Mechanisms of Infectious Disease

Terminology in Infectious Disease

Understanding key terms is essential for studying infectious diseases and their impact on hosts.

• Host: Any organism capable of supporting the nutritional and physical growth requirements of another organism.

• Infectious disease: The disease state brought about by interaction with another organism.

• Colonization: The presence and multiplication of a living organism on or within the host.

• Microflora: Bacteria inhabiting exposed surfaces of the body; also known as the normal microbiome.

• Virulence: The disease-inducing potential of a microorganism.

• Pathogens: Microorganisms so virulent that they are rarely found in the absence of disease.

• Saprophytes: Free-living organisms obtaining their growth from dead or decaying organic material from the environment.

Types of Host-Microbe Interactions

Microorganisms interact with hosts in various ways, influencing health and disease.

• Mutualism: Both the microorganism and the host benefit (e.g., Lactobacillus in the gut).

• Commensalism: Colonizing bacteria acquire nutrients and shelter, but the host is not affected (e.g., Staphylococcus aureus on skin).

• Opportunistic pathogens: Some commensals can become pathogenic under certain conditions, such as immunocompromised states.

• Parasitic relationship: Only the infecting organism benefits; includes external parasites (lice, ticks, fleas) and internal parasites (tapeworms, protozoa).

Triad of Infectious Disease Model

The development of infectious disease depends on the interaction between three factors:

• Host

• Agent

• Environment

These factors form a triangle, each influencing the occurrence and severity of disease.

Agents of Infectious Disease

Types of Infectious Agents

Various biological entities can cause infectious diseases in humans.

• Prions: Misfolded proteins causing fatal neurodegenerative diseases.

• Viruses: Smallest pathogens, lacking organized cellular structure, consisting of a protein coat and nucleic acid core (DNA or RNA). Require host cells for replication.

• Bacteria: Living organisms with a single cell.

• Rickettsiaceae: Obligate intracellular bacteria spread by arthropod vectors (e.g., lice, ticks).

• Chlamydiaceae: Gram-negative obligate intracellular bacteria (e.g., cause STDs).

• Fungi: Eukaryotic microorganisms (e.g., yeasts).

• Parasites: Organisms living on a host to obtain nourishment (e.g., Giardia, tapeworms).

Viruses

Viruses are unique infectious agents with distinct properties.

• Smallest pathogens

• No organized cellular structure

• Protein coat surrounding nucleic acid core (DNA or RNA)

• Incapable of replication outside a living cell

Classification and Categories of Bacteria

Classification of Bacteria

Bacteria are classified based on microscopic appearance and cell staining.

• Gram-positive organisms: Stained purple by crystal violet; e.g., MRSA (Methicillin-Resistant Staphylococcus aureus).

• Gram-negative organisms: Not stained by crystal violet, but counterstained red by safranin; e.g., Streptococcus pneumoniae, Staphylococcus aureus.

Note: Staphylococcus aureus is commonly found on skin and mucous membranes but can cause serious infections if it enters the bloodstream.

Categories of Bacteria

• Spirochetes: Spiral-shaped bacteria with unique movement (e.g., syphilis, Lyme disease).

• Mycoplasmas: Smallest bacteria, lack cell wall, resistant to antibiotics targeting cell wall synthesis (e.g., Mycoplasma pneumoniae).

Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, Coxiella

These organisms share features of both bacteria and viruses.

• Obligate intracellular pathogens

• Rigid peptidoglycan cell wall

• Asexual reproduction by cellular division

• Contain both RNA and DNA

Epidemiology and Disease Incidence

Epidemiology Terminology

Epidemiology studies the transmission and distribution of infectious diseases.

• Incidence: Number of new cases in a defined population.

• Prevalence: Number of active cases at any given time.

Incidence of Disease

• Endemic disease: Found in a particular geographic region; incidence and prevalence are stable.

