Pathogenic Gram-Negative Cocci and Bacilli: Structure, Physiology, and Clinical Significance

Pathogenic Gram-Negative Bacteria

Overview of Gram-Negative Bacteria

Gram-negative bacteria constitute the largest group of human bacterial pathogens. Their pathogenicity is largely attributed to the presence of lipid A in the bacterial cell wall, which acts as an endotoxin and triggers a range of host responses.

• Lipid A (Endotoxin): A component of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. It is responsible for many toxic effects during infection.

• Clinical Effects: Lipid A can induce fever, vasodilation, inflammation, shock, and disseminated intravascular coagulation (DIC).

• Pathogenicity: Most Gram-negative bacteria that breach skin or mucous membranes, grow at 37°C, and evade the immune system can cause disease in humans.

Potential Effects of Lipid A (Endotoxin)

Lipid A triggers a cascade of immune and physiological responses:

• Activation of leukocytes and release of cytokines (e.g., TNF, IL-1, IL-6, IL-8)

• Blood-clotting reactions and platelet activating factor (PAF)

• Complement activation (C3a, C5a)

• Clinical outcomes: Fever, Disseminated Intravascular Coagulation (DIC), Inflammation, Shock

Example: Endotoxemia in septic shock is a direct result of excessive lipid A release.

Pathogenic Gram-Negative Cocci: Neisseria

Structure and Physiology of Neisseria

Neisseria is the only genus of Gram-negative cocci that regularly causes disease in humans. These bacteria are aerobic, nonmotile, and typically arranged as diplococci.

• Oxidase Positive: This biochemical property distinguishes Neisseria from many other Gram-negative pathogens.

• Virulence Factors: Pathogenic strains possess fimbriae, a polysaccharide capsule, and a cell wall containing lipid A.

• Pathogenic Species:

  • N. gonorrhoeae

  • N. meningitidis

Example: N. gonorrhoeae causes gonorrhea, while N. meningitidis is a leading cause of bacterial meningitis.

Key Terms

• Diplococci: Spherical bacteria arranged in pairs.

• Fimbriae: Hair-like appendages that aid in attachment to host tissues.

• Polysaccharide Capsule: A protective layer that enhances resistance to phagocytosis.

Comparison Table: Key Features of Pathogenic Neisseria Species

Species	Primary Disease	Virulence Factors	Transmission
N. gonorrhoeae	Gonorrhea	Fimbriae, capsule, IgA protease, lipid A	Sexual contact
N. meningitidis	Meningitis	Fimbriae, capsule, lipid A	Respiratory droplets

Additional info: The oxidase test is a rapid biochemical assay used to distinguish Neisseria from other Gram-negative bacteria. Positive result indicates the presence of cytochrome c oxidase.