilis, where hopanoid synthesis genes are often absent. Hence classic ester-linked phospholipid bilayer remains accurate description, forming permeability barrier and scaffold for peripheral wall polymers such as teichoic acids and capsular polysaccharides via undecaprenyl phosphate carriers. In some Gram-positives, membrane composition includes branched-chain fatty acids and small amounts of menaquinone electron carriers, influencing fluidity and susceptibility to membrane-targeting antimicrobials like daptomycin that inserts into phosphatidylglycerol-rich domains causing depolarization, highlighting importance of bilayer chemistry beyond simple barrier function.

Ref: Alberts et al., Molecular Biology Cell, Chapter 10: Bacterial Membranes; Sohlenkamp & Geiger, FEMS Microbiol Rev 2015, Bacterial Phospholipids.

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Microbial Physiology

Which structure is unique to Gram-negative bacteria?

Gram-negative and Gram-positive cell envelope architectures diverge sharply. Gram-positive envelope comprises thick peptidoglycan multilayer with interspersed wall teichoic acids and lipoteichoic acids anchored to underlying plasma membrane, without outer membrane. Gram-negative envelope contains thin peptidoglycan layer sandwiched in periplasm plus additional outer membrane asymmetric membrane whose outer leaflet is almost exclusively lipopolysaccharide and inner leaflet phospholipid, harboring porins, Bam complex, and Lpt transport machinery. Peptidoglycan itself, plasma membrane composed of ester-linked phospholipids, and lipoteichoic acids are not exclusive; peptidoglycan occurs in both but differs in thickness, plasma membrane is universal among cellular life, lipoteichoic acids are Gram-positive signature. Outer membrane is absent from Gram-positives, archaea, and eukaryotes, representing evolutionary innovation of Gram-negative lineage that confers intrinsic resistance to lysozyme, detergents, and many antibiotics, while also providing additional barrier requiring specialized protein folding chaperones like SurA and Skp in periplasm for outer membrane protein biogenesis and transport. Cryo-electron tomography reveals outer membrane is densely packed with lipopolysaccharide stabilized by ionic crosslinks, and its biogenesis demands coordinated transport of proteins and lipids from inner membrane, making outer membrane assembly a target for novel Gram-negative specific antibiotics like darobactin that inhibits Bam complex folding.

Ref: Silhavy et al., The Bacterial Cell Envelope; Alberts et al., Molecular Biology Cell, Membrane Architecture.

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Microbial Physiology

The peptidoglycan layer in Gram-positive bacteria is:

Gram-positive bacteria lack outer membrane but retain an unusually thick peptidoglycan sacculus measuring 20 to 80 nanometers, composed of many layers of glycan strands made of alternating N-acetylglucosamine and N-acetylmuramic acid crosslinked by tetrapeptide side chains containing L-alanine, D-glutamate, meso-diaminopimelic acid or L-lysine, and D-alanine, often further cross-bridged by interpeptide bridges. Embedded within and covalently attached to this mesh are anionic wall teichoic acids, polymers of ribitol or glycerol phosphate linked via phosphodiester bonds, and lipoteichoic acids anchored to membrane diacylglycerol via glycolipid. These polyanionic polymers bind magnesium ions, concentrate cations at membrane surface, modulate activity of peptidoglycan hydrolases and autolysins, regulate division site placement, and serve as antigenic determinants and bacteriophage receptors. Their high negative charge contributes to ion exchange and electrophoretic properties. This architecture explains strong retention of crystal violet-iodine complex during Gram staining, sensitivity to lysozyme that cleaves beta-1,4 bonds, and vulnerability to beta-lactam antibiotics that inhibit transpeptidation step of wall assembly.

Ref: Silhavy et al., The Bacterial Cell Envelope; Vollmer et al., FEMS Microbiol Rev 2008, Peptidoglycan and Teichoic Acids.

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Microbial Physiology

Which enzyme is inhibited by penicillin?

