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

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

View Details

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, teich

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

View Details

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

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

View Details

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 ste

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

View Details

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 ami

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

View Details

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 glyc

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

View Details

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. Arch

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

View Details

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 me

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

View Details

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 w

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

View Details

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

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

View Details

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. Chloroph

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

View Details

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. A

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

View Details