Microbiology Chapter 3: Principles of Disease and Epidemiology

Virulence and Disease

• Virulence: an organism's capacity to cause disease, with virulent agents causing disease when introduced into the host in small numbers.
• Adherence: the process by which bacteria stick to the surfaces of host cells.
• Carrier: a person or animal with asymptomatic infection that can be transmitted to another susceptible person or animal.
• Infection: multiplication of an infectious agent within the body, involving invasion or colonization of the body by pathogenic organisms.

Pathogens and Disease Development

• Pathogen: a microorganism capable of causing disease.
• Nonpathogen: a microorganism that does not cause disease, may be part of the normal microbiota.
• Opportunistic Pathogen: an agent capable of causing disease only when the host's resistance is impaired.
• Pathogenicity: the ability of an infectious agent to cause disease.
• Pathology: the scientific study of disease.
• Etiology: the cause of disease.
• Pathogenesis: the study of how disease develops.

Microbiota and Normal Flora

• Microbiota: microbial flora harbored by normal, healthy individuals.
• Normal microbiota: abundant normal microbiota that usually do not produce disease, achieving a balance that ensures the survival, growth, and propagation of both the bacteria and the host.
• Resident Microbiota: microorganisms that remain throughout an individual's life.
• Transient Microbiota: microorganisms that are present at a certain time then disappear.

Portals of Entry and Pathogenicity Islands

• Portals of entry: respiratory, gastrointestinal, genital, and urinary tract.
• Pathogenicity Islands: large groups of genes associated with pathogenicity, located on the bacterial chromosome.

Bacterial Virulence Factors

• Adherence factors: necessary for bacteria to adhere to cells of a tissue surface.
• Pili: thick rodlike appendages.
• Fimbriae: shorter, hairlike structures that help mediate adherence of bacteria to host cell surfaces.
• Lipoteichoic acid: causes adherence of streptococci to buccal epithelial cells.
• Invasion plasmid antigens (IpA-D): proteins that contribute to the invasion process.

Toxins

• Toxins: produced by bacteria, classified into two groups: endotoxin and exotoxin.
• Exotoxin: excreted by living cells, produced by both Gram-positive and Gram-negative bacteria.
• Endotoxin: integral part of the cell wall of Gram-negative bacteria, released on bacterial death and in part during growth.

Additional Virulence Factors

• Collagenase: degrades collagen, promoting spread of infection in tissue.
• Coagulase: contributes to the formation of fibrin walls around staphylococcal lesions.
• Hyaluronidases: enzymes that hydrolyze hyaluronic acid, a constituent of the ground substance of connective tissue.
• Streptokinase (fibrinolysin): activates a proteolytic enzyme of plasma.
• Hemolysins: cytolysins that dissolve red blood cells.
• Leukocidins: cytolysins that kill tissue cells or leukocytes.

Immune Response

• IgA: immunoglobulin A, the secretory antibody on mucosal surfaces.
• IgA1: has a series of amino acids in the hinge region that are not present in IgA2.
• IgA1 proteases: split IgA1 at specific proline--threonine or proline--serine bonds in the hinge region, inactivating its antibody activity.

Elimination and Damage

• Elimination: removal of the microbe from the host by physical factors, an immune response, therapy, or out competition by existing microbes, or if the microbe causes sufficient damage to the host, death.
• Damage: interruption of normal tissue structure and/or function at the cellular, tissue, or organ level.