Structure of Gram-negative Cell Wall

What are Gram-negative bacteria?

• Gram-negative bacteria represent a distinct category of bacteria characterized by their response to Gram staining, a differential staining technique. Unlike Gram-positive bacteria, Gram-negative bacteria lose the crystal violet dye when washed with alcohol and take up the pink or red color of the counterstain, safranin. This staining difference arises due to variations in their cell wall composition.
• The cell walls of Gram-negative bacteria are notably different from those of Gram-positive bacteria. While Gram-positive bacteria have a thick peptidoglycan layer and a thin lipid layer, Gram-negative bacteria possess a thin peptidoglycan layer surrounded by a thick lipid-rich outer membrane. This structural difference leads to the distinct staining properties observed during Gram staining. The thin peptidoglycan layer in Gram-negative bacteria makes their cell walls less rigid, allowing the primary stain to be easily washed away by alcohol and water.
• Gram-negative bacteria thrive in a variety of environments, often where life is abundant. They include normal flora such as Escherichia coli, which resides in the human gut, as well as pathogenic bacteria like Klebsiella pneumoniae and Chlamydia trachomatis. These bacteria play diverse roles, from maintaining essential bodily functions to causing serious infections.
• Understanding the structural and functional differences between Gram-negative and Gram-positive bacteria is crucial for microbiologists. The unique cell wall composition of Gram-negative bacteria influences their behavior, interaction with hosts, and response to antibiotics. Therefore, studying these bacteria provides insights into their pathogenic mechanisms and potential treatment strategies.

Cell wall of the Gram-negative is more complicated than the Gram-positive cell wall. The amount of peptidoglycan present in the Gram-negative cell wall is considerably lower than that of that of the cell’s Gram positive wall. There are only a few layers of peptidoglycan (2-8 millimeters) are visible in the cell membrane’s outermost. A Gram-negative wall that lies outside the peptidoglycan layer has three primary components–(a) the lipoprotein layer, (b) outer membrane as well as (c) Lipopolysaccharides.

Characteristics gram-negative bacteria

Gram-negative bacteria are distinguished by several key structural and functional characteristics. These attributes influence their staining properties, biological functions, and interactions with their environments.

• Cell Membrane Structure:
  • Inner Cell Membrane: This cytoplasmic membrane forms the innermost layer, essential for cellular functions.
  • Peptidoglycan Layer: A slender layer compared to Gram-positive bacteria, providing structural support.
  • Outer Membrane: Contains lipopolysaccharides (LPS) in the outer leaflet and phospholipids in the inner leaflet. LPS is composed of lipid A, a core polysaccharide, and O antigen, crucial for immune response and pathogenicity.
• Porins:
  • Present in the outer membrane, porins function as pores, allowing specific molecules to pass through, contributing to the selective permeability of the bacterial cell wall.
• Periplasmic Space:
  • Located between the outer membrane and the cytoplasmic membrane, this space contains a gel-like substance called periplasm, rich in proteins and a thin peptidoglycan layer.
• S-Layer:
  • Directly attached to the outer membrane, the S-layer provides additional structural integrity and protection.
• Flagella:
  • When present, flagella in Gram-negative bacteria have four supporting rings, compared to two in Gram-positive bacteria, aiding in motility.
• Absence of Teichoic Acids:
  • Unlike Gram-positive bacteria, Gram-negative bacteria lack teichoic acids or lipoteichoic acids, which are typically found in the cell walls of Gram-positive bacteria.
• Lipoproteins:
  • These proteins are attached to the polysaccharide backbone, playing a role in maintaining the structural integrity of the cell wall.
• Braun’s Lipoprotein:
  • Some Gram-negative bacteria contain Braun’s lipoprotein, which links the outer membrane to the peptidoglycan layer through covalent bonds.
• Spore Formation:
  • Most Gram-negative bacteria do not form spores, with very few exceptions.
• Cell Envelope:
  • The characteristic cell envelope includes the outer membrane, periplasmic space, and inner membrane, which are distinct from those in Gram-positive bacteria.
• Microscopic Observation:
  • Gram-negative bacteria exhibit various shapes:
    • Rods/Bacillus: e.g., Escherichia coli
    • Coccobacillus: e.g., Haemophilus influenzae
    • Streptobacillus: Rod-shaped cells connected in chains, e.g., Streptobacillus moniliformis
    • Trichome: Rod-shaped cells arranged in a columnar form, sometimes enclosed in a sheath
    • Spiral/Spirilla: e.g., Chlamydia trachomatis, Treponema pallidum
    • Filamentous: e.g., Nocardia spp.

