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.