Ch. 6 – Viruses and Prions

Introduction to Viruses

Viruses are unique infectious agents that differ fundamentally from prokaryotic and eukaryotic cells. Understanding their structure, classification, and replication is essential for microbiology students.

• Viruses as Nonliving Microbes: Viruses lack cellular structure, do not carry out metabolism independently, and require a host cell for replication. They are considered obligate intracellular parasites.

• Comparison to Cells: Unlike prokaryotic and eukaryotic cells, viruses do not have organelles, cytoplasm, or a plasma membrane. They consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid), and sometimes a lipid envelope.

• Five General Tasks of Pathogens: To cause disease, a pathogen must (1) enter the host, (2) adhere to host tissues, (3) invade and evade host defenses, (4) obtain nutrients, and (5) exit the host to infect new hosts.

• Bacteriophages: These are viruses that infect bacteria. They are beneficial in molecular biology (e.g., cloning vectors) and can be used in phage therapy to target antibiotic-resistant bacteria.

• Antigenic Shift vs. Antigenic Drift:

  • Antigenic Drift: Minor genetic mutations in viral genes, leading to small changes in surface proteins. Common in influenza viruses, causing seasonal epidemics.

  • Antigenic Shift: Major genetic reassortment, resulting in new viral subtypes. Can lead to pandemics due to lack of population immunity.

• Pathogenicity and Types of Infections:

  • Pathogenicity: The ability of a microbe to cause disease.

  • Chronic Infection: Persistent infection with continuous viral production (e.g., hepatitis B).

  • Latent Infection: Virus remains dormant in host cells and can reactivate (e.g., herpesviruses).

  • Persistent Infection: Virus is not cleared and remains in the body for long periods.

• Modes of Transmission: Viruses can be transmitted via respiratory droplets, fecal-oral route, direct contact, vectors (e.g., mosquitoes), or contaminated surfaces.

Example:

Influenza virus undergoes antigenic drift annually, requiring updates to the flu vaccine.

Ch. 17 – Skin Infections

Anatomy and Microbiome of the Skin

The skin is the largest organ of the body and serves as a primary barrier to infection. Its structure and resident microbiota play crucial roles in defense against pathogens.

• Skin Anatomy: Composed of the epidermis (outer layer), dermis (middle layer), and hypodermis (deepest layer).

• Normal Microbiome: The skin hosts diverse microbes, including Staphylococcus epidermidis and Propionibacterium acnes, which protect against pathogens by competing for resources and producing antimicrobial substances.

• Portals of Entry and Exit: Pathogens can enter through cuts, abrasions, hair follicles, or insect bites. Exit can occur via shedding of skin cells or secretions.

Skin Lesions and Rashes: Terminology

• Vesicle: Small, fluid-filled blister.

• Macule: Flat, discolored spot.

• Papule: Raised, solid lesion.

• Pustule: Raised lesion filled with pus.

• Cyst: Closed sac containing fluid or semisolid material.

• Vesicular Rash: Rash with vesicles.

• Papular Rash: Rash with papules.

• Maculopapular Rash: Rash with both macules and papules.

• Folliculitis: Inflammation of hair follicles, often presenting as pustules.

Primary lesions are the initial manifestation; secondary lesions result from modification of primary lesions (e.g., scratching, infection).

Common Skin Infections

• Chickenpox vs. Shingles: Both caused by Varicella-zoster virus. Chickenpox is the primary infection; shingles is reactivation later in life.

• Measles, Roseola, Rubella: Viral diseases with distinct rashes and complications. Measles is highly contagious and can cause severe complications; rubella is dangerous in pregnancy; roseola typically affects infants.

Key Bacterial Pathogens

Note: Clostridium difficile (C-diff) is primarily associated with digestive tract infections.

Ch. 19 – Digestive System Infections

Digestive System and Microbiota

The digestive system processes food, absorbs nutrients, and eliminates waste. Its normal microbiota aid in digestion and vitamin production.

• Role of Digestive System: Breaks down food, absorbs nutrients, and protects against pathogens.

• Normal Microbiota: Bacteria such as Lactobacillus and Bifidobacterium synthesize vitamins (e.g., vitamin K, B vitamins).

Viral Hepatitis

Norovirus

• Etiology: Norovirus is a highly contagious RNA virus causing acute gastroenteritis.

• Transmission: Fecal-oral route, contaminated food/water, person-to-person contact.

• Impact: Causes vomiting, diarrhea, and abdominal pain; significant cause of outbreaks in closed settings (e.g., cruise ships).

Foodborne Infection vs. Food Poisoning

• Foodborne Infection: Illness caused by ingestion of food containing pathogenic microbes that colonize the gut (e.g., Salmonella).

• Food Poisoning (Intoxication): Illness caused by ingestion of toxins produced by microbes in food (e.g., Staphylococcus aureus enterotoxin).

Common Pathogenic Bacteria in the Digestive Tract

Example:

Helicobacter pylori infection is a major cause of peptic ulcers and can be diagnosed by urea breath test.

Additional info:

• Students should focus on Section 1 of each chapter and the learning outcomes for exam preparation.

• Practice questions at the end of chapters are useful but not all are relevant to the exam.

• Active recall and group study are recommended strategies.