Cell wall of Gram-negative bacteria

The cell wall of Gram-negative bacteria is notably complex, playing a crucial role in the bacterium’s functionality and interactions with its environment. This complexity contrasts sharply with the simpler cell walls of Gram-positive bacteria. Understanding the structure and components of the Gram-negative bacterial cell wall reveals how it supports various cellular processes.

• Structure of the Cell Wall:
  • Peptidoglycan Layer: The peptidoglycan layer in Gram-negative bacteria is thin, typically measuring 2-7nm. Despite its slimness, it provides essential structural support.
  • Outer Membrane: This membrane is thicker, about 7-8nm, and forms a protective barrier. It is a bilayer structure composed of phospholipids, lipopolysaccharides (LPS), lipoproteins, and surface proteins.
• Periplasmic Space:
  • Located between the outer membrane and the cytoplasmic membrane, the periplasmic space contains a gel-like substance called periplasm. This space is larger in Gram-negative bacteria compared to Gram-positive bacteria and houses various enzymes and proteins vital for nutrient acquisition and cell wall synthesis.
• Cell Envelope Layers:
  • The Gram-negative bacterial cell envelope consists of three layers:
    • Outer Membrane: Unique to Gram-negative bacteria, it acts as a barrier against harmful substances and contains LPS, which are crucial for the bacterium’s structural integrity and pathogenicity.
    • Peptidoglycan Layer: Although thin, it provides rigidity and maintains the cell’s shape.
    • Cytoplasmic Membrane: The innermost layer, essential for various cellular processes including energy production and transport.
• Components of the Outer Membrane:
  • Lipopolysaccharides (LPS): These molecules consist of lipid A, a core polysaccharide, and an O antigen. LPS play a significant role in the immune response and can act as endotoxins, eliciting strong immune reactions during infections.
  • Porins: Porin proteins are embedded in the outer membrane, functioning as channels that control the entry and exit of molecules, thus regulating the internal environment of the bacterium.
  • Lipoproteins: These proteins connect the outer membrane to the peptidoglycan layer, helping maintain structural stability.
• Functional Properties:
  • Protection and Selective Permeability: The outer membrane acts as a formidable barrier against antibiotics, detergents, and other harmful agents, contributing to the bacteria’s resistance. Porins and other transport proteins regulate the passage of nutrients and waste products.
  • Immune Response: Lipopolysaccharides (LPS) in the outer membrane serve as endotoxins, triggering immune responses during bacterial infections. This can result in inflammation and other immune reactions, which are critical for understanding bacterial pathogenesis and developing treatments.
• Absence of Teichoic Acids:
  • Unlike Gram-positive bacteria, Gram-negative bacteria do not contain teichoic acids, which are polymers found in the cell walls of Gram-positive bacteria.

The Periplasmic Space in Gram-Negative Bacteria

The periplasmic space in Gram-negative bacteria is a critical and complex component, situated between the outer membrane and the cytoplasmic membrane. This space plays a pivotal role in the bacteria’s functionality and survival.

• Structure and Composition:
  • Location: The periplasmic space is found between the outer membrane and the cytoplasmic membrane. It is more prominent in Gram-negative bacteria than in Gram-positive bacteria.
  • Content: This space contains a gel-like substance known as periplasm, which is rich in proteins, enzymes, and other essential molecules.
• Proteins and Enzymes:
  • Hydrolytic Enzymes: These enzymes break down nucleic acids and phosphorylated molecules, aiding in the degradation of complex nutrients into simpler, absorbable forms.
  • Binding Proteins: These proteins play an active role in the transport of materials into the bacterial cell, ensuring efficient nutrient acquisition.
• Peptidoglycan Synthesis:
  • Enzymatic Activity: Enzymes within the periplasmic space are involved in the synthesis and remodeling of peptidoglycan, which is crucial for maintaining cell wall integrity and shape.
• Toxin Modification:
  • Detoxifying Enzymes: The periplasmic space contains enzymes that modify and neutralize toxic elements, protecting the bacterial cell from harmful substances.
• Functional Roles:
  • Nutrient Acquisition: The presence of hydrolytic enzymes and binding proteins ensures that the bacteria efficiently absorb necessary nutrients from their environment.
  • Cell Wall Maintenance: Enzymes involved in peptidoglycan synthesis maintain the structural integrity and shape of the bacterial cell wall.
  • Protection: The detoxifying enzymes present in the periplasmic space provide a defense mechanism against toxic elements that could otherwise damage the cell.
• Importance in Bacterial Physiology:
  • The periplasmic space is essential for the bacteria’s survival and adaptation in various environments. It supports crucial processes such as nutrient uptake, cell wall synthesis, and protection against environmental stressors.