• Epidemic: Abrupt and unexpected increase in disease incidence over endemic rates.

• Pandemic: Spread of disease beyond continental boundaries.

Transmission and Sources of Infection

Modes of Transmission

Pathogens can be transmitted through various routes:

• Penetration: Direct physical contact or indirect transmission via contaminated objects.

• Direct contact: Person-to-person transfer via touching, kissing, sexual contact, or body fluids.

• Indirect (fomites): Non-living objects that harbor and transmit infectious agents (e.g., touching a contaminated surface then one's mouth).

• Ingestion: Entry via contaminated food or water.

• Inhalation: Airborne transmission via droplets or particles.

Source of Infectious Disease

The reservoir is the source from which an infectious agent is acquired.

• Location: Nosocomial (hospital-acquired) or community-acquired (outside healthcare facilities).

• Host: May be endogenous (from host's normal flora) or exogenous (from environment or other individuals).

Clinical Features and Disease Progression

Symptomatology

Symptoms of infectious diseases can be classified as:

• Specific: Reflect the site of infection (e.g., diarrhea, rash, pneumonia).

• Nonspecific: Shared by diverse diseases (e.g., fever, headache).

• Obvious: Predictable patterns (e.g., chickenpox, measles).

• Covert: Require laboratory testing (e.g., hepatitis, increased WBC count).

Disease Course in Infection

Infectious diseases typically progress through several stages:

• Incubation period: Time from exposure to first appearance of symptoms.

• Prodromal stage: Early, non-specific symptoms as the body responds.

• Acute stage: Severe, infection-specific symptoms/signs.

• Convalescent stage: Recovery phase.

• Resolution stage: Return to pre-illness state.

Factors Influencing Site of Infection

• Type of pathogen

• Portal of entry

• Competence of host's immune system

Virulence Factors

Bacterial Toxins

Bacterial toxins contribute to virulence and disease severity.

• Exotoxins: Proteins secreted by bacteria, act locally or at a distance, trigger inflammation and fever.

• Endotoxins: Part of the cell wall of gram-negative bacteria, released upon cell death (lysis), can cause inflammation and septic shock.

Septic Shock: Life-threatening condition from widespread inflammation and organ failure due to large amounts of endotoxins.

Diagnosis and Laboratory Techniques

Criteria for Diagnosis

• Recovery of a probable pathogen or evidence from infected sites

• Accurate documentation of clinical signs and symptoms compatible with infection

Laboratory Diagnosis Techniques

• Culture and sensitivity: Isolation and identification of microorganisms; susceptibility testing.

• Serology: Detection of antibodies or antigens from host immune response.

• DNA and RNA sequencing: Analysis of genetic material for identification and epidemiological tracking.

Treatment and Intervention

Types of Antimicrobial Agents

• Antibacterial agents: Classified as bactericidal (lethal damage) or bacteriostatic (inhibitory, reversible effects).

• Antiviral agents

• Antifungal agents

• Antiparasitic agents

Antibiotic Mechanisms

• Interference with bacterial cell wall synthesis

• Inhibition of bacterial protein synthesis

• Interruption of bacterial nucleic acid synthesis

• Interference with normal bacterial metabolism

Immunotherapy

• Supplementing or stimulating host immune response to limit or reverse pathogen spread

• Infusion of pathogen-specific antibodies to enhance neutralization, phagocytosis, and clearance

Nonpharmacological Interventions

• Surgical interventions to access infected sites (e.g., abscess drainage)

• Cleaning of infected sites (debridement)

• Removal of infected organs or tissue (e.g., appendectomy)

• Surgical therapy often used alongside antibiotics in severe infections

Summary Table: Key Agents of Infectious Disease

Key Equations

• Incidence Rate:

• Prevalence Rate:

Additional info: These notes expand upon the original slides and text, providing definitions, examples, and context for each concept. The summary table and equations are included for exam preparation and conceptual clarity.