Cell Biology and Parasites

Microorganisms

• Must take over the machinery of another living cell (eukaryote/prokaryote) to replicate
• Bacteria have a cell wall but lack eukaryotic organelles
• Fungi are eukaryotic with a rigid external wall, similar to plants
• Parasites exist as single or multicellular structures with eukaryotic cell plan

Introduction to Microbiology

• Study of micro-organism interaction with human host resulting in disease
• Resultant diseases are termed infectious diseases
• Micro-organism = BUG, Host = MUG, Antibiotic = DRUG
• Normal presence of micro-organisms on or in humans, with 10x more microbial cells than human cells
• Microorganisms play a role in both human health and disease

Microorganisms' Roles

• Commensal/Normal flora
• Degeneration of organic matter
• Biotechnological areas – medicines, vaccines
• Production of cheese, yogurt, beer, and wine

Definitions

• Commensal organisms = Normal flora = Microbiota
• Residents: strains that occupy a niche at a body site indefinitely
• Transients: acquired from environment, establish briefly, but are excluded by competition or host defense
• Pathogens: microorganisms that possess the ability to cause infections
• Contamination: presence of micro-organisms for a short period without damage or invasion
• Colonization: continued presence of organisms for long periods without causing damage or invasion
• Contagious: can spread between people
• Carrier state: pathogenic organisms establish themselves in a host without symptoms, and can transmit the infection
• Reservoir: where the organism/pathogen lives & multiplies (e.g. humans, animals, soil, water)
• Source: site from which the organism/pathogen spreads to the host (e.g. soil, contaminated vegetables)

Infection and Disease

• Infection: damage or invasion of host tissues by organisms
• Occurs with primary/professional pathogens or opportunistic pathogens
• Can be life-threatening, but many are treatable with anti-infective agents
• Prevention is possible for many infectious diseases with vaccines
• Epidemics: increase in the number of cases of a disease above what is normally expected
• Pandemics: worldwide spread of a new disease

Pathogenicity

• Pathogenicity: ability of an organism to cause disease in a susceptible individual
• Pathogens may be primary/professional or opportunistic
• Virulence: degree of damage caused by a microbe to its host
• Multiple virulent factors are required for an organism to attach, persist, cause disease, and escape

Virulence Factors

• Attachment/adherence and persistence
• Involves specialized surface molecules/structures that correspond to receptors on human cells
• Secretory antibody, lactoferrin, and lysozyme produced by the host as a way to protect against infection
• Lactoferrin competes with bacteria for free iron
• Adhesions: fimbriae (pili), surface polysaccharides

Invasion and Evasion

• Invasion into or beyond the surface mucosal cells
• Ability to penetrate and grow in tissues
• May be localized or involve deep tissues
• Invading organisms should be able to adapt to the new environment
• Evasion of the immune system: phagocytosis evasion, phagocytic cell killing, inhibit activation of complement pathway, polysaccharide capsule/protein A/leukocidins