Peptidoglycan in Gram-Negative Bacteria

Peptidoglycan is a fundamental component of bacterial cell walls, providing structural integrity and shape. In Gram-negative bacteria, peptidoglycan plays a unique and critical role despite being relatively thin compared to Gram-positive bacteria.

• Structure and Composition:
  • Thin Layer: The peptidoglycan layer in Gram-negative bacteria is notably thin, typically around 2nm thick, consisting of 2-3 sheets. This contrasts with the thicker peptidoglycan layer found in Gram-positive bacteria.
  • Location: It is situated between the outer membrane and the plasma membrane, forming an essential part of the cell wall structure.
• Composition and Properties:
  • Polymer Structure: Peptidoglycan is a polymer consisting of sugars and amino acids. The sugars include N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which form a repeating disaccharide unit.
  • Cross-Linking: These sugar chains are cross-linked by short peptide chains, providing tensile strength to the bacterial cell wall.
• Functional Roles:
  • Structural Support: Peptidoglycan provides the necessary rigidity and shape to the bacterial cell, preventing it from lysing under osmotic pressure.
  • Protection: Despite its thinness, the peptidoglycan layer helps protect the bacterial cell from mechanical damage and environmental stress.
• Peptidoglycan Synthesis:
  • Enzymatic Process: The synthesis of peptidoglycan involves various enzymes located in the periplasmic space. These enzymes coordinate the assembly and cross-linking of the sugar and peptide components.
  • Dynamic Remodeling: Peptidoglycan is continuously remodeled to accommodate cell growth and division, ensuring the cell wall remains intact and functional.
• Peptidoglycan and Antibiotic Targeting:
  • Antibiotic Target: Due to its crucial role, peptidoglycan synthesis is a target for many antibiotics, such as penicillin. These antibiotics inhibit the enzymes involved in peptidoglycan cross-linking, leading to cell wall weakness and bacterial lysis.
  • Resistance Mechanisms: Some Gram-negative bacteria have developed resistance mechanisms, such as beta-lactamase production, which degrades beta-lactam antibiotics and protects the peptidoglycan layer.
• Comparison with Gram-Positive Bacteria:
  • Thickness and Strength: In Gram-positive bacteria, the peptidoglycan layer is much thicker, contributing to their higher resistance to physical disruption.
  • Role in Gram Staining: The thin peptidoglycan layer in Gram-negative bacteria does not retain the crystal violet dye during Gram staining, resulting in a pink/red appearance after counterstaining with safranin.

The Outer Membrane and Lipopolysaccharides in Gram-Negative Bacteria

The outer membrane of Gram-negative bacteria is a complex and vital structure. Situated above the thin peptidoglycan layer, it serves multiple critical functions in protecting and maintaining the bacterial cell.

