Microbiology Study Guide: Viruses, Skin Infections, and Digestive System Infections

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 to reproduce. Therefore, they are classified as nonliving entities.

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

Pathogenicity and Viral Life Cycle

• 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 biotechnology (e.g., phage therapy) and in regulating bacterial populations in nature.

Viral Evolution and Infection Types

• Antigenic Shift vs. Antigenic Drift:

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

  • Antigenic Shift: Major genetic reassortment, often when two different viruses infect the same cell, resulting in new viral strains. Can lead to pandemics.

• Types of Infections:

  • Acute: Rapid onset, short duration (e.g., influenza).

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

  • Latent: Virus remains dormant in host cells, can reactivate (e.g., herpes simplex).

  • Persistent: General term for infections lasting long periods, including chronic and latent types.

• Modes of Transmission: Viruses can be transmitted via respiratory droplets, bodily fluids, contaminated food/water, vectors (e.g., mosquitoes), or direct contact.

Example: Influenza virus undergoes antigenic drift annually, requiring updated vaccines.

Ch. 17 – Skin Infections

Anatomy and Microbiome of the Skin

The skin is the largest organ of the body, serving as a barrier and hosting a diverse microbiome that protects against pathogens.

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

• Normal Microbiome: Resident microbes (e.g., Staphylococcus epidermidis) occupy the skin, preventing colonization by harmful organisms.

Portals of Entry and Exit

• Entry: Pathogens can enter through cuts, abrasions, hair follicles, or insect bites.

• Exit: Pathogens may exit via skin lesions, secretions, or shedding skin cells.

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.

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; rubella is dangerous in pregnancy; roseola affects young children.

Bacterial Skin Pathogens

Bacterium	Features
Staphylococcus aureus	Gram-positive cocci; causes boils, impetigo, MRSA infections.
Pseudomonas aeruginosa	Gram-negative rod; associated with wound and burn infections, resistant to many antibiotics.
Clostridium perfringens (C-diff)	Gram-positive, spore-forming rod; causes gas gangrene in skin, and C-diff causes colitis in the gut.

Example: Staphylococcus aureus can cause folliculitis, leading to pustular lesions.

Ch. 19 – Digestive System Infections

Digestive System and Microbiota

The digestive system processes food, absorbs nutrients, and houses a complex microbiota that contributes to health, including vitamin production.

• Role in the Body: Digestion, absorption, and immune defense.

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

Viral Hepatitis

Type	Transmission	Features
Hepatitis A	Fecal-oral	Acute, self-limiting, no chronic infection
Hepatitis B	Blood, sexual	Can become chronic, risk of liver cancer
Hepatitis C	Blood	Often chronic, risk of cirrhosis and cancer
Hepatitis D	Blood, with HBV	Requires HBV coinfection, worsens disease
Hepatitis E	Fecal-oral	Acute, dangerous in pregnancy

Norovirus

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

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

• Impact: Causes vomiting, diarrhea, and dehydration; common in outbreaks (e.g., cruise ships).

Foodborne Infection vs. Food Poisoning

• Foodborne Infection: Illness caused by ingesting pathogens that grow in the host (e.g., Salmonella).

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

Pathogenic Bacteria in the Digestive Tract

Bacterium	Associated Disease
Escherichia coli (E. coli)	Diarrhea, hemolytic uremic syndrome
Helicobacter pylori (H. pylori)	Peptic ulcers, gastritis
Staphylococcus aureus	Food poisoning (enterotoxin)
Clostridioides difficile (C-diff)	Antibiotic-associated colitis
Clostridium perfringens	Food poisoning, gas gangrene
Vibrio cholerae	Cholera (severe watery diarrhea)

Example: H. pylori infection is a major cause of peptic ulcers.

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 for self-assessment but not graded.