Bacterial Adherence and Virulence

Bacterial Adherence to Host Cells

• Adhesins are bacterial surface molecules that bind to host cells.
• Fibrilla cover the surface of Gram-positive bacteria, such as Streptococcus pyogen.
• Fimbria (or pili) are filamentous proteins on Gram-negative bacteria, such as E. coli and N. gonorrhea.

Virulence of Intracellular Bacteria

• Facultative intracellular bacteria infect epithelial cells, macrophages, or both.
• Virulence factors concern the ability to bind and enter cells, and survive within them.
• Examples of intracellular bacteria include Shigella, enteroinvasive E. coli, M. tuberculosis, and M. leprae.

Bacterial Endotoxin

• Bacterial endotoxin is a lipopolysaccharide (LPS) found in the outer cell wall of Gram-negative bacteria.
• LPS is composed of a long-chain fatty acid anchor (lipid A) connected to a core sugar chain.
• Free LPS attaches to a circulating LPS-binding protein, which binds to a specific receptor (CD14) on monocytes, macrophages, and neutrophils.
• Engagement of CD14 results in intracellular signaling, causing cell activation and production of effector cytokines.

Host Response to LPS

• Low LPS concentration: cytokines >> activation of T lymphocytes.
• High LPS concentration: cytokines (TNF, IL1, IL12) >> septic shock, DIC, ARDS.

Bacterial Exotoxins

• Exotoxins are secreted proteins that directly cause cellular injury and frequently underlie disease manifestations.
• Some exotoxins are bacterial enzymes (proteases, hyaluronidases, coagulases, fibrinolysins).
• Many exotoxins alter intracellular signaling or regulatory pathways.
• Examples of exotoxins include neurotoxins produced by Clostridium botulinum and Clostridium tetani.

Patterns of Inflammatory Responses to Infection

• Neutrophil-rich acute suppurative inflammation is typical of many bacteria ("pyogenic" bacteria) and some fungi.
• Mononuclear cell infiltrates are common in many chronic infections and some acute viral infections.
• Granulomatous inflammation is the hallmark of M. tuberculosis and certain fungi.
• Cytopathic and proliferative lesions are caused by some viruses.

Suppurative Inflammation

• This pattern is the reaction to acute tissue damage, characterized by increased vascular permeability and leukocytic exudates, predominantly neutrophils.
• Collections of neutrophils may give rise to localized liquefactive necrosis, forming abscesses.
• The necrotic tissue and inflammatory cells constitute pus, and bacteria that evoke pus formation are called "pyogenic."

Diagnosing Infectious Agents

• The histopathology of various infections provides an important clue as to etiology.
• Some infectious agents can be directly identified in H&E-stained sections.
• Special stains can identify organisms based on their cell wall or cell coat characteristics.
• Microorganisms can also be identified after labeling with specific antibody probes.
• Organisms are usually best visualized at the advancing edge of a lesion rather than at its center, particularly if there is necrosis.

This quiz covers the mechanisms of bacterial adherence to host cells, including adhesins, fibrils, and fimbria. It also explores the virulence of intracellular bacteria, including facultative intracellular bacteria that infect epithelial cells and macrophages.