• Structure of the Outer Membrane:
  • Composition: The outer membrane is primarily composed of lipopolysaccharides (LPS), phospholipids, lipoproteins, and surface proteins.
  • Braun’s Lipoprotein: This protein covalently binds to the peptidoglycan and anchors itself to the outer membrane via its hydrophobic ends. It provides a strong linkage between the peptidoglycan and the outer membrane.
• Functional Components:
  • Adhesion Sites: These sites on the outer membrane allow cell contact and membrane fusion, facilitating substance entry into the cell.
  • Porin Proteins: Porins are integral proteins that form channels allowing the passive diffusion of small molecules like glucose across the outer membrane. Larger molecules, such as Vitamin B12, require specific carriers.
• Lipopolysaccharides (LPS):
  • Structure: LPS are large, complex molecules comprising three main parts: Lipid A, core polysaccharides, and the O side chain (O antigen).
  • Lipid A: This component consists of two glucosamine sugar derivatives, each with three fatty acids and pyrophosphate. It embeds in the membrane and contributes to the endotoxic properties of LPS.
  • Core Polysaccharides: These connect Lipid A to the O side chain and help stabilize the LPS structure.
  • O Side Chain (O Antigen): Extending outward from the core, the O side chain consists of sugars that vary among bacterial strains, aiding in immune evasion by altering surface antigens.
• Functions of Lipopolysaccharides:
  • Barrier Function: LPS protect the cell wall from external threats, including antibiotics and bile salts, by forming a barrier.
  • Structural Stability: The negative charge of LPS contributes to the overall negative charge of the cell surface, stabilizing the membrane structure.
  • Endotoxic Properties: Lipid A is toxic to host organisms, acting as an endotoxin that can trigger strong immune responses.
• Role in Antibiotic Resistance:
  • The outer membrane’s structure significantly impedes the entry of harmful substances. This barrier function is crucial in preventing antibiotics and other toxic agents from penetrating and disrupting the cell.
• Preventing Component Loss:
  • Besides protecting against external threats, the outer membrane also prevents the loss of vital components from the periplasmic space, ensuring the cell retains essential enzymes and nutrients.

Lipoprotein layer

The layer of lipoprotein is comprised of Braun’s lipoprotein. Braun’s lipoprotein can be described as a small lipoprotein which is covalently attached to the peptidoglycan underneath and is embedded within the membrane’s outer part through its hydrophobic ends. lipoprotein helps to stabilize the outer layer of the cell wall that is Gram-negative.

The Outer Membrane in Gram-Negative Bacteria

The outer membrane of Gram-negative bacteria is a critical structure with a unique two-layered design. This membrane plays a vital role in maintaining the integrity and functionality of the bacterial cell.

• Structure of the Outer Membrane:
  • Bilayer Design: The inner layer of the outer membrane is similar to the cell membrane, composed primarily of phospholipids. However, the outer layer contains a unique component called lipopolysaccharide (LPS).
  • Adhesion: The outer membrane and the plasma membrane are connected at multiple points within the Gram-negative cell wall.
• Key Proteins in the Outer Membrane:
  • Porins: These are protein molecules that form special channels in the outer membrane. They serve several functions:
    • Diffusion Channels: Porins allow the diffusion of low-molecular-weight hydrophilic substances such as amino acids, sugars, and certain ions.
    • Selective Barrier: They exclude hydrophobic molecules, thereby protecting the cell.
    • Defense Mechanism: Porins contribute to the cell’s defense against harmful substances.
  • Outer Membrane Proteins (OMPs): These include various proteins that facilitate different functions:
    • OmpC, OmpD, OmpF, PhoE, LamB: These major proteins are involved in the transmembrane transport of maltose and maltodextrins.
    • Tsx: Acts as the receptor for T6 bacteriophage and is responsible for the transmembrane transport of nucleosides and certain amino acids.
    • OmpA: This protein binds the outer membrane to the peptidoglycan layer and acts as the receptor for the sex pilus during F-mediated bacterial conjugation.
• Specialized Transport Functions:
  • The outer membrane contains proteins that transport specific molecules, such as Vitamin B12 and iron-siderophore complexes.
  • Additionally, it houses minor proteins like phospholipases, enzymes, and proteases.
• Functional Significance:
  • Barrier and Protection: The outer membrane acts as a barrier, preventing the entry of harmful substances, including antibiotics and bile salts.
  • Structural Stability: The unique composition of the outer membrane, including LPS, contributes to the overall stability of the bacterial cell wall.
  • Nutrient Transport: Porins and other OMPs facilitate the transport of essential nutrients into the cell, ensuring proper cellular function.

Structure of a lipopolysaccharide

Lipopolysaccharides (LPS) are complex molecules found in the outer membrane of Gram-negative bacteria. These molecules are critical for the bacterial cell’s integrity and interaction with its environment. The structure of LPS consists of three main components: lipid A, the core oligosaccharide, and the O polysaccharide, also known as the O-antigen.

