Microbial Physiology

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

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

The permeability of the Gram-negative outer membrane is controlled by:

Gram-negative outer membrane serves as molecular sieve preventing entry of large hydrophilic and hydrophobic antibiotics like vancomycin, daptomycin, and bile salts. Permeability is governed primarily by porins, abundant trimeric beta-barrel proteins forming water-filled diffusion channels with constrictions determined by internal loop L3. General porins OmpF and OmpC allow passive diffusion of molecules below about 600 daltons, including nutrients and some beta-lactams, dependent on charge and size. Specific porins like LamB and ScrY facilitate uptake of maltodextrins and sucrose via binding site within channel, increasing efficiency. Regulation of porin abundance through two-component system EnvZ-OmpR and small RNAs like MicF enables adaptation to osmolarity, pH, and antibiotic pressure, with porin loss conferring resistance to carbapenems. Lipoteichoic acids are Gram-positive polymers, peptidoglycan thickness controls lysozyme sensitivity in Gram-positives, and ether bonds characterize archaeal lipids not controlling Gram-negative permeability. Hence porins act as major determinants of outer membrane exclusion limit, linking envelope permeability to nutrient acquisition and intrinsic drug resistance mechanisms studied by Nikaido pioneering work.

Ref: Nikaido, Microbiol Mol Biol Rev 2003, Outer Membrane Porins and Resistance; Delcour, BBA 2009, Porin Regulation.

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Microbial Physiology

The pentapeptide bridge in Gram-positive bacteria consists mainly of:

Crosslinking between glycan strands in many Gram-positive pathogens involves pentaglycine bridge characteristic of Staphylococcus aureus, intensively studied because it influences vancomycin sensitivity. After polymerization, tetrapeptide side chains protrude from MurNAc. In S. aureus, FemXAB family nonribosomal peptidyl transferases add glycine residues one by one to epsilon amino group of L-lysine at position three, using glycyl-tRNA as donor, generating pentaglycine. This flexible extension then becomes substrate for transpeptidase that links glycine terminal to D-alanine of neighboring stem, creating pentaglycine cross-bridge. Composition varies: Streptococcus pneumoniae uses dipeptide L-Ala-L-Ala, Enterococcus uses L-Ala-L-Ala or L-Ala-L-Ser. Glycine as smallest amino acid provides flexibility facilitating long crosslinks, increasing cell wall thickness and resistance to lysozyme. Interpeptide bridges containing alanine, serine, or proline also occur, but glycine dominance in this species is textbook example of how nonribosomal amino acid incorporation shapes wall architecture and antibiotic resistance through Fem-mediated bridging and altered penicillin-binding protein 2a affinity. The pentaglycine bridge length influences susceptibility to lysostaphin, an endopeptidase from Staphylococcus simulans that specifically cleaves Gly-Gly bonds, widely used in laboratory to lyse staphylococci, and to host immunity protein FemX-mediated resistance mechanisms that replace glycine with serine in some methicillin-resistant isolates, altering bridge flexibility and vancomycin binding.

Ref: Schneider et al., Mol Microbiol 2004, FemABX Pentaglycine; Pinho et al., Nature Rev Microbiol 2013, Staphylococcal Cell Wall.

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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

Which feature distinguishes the plasma membrane of Archaea from Bacteria?

Although both bacterial and archaeal plasma membranes adopt fluid mosaic architecture with proteins diffusing within lipid matrix, chemical composition diverges dramatically. Bacteria synthesize phospholipid fatty acid esters attached to glycerol-3-phosphate, containing straight-chain fatty acids that can be saturated or unsaturated, regulated to maintain fluidity via desaturases and branched chain synthesis. They sometimes produce hopanoids or carotenoids for ordering, while sterols, cholesterol, and sphingolipids are generally absent except in some mycoplasmas that scavenge cholesterol. Archaea uniquely employ isoprenoid hydrocarbon chains, often C20 phytanyl or C40 biphytanyl, joined via ether bonds to glycerol-1-phosphate enantiomer, opposite stereochemistry to bacteria. These ether bonds resist hydrolysis by heat, acid, and phospholipases, and when tetraether forms span membrane as monolayer, proton leak is dramatically reduced. Hence presence of ether-linked isoprenoid lipids is defining chemical distinction, used as biomarker for archaeal domain in environmental lipidomics and underlying ability to thrive at extremes. Phospholipid concept remains, but linkage type and chain chemistry are inverted relative to bacteria.

Ref: Koga & Morii, Microbiol Mol Biol Rev 2007, Archaeal Membrane Lipids; Siliakus et al., Extremophiles 2017, Lipid Adaptation.

