#Gram-negative bacteria

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.

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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 me

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

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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

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

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Microbial Physiology

Which component of the Gram-negative bacterial cell wall is responsible for its endotoxin activity?

Lipopolysaccharide defines outer leaflet of Gram-negative outer membrane and accounts for barrier properties and immune activation. Its lipid anchor, lipid A, determines endotoxin activity, while core polysaccharide and distal O-antigen provide antigenic variation. Lipid A consists of diglucosamine bisphosphate hexa-acylated with hydroxy fatty acids, a unique pathogen-associated molecular pattern detected by host innate receptor TLR4/MD-2 complex. Upon release during cell division, death, or antibiotic-induced lysis, lipid A stimulates macrophages to secrete potent mediators like tumor necrosi

Ref: Raetz & Whitfield, Ann Rev Biochem 2002, LPS Endotoxins; Erridge et al., Microbes Infect 2002, Endotoxin Structure.

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Microbial Physiology

The periplasmic space is a characteristic of which group of microorganisms?

Periplasmic space is compartment created by double membrane organization, bounded internally by inner plasma membrane containing phospholipid bilayer and respiratory proteins and externally by outer membrane containing lipopolysaccharide. This intervening aqueous compartment, ranging from 10 to 20 nanometers, contains thin peptidoglycan sacculus, binding proteins for nutrient uptake, beta-lactamases, and oxidation-reduction folding enzymes like DsbA. It is hallmark of Gram-negative bacteria such as Escherichia coli and Pseudomonas aeruginosa, enabling distinct redox environment more oxidizing

Ref: Silhavy et al., The Bacterial Cell Envelope, 2nd ed.; Alberts et al., Molecular Biology Cell, Chapter 10: Gram-Negative Envelope.

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Microbial Physiology

The Porin proteins in Gram-negative bacteria facilitate:

In Gram-negative organisms, outer membrane asymmetry places lipopolysaccharide in outer leaflet and phospholipid inside, rendering it intrinsically impermeable to many hydrophilic compounds, including bile salts and antibiotics like vancomycin. Nutrient acquisition therefore depends on abundant trimeric beta-barrel proteins known as porins, exemplified by general porins OmpF, OmpC, and PhoE in Escherichia coli. Each monomer folds as a 16-stranded antiparallel barrel creating a water-filled channel lined with charged residues forming eyelet constriction that determines size exclusion near 600 d

Ref: Alberts et al., Molecular Biology Cell, 7th ed., Chapter 10: Porins; Delcour, Biochim Biophys Acta 2009, Outer Membrane Permeability.

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Microbial Physiology

Which component gives Gram-negative bacteria endotoxic properties?

The outer leaflet of Gram-negative outer membrane is dominated by lipopolysaccharide, a high-molecular-weight amphipathic complex consisting of variable O-antigen polysaccharide, conserved core oligosaccharide containing 3-deoxy-D-manno-octulosonate (Kdo), and hydrophobic lipid A anchor. Lipid A is the endotoxic principle. Chemically it is a beta-1,6-linked D-glucosamine disaccharide phosphorylated at positions 1 and 4 prime and hexa-acylated with 3-hydroxy fatty acids in Escherichia coli, a structure absent from mammalian membranes. This moiety serves as a classic pathogen-associated molecula

Ref: Raetz & Whitfield, Ann Rev Biochem 2002, LPS Endotoxins; Janeway's Immunobiology, 9th ed., Recognition of LPS.

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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

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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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 l

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. Serologi

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

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Microbial Physiology

The bacterial Gram-negative outer membrane contains:

Envelope architecture differentiates Gram staining groups morphologically and chemically. Gram-negative bacteria possess dual membrane system separated by periplasm containing thin peptidoglycan two to six nanometers and proteins, hydrolytic enzymes. Inner membrane phospholipid bilayer standard contains respiratory chain. Outer membrane asymmetric: inner leaflet phosphatidylethanolamine phosphatidylglycerol, outer leaflet predominantly lipopolysaccharides LPS molecules comprising lipid A glucosamine disaccharide with six acyl chains anchoring membrane and conferring endotoxin activity stimulat

Ref: Silhavy et al., Cold Spring Harbor Perspect Biol 2010, Gram-negative outer membrane contains LPS.

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Microbial Classification

Which bacteria stain Gram-negative despite being Firmicutes?

Veillonella represents anaerobic Gram-negative cocci within class Negativicutes of phylum Firmicutes exhibiting diderm envelope architecture that stains Gram-negative despite firmicute phylogenetic affiliation. Genomic analysis shows presence of outer membrane with lipopolysaccharide atypical for Gram-positives, interpreted as secondary acquisition within Firmicutes. These commensals metabolize lactate to propionate in oral cavity and gut. Exception demonstrates that Gram stain reaction does not always reflect phylogeny, challenging classical determinative criteria, as Veillonella clusters wit

Ref: Campbell Biology Chapter 27 Negativicutes Veillonella Gram-negative Firmicutes; Bergey's Manual Volume 3 Veillonella diderm exception

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