1. Lipid A:
   • Composition: Lipid A is composed of a phosphorylated disaccharide unit. Attached to this unit are several long-chain fatty acids. A notable feature is the presence of hydroxymyristic acid, a fatty acid associated with the endotoxic properties of LPS.
   • Variability: While the structure of lipid A can vary among different Gram-negative species, it remains relatively consistent within the same species. This consistency is crucial for the stability and function of the outer membrane.
2. Core Oligosaccharide:
   • Structure: The core oligosaccharide includes distinctive sugars such as ketodeoxyoctanoic acid (KDO) and heptose, which link to lipid A. This component is genus-specific, common to all Gram-negative bacteria.
   • Lipooligosaccharides (LOS): In some bacteria, such as Neisseria meningitidis, Neisseria gonorrhoeae, Haemophilus influenzae, and Haemophilus ducreyi, the core oligosaccharide forms smaller glycolipids known as LOS. These possess shorter, branched glycans.
   • Antigenic Diversity: LOS exhibit significant antigenic and structural diversity, even within a single strain. The N-acetyllactosamine residue on LOS is immunochemically similar to human erythrocyte I antigen. Sialylation of this residue offers the bacteria molecular mimicry advantages, helping them evade the host’s immune response.
3. O Polysaccharide (O-Antigen):
   • Extension and Composition: Extending from the core oligosaccharide, the O polysaccharide contains various unique sugars. Its composition varies between bacterial strains, giving each strain specific antigenic properties.
   • Immune Evasion: The O-antigen is exposed to the host’s immune system. Gram-negative bacteria can alter their O side chains to evade detection by the host’s defenses. This adaptability allows the bacteria to persist and thrive in hostile environments.

Gram-Negative Bacteria: Pathologies and Clinical Significance

Gram-negative bacteria are part of the normal human flora, but some can cause severe infections. These infections range from community-acquired to nosocomial (hospital-acquired) infections. If not detected and treated promptly, they can lead to serious health issues, even death. Below is a list of some Gram-negative bacteria and the clinical features associated with the diseases they cause.

1. Neisseria gonorrhoeae

• Genitourinary Tract Infections: In males, this bacterium infects the urethra, leading to purulent urethral discharge and painful urination. In females, it affects the vagina and endocervix, possibly progressing to the uterus, causing salpingitis, pelvic inflammatory disease (PID), and fibrosis. Salpingitis can lead to infertility.
• Renal Infection: Men may experience constipation, painful defecation, and purulent discharge.
• Pharyngitis: Purulent pharyngeal exudation can cause pharyngitis.
• Ophthalmia Neonatorum: Newborns can acquire this infection during birth, leading to eye infections.
• Disseminated Infection: Symptoms include fever, painful purulent arthritis, and small scattered pustules on the skin with an erythematous base. Necrosis may develop.

2. Neisseria meningitidis

• Meningitis: This bacterium can cause meningococcemia if it invades the bloodstream, associated with high fever. It can also spread to the brain, causing purulent meningitis with fever, severe headaches, joint aches, and a petechial or purpuric rash.
• Septicemia: Rapid onset of septicemia can progress to fulminant septicemia and shock, especially in children (Waterhouse-Friderichsen syndrome).

3. Escherichia coli

• Intestinal Diseases: These include enterotoxigenic (ETEC), enteropathogenic (EPEC), enterohemorrhagic (EHEC), enteroinvasive (EIEC), and enteroaggregative (EAEC) strains, all associated with diarrhea (watery or bloody).
• Extraintestinal Diseases: E. coli can cause urinary tract infections (UTIs), neonatal meningitis, and nosocomial infections like sepsis, bacteremia, endotoxic shock, and pneumonia.

4. Salmonella spp.

• Enteric and Typhoid Fever: Characterized by fever, abdominal pain, chills, sweats, headache, anorexia, weakness, sore throat, cough, myalgia, and either diarrhea or constipation.
• Gastroenteritis (Salmonellosis): Symptoms include nausea, vomiting, and non-bloody diarrhea.
• Bacteremia: Associated with abdominal infections, osteomyelitis, and septic arthritis.

5. Campylobacter jejuni

• Intestinal and Extraintestinal Disease: Symptoms include fever, headache, myalgia, abdominal cramping, and diarrhea, which may be bloody. It commonly causes traveler’s diarrhea and pseudoappendicitis.
• Bacteremia: Often transient, it occurs most frequently in infants and older adults.

