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

Viruses

Classification and Characteristics of Viruses

Viruses are unique infectious agents that differ fundamentally from cellular life forms. Understanding their classification and properties is essential for microbiology students.

• Nonliving Microbes: Viruses lack cellular structure, metabolism, and independent reproduction, so they are considered nonliving.

• Comparison to Cells: Unlike prokaryotic and eukaryotic cells, viruses do not possess organelles, cytoplasm, or a cell membrane. They require a host cell to replicate.

• Bacteriophage: A bacteriophage is a virus that infects bacteria. They are beneficial in research and medicine, such as in phage therapy to treat bacterial infections.

• Antigenic Shift and Drift:

  • Antigenic drift: Small, gradual mutations in viral genes, often seen in influenza viruses, leading to new strains.

  • Antigenic shift: Major genetic changes due to reassortment of viral genomes, resulting in new viral subtypes.

• Pathogenicity: The ability of a microbe to cause disease. Viruses can cause acute, chronic, latent, or persistent infections.

• Modes of Transmission: Viruses can be transmitted via respiratory droplets, direct contact, contaminated surfaces, blood, or vectors.

Example: Influenza virus undergoes antigenic drift and shift, leading to seasonal epidemics and occasional pandemics.

Five General Tasks of Pathogens

To cause disease, a pathogen must:

1. Enter the host

2. Adhere to host tissues

3. Evade host defenses

4. Obtain nutrients

5. Exit the host to spread

Additional info: These steps are often referred to as the "pathogenesis process."

Skin Infections

Anatomy and Microbiome of the Skin

The skin is the body's largest organ and serves as a barrier against pathogens. Its microbiome plays a crucial role in health and disease.

• Anatomy: The skin consists of the epidermis, dermis, and subcutaneous layer.

• Microbiome: Normal skin microbiota are located primarily on the epidermis and hair follicles, providing protection against pathogenic microbes.

Portals of Entry and Exit

• Entry: Skin can be breached by cuts, abrasions, insect bites, or medical procedures.

• Exit: Pathogens may exit via pus, blood, or skin shedding.

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 with fluid or semi-solid material.

• Vesicular rash: Rash with vesicles.

• Papular rash: Rash with papules.

• Maculopapular rash: Rash with both macules and papules.

• Folliculitis: Inflammation of hair follicles.

Primary lesions are the initial manifestation; secondary lesions result from changes in primary lesions.

Common Skin Diseases

• Chickenpox (Varicella): Caused by Varicella-zoster virus; characterized by vesicular rash.

• Shingles (Herpes zoster): Reactivation of Varicella-zoster virus; causes painful, localized rash.

• Measles: Caused by Measles virus; presents with maculopapular rash.

• Roseola: Caused by Human herpesvirus 6; high fever followed by rash.

• Rubella: Caused by Rubella virus; mild fever and rash.

Common Bacterial Skin Pathogens

Bacterium	Features	Associated Diseases
Staphylococcus aureus	Gram-positive cocci; often antibiotic-resistant	Folliculitis, boils, impetigo
Pseudomonas aeruginosa	Gram-negative rod; opportunistic pathogen	Wound infections, burns
Clostridium perfringens (C-diff)	Gram-positive, spore-forming anaerobe	Gas gangrene, skin necrosis

Additional info: C-diff is more commonly associated with digestive infections, but Clostridium perfringens can cause skin infections.

Digestive System Infections

Role and Microbiota of the Digestive System

The digestive system is responsible for nutrient absorption and waste elimination. Its microbiota contributes to health by producing vitamins and protecting against pathogens.

• Vitamin Production: Normal microbiota synthesize vitamins such as vitamin K and some B vitamins.

Common Digestive Pathogens

Bacterium	Features	Associated Diseases
Escherichia coli (E-coli)	Gram-negative rod; some strains pathogenic	Diarrhea, gastroenteritis
Helicobacter pylori (H-pylori)	Gram-negative spiral; survives stomach acid	Peptic ulcers, gastritis
Staphylococcus aureus (Staph-aureus)	Gram-positive cocci; produces enterotoxins	Food poisoning
Clostridioides difficile (C-diff)	Gram-positive, spore-forming anaerobe	Antibiotic-associated diarrhea
Clostridium perfringens	Gram-positive, spore-forming anaerobe	Food poisoning, gas gangrene
Vibrio cholerae	Gram-negative, comma-shaped	Cholera (severe watery diarrhea)

Hepatitis Viruses

• Hepatitis A: Fecal-oral transmission; acute infection.

• Hepatitis B: Bloodborne; can cause chronic infection.

• Hepatitis C: Bloodborne; often leads to chronic infection.

• Hepatitis D: Requires Hepatitis B for infection.

• Hepatitis E: Fecal-oral transmission; acute infection.

Norovirus

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

• Transmission: Spread via contaminated food, water, or surfaces.

• Impact: Causes vomiting, diarrhea, and abdominal pain.

Foodborne Infection vs. Food Poisoning

• Foodborne Infection: Caused by ingestion of pathogens that multiply in the digestive tract (e.g., Salmonella).

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

Example: Eating contaminated chicken may cause foodborne infection, while eating food left out too long may cause food poisoning.

Additional info: The distinction is important for diagnosis and treatment.