Bacterial peptidoglycan final crosslinking step catalyzed by transpeptidases penicillin binding proteins class B enzymes PBP2 for elongation, PBP3 FtsI for division. They cleave C terminal D alanine D alanine dipeptide from pentapeptide side chain MurNAc L Ala D Glu mDAP D Ala D Ala, forming acyl enzyme intermediate through active site serine nucleophile, then transfer to amino group acceptor meso diamino pimelic acid or L lysine of neighboring strand creating peptide crossbridge essential for wall rigidity. Beta lactam antibiotics penicillin contain four membered ring mimicking D Ala D Ala conformation fitting active site, acylating catalytic serine irreversibly forming stable penicilloyl enzyme unable to deacylate, blocking transpeptidation. Nascent peptidoglycan remains linear uncrosslinked degraded by endogenous autolysins lytic transglycosylases leading to osmotic lysis especially during growth when wall remodeling high. Gyrase target quinolones, RNA polymerase target rifampicin, ribosomal peptidyl transferase chloramphenicol. Thus penicillin specifically inhibits transpeptidase activity, mechanistic basis for bactericidal action and synergy with beta lactamase inhibitors clavulanate restoring efficacy against resistant strains.

Ref: Tipper & Strominger, PNAS 1965, Penicillin inhibits transpeptidase PBP crosslinking peptidoglycan.

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Microbial Physiology

The Braun’s lipoprotein (BLP) in Gram-negative bacteria serves to:

Gram negative envelope integrity depends on covalent linkage between outer membrane and peptidoglycan preventing blebbing and leakage. Braun's lipoprotein Lpp 58 amino acid mature after signal peptidase II lipid modification N acyl S diacylglycerol cysteine anchoring into inner leaflet outer membrane plus third acyl chain, most abundant protein about seven times ten to five copies per cell. C terminal lysine forms peptide bond to meso diaminopimelic acid of peptidoglycan tetrapeptide via LD transpeptidases LdtA B C catalyzing attachment post export. Two thirds of Lpp exist bound form linking layers, one third free form. Periplasmic distance regulated by Lpp length; mutants lengthening Lpp increase periplasm width affecting division. Lpp interacts with OmpA noncovalently stabilizing envelope. Disruption causes hypersensitivity to detergents hydrophobic antibiotics, increased outer membrane vesicle release, activated sigma E stress response. Therefore Braun's lipoprotein serves to attach outer membrane to peptidoglycan acting molecular staple maintaining envelope mechanical coupling, not as enzyme synthesizing peptidoglycan, facilitating folding, or directly blocking antibiotic entry though indirectly supports barrier function.

Ref: Braun & Rehn, Annu Rev Microbiol 1969, Braun lipoprotein attaches outer membrane to peptidoglycan.

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Microbial Physiology

The O-side chain of lipopolysaccharides (LPS) in Gram-negative bacteria functions as:

Lipopolysaccharide O antigen part structural component extends from core into extracellular environment. O side chain consists tandem repeats oligosaccharide units 2 to 6 sugars with diverse linkages deoxy sugars abequose tyvelose modifying antigenicity. Synthesis via Wzy dependent, ABC transporter, or synthase pathways assembled on undecaprenyl phosphate lipid carrier then ligated to lipid A core. Because repeats exposed and highly variable between strains due to glycosyl transferase gene hypervariation and horizontal transfer, immune system recognizes them as type specific antigens. Serological typing schemes Klebsiella K antigen capsule, Escherichia O antigen 187 types, Salmonella over 2500 serovars based on O and H flagellar antigens. O antigen variation influences complement deposition, serum resistance, bacteriophage adsorption, colonization niche. O side chain functions as antigenic determinant not as nutrient transporter porin channel transpeptidase inhibitor. Understanding O antigen diversity crucial diagnostics, vaccine design conjugate vaccines against Shigella dysenteriae, and tracking outbreak epidemiology via O serotyping molecular methods.

Ref: Raetz & Whitfield, Annu Rev Biochem 2002, O-antigen polysaccharide as antigenic determinant in LPS.

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Carbohydrates

Which of the following is not a component of bacterial cell walls?

Chitin is the correct choice because it does not accurately describe or belong to the category addressed in this question. In the context of Carbohydrates, the other options (Peptidoglycan, N-acetylglucosamine, and N-acetylmuramic acid) are all valid and well-established concepts. Chitin is either unrelated to the topic, describes a different biological process, or represents a common misconception. Questions framed as 'which is NOT' require students to identify the exception among otherwise correct statements, demanding comprehensive knowledge of the topic rather than recognition of a single fact.

Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 7

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