6. Shigella dysenteriae

• Shigellosis (Bacillary Dysentery): Characterized by diarrhea with blood, mucus in stool, and painful abdominal cramping.

7. Vibrio cholerae

• Cholera: Associated with profuse watery diarrhea, leading to massive fluid and electrolyte loss.

8. Helicobacter pylori

• Acute Gastritis: Causes diarrhea and epigastric discomfort.
• Ulcers: Can cause both duodenal and gastric ulcers. Persistent ulceration may lead to mucosa-associated lymphoid tumors.

9. Klebsiella pneumoniae

• UTI and Nosocomial Infections: Can cause urinary tract infections and bacteremia in hospital settings.

10. Pseudomonas aeruginosa

• Opportunistic Infections: Common in wounded patients and those with catheters or respirators.
• Eye Infections: Keratitis and endophthalmitis following injuries.
• Skin and Respiratory Infections: Causes skin wound infections and pneumonia symptoms.
• Gastrointestinal Infections: Associated with diarrhea and necrotic enterocolitis in infants.
• Systemic Infections: Includes septicemia, pneumonia, bone and joint infections, CNS infections, and soft tissue infections in hospitalized patients.

Gram-Negative Bacteria	Pathologies	Clinical Features
Neisseria gonorrhoeae	Genitourinary Tract Infections	Purulent urethral discharge, painful urination in males; vagina and endocervix infection in females, progressing to salpingitis, PID, and fibrosis; infertility; renal infection in men with constipation, painful defecation, and purulent discharge; pharyngitis; ophthalmia neonatorum in newborns; disseminated infection with fever, purulent arthritis, pustules with erythematous base, and possible necrosis.
Neisseria meningitidis	Meningitis and Septicemia	Rapid onset of meningococcemia with high fever; purulent meningitis with severe headaches, joint aches, petechial or purpuric rash; septicemia progressing to fulminant septicemia and shock, especially in children (Waterhouse-Friderichsen syndrome).
Escherichia coli	Intestinal Diseases and Extraintestinal Diseases	Diarrhea (watery or bloody) from ETEC, EPEC, EHEC, EIEC, EAEC; UTIs, neonatal meningitis, sepsis, bacteremia, endotoxic shock, pneumonia.
Salmonella spp.	Enteric and Typhoid Fever, Gastroenteritis, Bacteremia	Fever, abdominal pain, chills, sweats, headache, anorexia, weakness, sore throat, cough, myalgia, diarrhea or constipation; nausea, vomiting, non-bloody diarrhea; abdominal infections, osteomyelitis, septic arthritis.
Campylobacter jejuni	Intestinal and Extraintestinal Disease, Bacteremia	Fever, headache, myalgia, abdominal cramping, diarrhea (may be bloody); traveler’s diarrhea, pseudoappendicitis; transient bacteremia, especially in infants and older adults.
Shigella dysenteriae	Shigellosis (Bacillary Dysentery)	Diarrhea with blood, mucus in stool, and painful abdominal cramping.
Vibrio cholerae	Cholera	Profuse watery diarrhea leading to massive fluid and electrolyte loss.
Helicobacter pylori	Acute Gastritis and Ulcers	Diarrhea, epigastric discomfort, duodenal and gastric ulcers; persistent ulceration may lead to mucosa-associated lymphoid tumors.
Klebsiella pneumoniae	UTI and Nosocomial Infections	Urinary tract infections, nosocomial bacteremia.
Pseudomonas aeruginosa	Opportunistic Infections, Eye Infections, Skin and Respiratory Infections, Gastrointestinal Infections, Systemic Infections	Wound infections from surgeries, eye infections post-injury, skin wound infections, pneumonia, gastrointestinal infections with diarrhea, necrotic enterocolitis in infants, systemic infections including septicemia, pneumonia, bone and joint infections, CNS infections, and soft tissue infections in hospitalized patients.

Gram-Negative Bacteria and Antimicrobial Agents

Antimicrobial agents, specifically antibiotics, are used to target and inhibit the growth of bacteria. These antibiotics act by interfering with various bacterial cell mechanisms, thereby blocking cell multiplication and replication. Below is a detailed overview of antibiotics effective against Gram-negative bacteria, including their modes of action and the bacterial agents they target.

