Chapter 14: Infectious Disease and Epidemiology

Symbiotic Relationships

Symbiosis refers to the close association between two different species. In microbiology, these relationships are crucial for understanding host-microbe interactions.

• Mutualism: Both organisms benefit. Example: Escherichia coli in the human gut synthesizes vitamin K for the host and receives nutrients.

• Commensalism: One organism benefits, the other is unaffected. Example: Skin microbiota living on human skin.

• Parasitism: One organism benefits at the expense of the other. Example: Pathogenic bacteria causing disease in humans.

Resident Microbiota

Resident microbiota are the microorganisms that colonize a host without causing disease under normal conditions.

• Acquisition: Acquired during birth and from the environment (e.g., contact with caregivers, food, air).

• Locations: Skin, mucous membranes, gastrointestinal tract, upper respiratory tract, and urogenital tract.

Opportunistic Pathogens

Opportunistic pathogens are normally harmless microbes that can cause disease when the host's defenses are compromised.

• Conditions allowing disease: Immune suppression, changes in normal microbiota, or introduction into unusual body sites.

• Example: Staphylococcus aureus causing infection in wounds.

Pathogen Reservoirs

Reservoirs are sites where pathogens are maintained as a source of infection.

• Animal reservoirs: Zoonoses (diseases transmitted from animals to humans).

• Human reservoirs: Carriers who may not show symptoms.

• Nonliving reservoirs: Soil, water, and food.

Koch’s Postulates

Koch’s postulates are a set of criteria to establish a causative relationship between a microbe and a disease.

1. The suspected pathogen must be present in every case of the disease.

2. The pathogen must be isolated and grown in pure culture.

3. The cultured pathogen must cause the disease when introduced into a healthy host.

4. The same pathogen must be re-isolated from the diseased experimental host.

Exceptions: Some pathogens cannot be cultured in the lab, and some diseases are caused by a combination of pathogens or by pathogens found in both sick and healthy individuals.

Portals of Entry and Exit

Pathogens enter and exit the host through specific routes called portals.

• Entry: Skin, mucous membranes (respiratory, gastrointestinal, urogenital tracts), and placenta.

• Exit: Often the same as entry; includes respiratory droplets, feces, urine, blood, and secretions.

Pathogenicity and Virulence

Pathogenicity is the ability of a microbe to cause disease. Virulence refers to the degree of pathogenicity.

• Factors increasing virulence: Toxins, adhesion factors, enzymes, and evasion of host immunity.

Exotoxins vs. Endotoxins

Both are toxins produced by bacteria, but they differ in structure, origin, and effects.

Stages of Infectious Disease

Infectious diseases progress through several stages:

1. Incubation period: Time between infection and first symptoms.

2. Prodromal period: Mild, nonspecific symptoms.

3. Illness: Most severe symptoms.

4. Decline: Symptoms subside as immune response overcomes pathogen.

5. Convalescence: Recovery and return to normal function.

Signs vs. Symptoms

• Signs: Objective, measurable indicators (e.g., fever, rash).

• Symptoms: Subjective experiences reported by the patient (e.g., pain, fatigue).

Methods of Disease Transmission

Diseases can be transmitted through various routes:

• Contact transmission: Direct (person-to-person), indirect (fomites), or droplet.

• Vehicle transmission: Airborne, waterborne, or foodborne.

• Vector transmission: Biological (e.g., mosquitoes) or mechanical (e.g., flies).

Chapter 11: Prokaryotes

Methods of Reproduction in Prokaryotes

Prokaryotes reproduce asexually by several methods:

• Binary fission: Most common; cell divides into two identical daughter cells.

• Snapping division: Inner cell wall layer forms a cross wall, then the outer wall snaps, releasing daughter cells.

• Budding: A small outgrowth (bud) forms and detaches as a new cell.

Cell Shapes and Arrangements

• Shapes: Coccus (spherical), bacillus (rod-shaped), spirillum (spiral), vibrio (comma-shaped), spirochete (flexible spiral).

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-), tetrads, sarcinae.

• Genus-specific arrangements: Staphylococcus (clusters), Streptococcus (chains).

Cyanobacteria and Chloroplasts

Cyanobacteria are photosynthetic bacteria important for oxygen production and nitrogen fixation.

• Characteristics: Contain chlorophyll a, perform oxygenic photosynthesis, some fix nitrogen in heterocysts.

• Importance: Major contributors to Earth's oxygen; ancestors of chloroplasts (endosymbiotic theory).

• Similarity to chloroplasts: Both have similar photosynthetic pigments and thylakoid membranes.

Selected Bacteria: Characteristics and Importance

• Cyanobacteria: See above.

• Lactobacillus: Gram-positive rods, important in fermentation (yogurt, cheese), part of normal microbiota.

• Streptomyces: Filamentous, soil-dwelling, produce antibiotics (e.g., streptomycin).

• Campylobacter: Gram-negative curved rods, cause gastroenteritis.

• Helicobacter pylori: Gram-negative spiral, colonizes stomach, causes ulcers.

Gram-Positive Bacteria: Firmicutes vs. Actinobacteria

Endospore Producers

• Genera: Bacillus and Clostridium are notable endospore producers.

• Significance: Endospores are highly resistant to heat, chemicals, and desiccation.

Tests for Mycobacterium tuberculosis/Leprae

• Acid-fast staining: Detects mycolic acid in cell walls.

• Tuberculin skin test (Mantoux test): Detects immune response to M. tuberculosis antigens.

• Culture and PCR: Confirmatory tests for diagnosis.

Staphylococci: Characteristics and Diseases

• Arrangement: Clusters (staphylo-).

• Skin colonizer: Staphylococcus epidermidis is common on skin.

• Diseases by S. aureus: Skin infections, food poisoning, toxic shock syndrome.

Streptococcus: Characteristics and Diseases

• Arrangement: Chains (strepto-).

• Diseases by S. pyogenes: Strep throat, scarlet fever, rheumatic fever, necrotizing fasciitis.

Proteobacteria and Their Classification

Proteobacteria are a major phylum of Gram-negative bacteria, classified into six groups based on genetic and metabolic differences.

• Alpha Proteobacteria: Rickettsia (Rocky Mountain spotted fever), Rhizobium (nitrogen fixation).

• Beta Proteobacteria: Neisseria (gonorrhea, meningitis), Bordetella (whooping cough).

• Gamma Proteobacteria: Most diverse; Vibrio cholerae (cholera), enterics (E. coli, Salmonella, Yersinia pestis).

• Delta Proteobacteria: Bdellovibrio (predatory life cycle).

• Epsilon Proteobacteria: Campylobacter, Helicobacter pylori.

Bacteroides vs. Enteric Bacteria

• Bacteroides: Anaerobic, Gram-negative rods, abundant in the human gut, generally non-pathogenic but can cause infections if displaced.

• Enteric bacteria (e.g., E. coli): Facultative anaerobes, Gram-negative rods, can be pathogenic or commensal, ferment lactose (some species).

Practice Problems: Exotoxins vs. Endotoxins

Practice Problems: Significance of Endospores

• Survival mechanism: Endospores allow bacteria to survive harsh conditions (heat, desiccation, chemicals).

• Transmission: Endospores can spread diseases such as tetanus, botulism, and anthrax.

• Longevity: Endospores can survive for decades or longer due to their protective coating and sporulation process.

• Sterilization challenge: Their resistance makes them difficult to eliminate in medical and food settings.

Example: Bacillus anthracis endospores can remain viable in soil for years, causing outbreaks when conditions allow.