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Microbial Physiology

The Gram-negative bacterial cell wall consists of:

Gram-negative cell wall is thin but complex, optimized for protection while maintaining permeability for nutrients. It comprises inner plasma membrane, narrow periplasmic space containing single or few layers of peptidoglycan only about 2 to 6 nanometers thick, and outer membrane distinguished by asymmetry. Outer membrane outer leaflet consists of lipopolysaccharide anchored by lipid A, while inner leaflet contains phospholipids; integral outer membrane proteins such as porins, TonB-dependent receptors, and OmpA form beta-barrels. Lipopolysaccharide confers negative charge, stabilizes outer membrane via bridging divalent cations, and provides endotoxin activity and serotype specificity. Thick peptidoglycan, characteristic of Gram-positives, is absent; instead, thin layer provides shape but contributes less mechanical strength than in Gram-positives. Monolayer lipid and teichoic acid-only models do not reflect Gram-negative dual membrane architecture. Assembly of outer membrane requires Lpt pathway transporting LPS from inner membrane across periplasm to outer leaflet and Bam complex folding beta-barrel proteins. This organization explains staining behavior, antibiotic susceptibility, and resistance to detergents and bile in enteric organisms.

Ref: Silhavy et al., The Bacterial Cell Envelope, 2nd ed.; Vollmer & Seligman, Trends Microbiol 2010, Gram-Negative Wall.

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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 Archaea domain lacks which of the following metabolic processes?

Archaea exhibit remarkable metabolic diversity including methanogenesis converting hydrogen and carbon dioxide or acetate into methane via unique cofactors like methanofuran, tetrahydromethanopterin, and coenzyme M, anaerobic respiration using sulfur, nitrate, or ferric iron as electron acceptors, and oxidative phosphorylation driven by archaeal A-type ATP synthase related to vacuolar ATPases. Some halophiles like Halobacterium salinarum perform light-driven proton pumping via bacteriorhodopsin, a retinal-based phototrophy fundamentally different from chlorophyll-based photosynthesis. Chlorophyll-based oxygenic photosynthesis requiring photosystems I and II, chlorophyll a, and Calvin cycle enzymes for carbon fixation is characteristic of cyanobacteria and eukaryotic algae and plants, not of archaeal domain. No archaeon synthesizes chlorophyll or assembles photosystem reaction centers containing chlorophyll. Archaeal phototrophy relies on simple rhodopsins, not chlorin pigments. Hence absence of chlorophyll-based photosynthesis is defining metabolic gap, while methanogenesis remains unique archaeal contribution to global carbon cycle and greenhouse gas production in anaerobic environments such as wetlands and rumen. Photosynthetic bacteria use chlorophyll to harvest light, generating reducing power and ATP via electron transport chains, while archaeal bacteriorhodopsin pumps protons using retinal isomerization without producing reducing equivalents, representing functional convergence but chemically distinct phototrophy that does not fix carbon via Calvin cycle and therefore not counted as true chlorophyll photosynthesis.

Ref: Madigan et al., Brock Biology, Methanogenesis and Phototrophy; Thauer et al., Nature Rev Microbiol 2008, Methanogenic Pathways.

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Microbial Physiology

Which factor differentiates Archaea from Bacteria?

While both Archaea and Bacteria appear as small cells lacking membrane-bound organelles under light microscope, molecular-level distinctions are profound. Most robust differentiation historically cited is membrane lipid chemistry. Bacterial membranes consist of unbranched fatty acids linked by ester bonds to glycerol-3-phosphate, forming bilayer susceptible to hydrolysis. Archaeal membranes use phytanyl or biphytanyl isoprenoid chains linked by ether bonds to glycerol-1-phosphate, the opposite enantiomer, forming bilayers or tetraether monolayers highly resistant to heat, acid, and salinity. Additional differentiators include archaeal histone-like proteins in Euryarchaeota, absence of peptidoglycan, presence of ether lipids, and transcription machinery more similar to eukaryotic RNA polymerase II. Circular DNA is common to both groups and not distinctive. Lack of nuclear membrane characterizes both domains and cannot separate them. Presence of ether-linked lipids therefore remains classic textbook criterion, supported by genomic presence of geranylgeranylglyceryl phosphate synthase and glycerol-1-phosphate dehydrogenase genes exclusive to Archaea that serve as molecular markers in metagenomic studies. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.

Ref: Woese et al., PNAS 1990, Domains; Koga & Morii, Microbiol Mol Biol Rev 2007, Archaeal Lipids.

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