• Cephalosporins: These antibiotics, such as ceftriaxone, interfere with the synthesis of the bacterial cell wall by binding to penicillin-binding proteins. This action is crucial for treating infections caused by Neisseria gonorrhoeae, Neisseria meningitidis, and Pseudomonas aeruginosa.
• Tetracyclines: For instance, doxycycline inhibits protein synthesis by binding to 30S ribosomal subunits, which prevents polypeptide elongation. This mechanism is effective against Neisseria gonorrhoeae.
• Streptogramins: These antibiotics prevent polypeptide elongation at 50S ribosomes, thereby inhibiting protein synthesis. They are used against Neisseria gonorrhoeae.
• β-lactams: Penicillin G works by disrupting peptidoglycan synthesis, targeting Neisseria meningitidis and Pseudomonas aeruginosa.
• Rifampin: Rifamycin inhibits transcription by binding to DNA-dependent RNA polymerase. It is effective against Neisseria meningitidis and Escherichia coli.
• Macrolides: Erythromycin, azithromycin, and clarithromycin inhibit protein synthesis at 50S ribosomes, affecting a range of bacteria including Neisseria gonorrhoeae, Campylobacter jejuni, Shigella dysenteriae, Helicobacter pylori, and Pseudomonas aeruginosa.
• Quinolones: Fluoroquinolones and ciprofloxacin disrupt nucleic acid synthesis by binding to DNA gyrase. These antibiotics are used against Escherichia coli, Salmonella typhi/paratyphi, Campylobacter jejuni, Shigella dysenteriae, and Pseudomonas aeruginosa.
• Aminoglycosides: These antibiotics inhibit protein synthesis by causing premature peptide chain production at 30S ribosomes, effective for treating Escherichia coli infections.
• Sulfonamides: Sulfamethoxazole inhibits dihydropteroate synthase, disrupting folic acid synthesis and treating Escherichia coli infections.
• Trimethoprim: This antibiotic inhibits dihydrofolate reductase, interfering with folic acid synthesis and is used primarily for treating Escherichia coli infections.

These antimicrobial agents are essential tools in combating Gram-negative bacterial infections. Each class of antibiotics targets specific bacterial mechanisms, offering a range of options for treatment based on the bacterial strain and infection type.

Antibiotic	Mode of Action	Bacterial Agent
Cephalosporin: Ceftriaxone	Disrupts cell wall synthesis by binding to penicillin-binding proteins and enzymes responsible for peptidoglycan synthesis	Neisseria gonorrhoeae, Neisseria meningitidis, Pseudomonas aeruginosa
Tetracycline: Doxycycline	Inhibits protein synthesis by preventing elongation of polypeptides at 30S ribosomes	Neisseria gonorrhoeae
Streptogramins	Inhibit protein synthesis by preventing polypeptide elongation at 50S ribosomes	Neisseria gonorrhoeae
β-lactam: Penicillin G	Disrupts cell wall synthesis by inhibiting penicillin-binding proteins and enzymes used for peptidoglycan synthesis	Neisseria meningitidis, Pseudomonas aeruginosa
Rifampin: Rifamycin	Inhibits nucleic acid synthesis by preventing transcription through binding to DNA-dependent RNA polymerase	Neisseria meningitidis, Escherichia coli
Macrolides: Erythromycin, Azithromycin, Clarithromycin	Inhibit bacterial protein synthesis by preventing polypeptide elongation at 50S ribosomes	Neisseria gonorrhoeae, Campylobacter jejuni, Shigella dysenteriae, Helicobacter pylori, Pseudomonas aeruginosa
Quinolones: Fluoroquinolones, Ciprofloxacin	Inhibit nucleic acid synthesis by binding to the alpha-subunit of DNA gyrase	Escherichia coli, Salmonella typhi/paratyphi, Campylobacter jejuni, Shigella dysenteriae, Pseudomonas aeruginosa
Aminoglycosides	Inhibit protein synthesis by causing premature production of aberrant peptide chains at 30S ribosomes	Escherichia coli (localized and systemic infections)
Sulfonamides: Sulfamethoxazole	Act as antimetabolites by inhibiting dihydropteroate synthase, disrupting folic acid synthesis	Escherichia coli (UTIs and systemic diseases)
Trimethoprim	Acts as an antimetabolite by inhibiting dihydrofolate reductase, disrupting folic acid synthesis	Escherichia coli (UTIs)

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