Microbiology Study Notes: Viruses, Pathogenicity, Skin Defenses, and Disease Transmission

Ch. 6 Viruses

Viruses as Nonliving Microbes

Viruses are unique infectious agents that are classified as nonliving because they lack the cellular machinery necessary for independent life.

• Definition: Viruses are acellular entities that require a host cell to replicate.

• Key Features:

  • No cells (acellular)

  • Cannot metabolize or grow independently

  • Require a host for replication

Comparison: Viruses, Prokaryotes, and Eukaryotes

Viruses differ fundamentally from prokaryotic and eukaryotic cells in structure and function.

Characteristic	Viruses	Prokaryotes	Eukaryotes
Cells?	No	Yes	Yes
Considered alive?	No	Yes	Yes
Cellular structure	No cell structure; protein coat (capsid) and genetic material	Cell wall, plasma membrane, cytoplasm, ribosomes	Plasma membrane, cytoplasm, membrane-bound organelles
Ribosomes?	No	Yes	Yes
Reproduction	Only inside host cell	Binary fission	Mitosis/meiosis
Genetic material	DNA or RNA	DNA	DNA

Pathogen Life Cycle: Five General Tasks

To cause disease, a pathogen must complete several key steps:

1. Pathogen must enter the host.

2. Pathogen must adhere to host tissues.

3. Pathogen must invade tissues and obtain nutrients.

4. Pathogen must evade host immune defenses to replicate.

5. Pathogen must be transmitted to a new host to repeat the cycle.

Bacteriophages: Definition and Benefits

Bacteriophages are viruses that infect bacteria. They are always named after the bacteria they infect.

• Ecological Role: Regulate bacterial populations, maintaining microbial balance.

• Genetic Evolution: Facilitate genetic exchange and diversity among bacteria.

• Medical Applications: Can be used to target antibiotic-resistant bacteria and prevent pathogenic bacteria from colonizing human tissues.

Antigenic Shift and Drift in Viruses

Influenza viruses can change their surface proteins, leading to new strains and affecting immunity.

• Antigenic Shift: Major genetic reassortment resulting in new, highly virulent influenza strains. Occurs when two different influenza viruses infect the same host cell and exchange genetic material.

• Antigenic Drift: Minor, frequent mutations in the viral genome, causing gradual changes in surface proteins (HA and NA spikes). Leads to seasonal flu variations.

Pathogenicity and Types of Infections

Pathogenicity

Pathogenicity is the ability of an organism to cause disease, determined by its genetic makeup and interaction with the host.

Chronic, Latent, and Persistent Infections

• Chronic Infections: Continuous release of viruses over time (months or years), with slow disease progression.

• Latent Infections: Periods of dormancy with intermittent flare-ups. Flare-ups can be triggered by stress, fever, or immune suppression.

• Persistent Infections: General term for infections that are not cleared by the immune system and remain in the host.

Examples: Herpesviridae family (HSV-1: cold sores, HSV-2: genital herpes, Varicella-zoster virus: chickenpox and shingles)

Oncogenic Viruses and Cancer

Oncogenic Viruses

Some viruses (oncoviruses) can cause cancer by integrating into the host genome and disrupting normal cell regulation.

• Oncogenic viruses cause 10-15% of human cancers.

• Mechanisms: Stimulate uncontrolled cell division, decrease cell death signals.

• Examples: Human papilloma viruses (HPVs), Human T-lymphotropic viruses (HTLV)

Table: Human Viruses Associated with Cancer

Virus	Genome	Integrates with Host Genome?	Cancer Link	Cancer-Causing Mechanism
Human papilloma virus (HPV)	DNA	Yes	Cervical, anal, oropharyngeal cancers	Viral genes disrupt cell cycle control
Epstein-Barr virus (EBV)	DNA	No	Nasopharyngeal carcinoma, Burkitt's lymphoma	Viral genes drive cell proliferation
Hepatitis B virus (HBV)	DNA	Yes	Liver cancer (hepatocellular carcinoma)	Chronic inflammation and integration
Hepatitis C virus (HCV)	RNA	No	Liver cancer	Chronic inflammation
Human T-lymphotropic virus (HTLV)	RNA (retrovirus)	Yes	Leukemia, lymphoma	Viral genes disrupt cell cycle

Transmission of Pathogens

Modes of Transmission

Pathogens can be transmitted by direct or indirect contact.

• Direct Contact: Person-to-person (touching, kissing, sexual contact), droplet transmission (coughing, sneezing), animal bites.

• Indirect Contact: Fomites (contaminated objects), airborne transmission, vector-borne (insects), contaminated food/water.

Skin Anatomy and Microbiome

Structure and Defenses of the Skin

The skin is the body's largest organ and serves as a primary barrier against infection.

• Layers: Epidermis (outer, keratinized), dermis (inner, living cells)

• Keratinocytes: Specialized cells producing keratin for protection

• Defenses: Tightly packed dead cells, antimicrobial peptides, acidic pH, sebum, sweat

Skin Microbiome

The skin hosts a diverse community of microbes that protect against pathogens and support immune function.

• Locations: Varies by body site (moist, dry, sebaceous areas)

• Functions: Competes with pathogens, modulates immune responses, produces antimicrobial substances

Factors Impacting Skin Microbiome

• Environmental exposure

• Occupation

• Antibiotic usage

• Age

• Cosmetics, moisturizers, soaps

• Clothing

• Hormone levels

Skin Defense System

• Melanin: Antimicrobial, protects against UV radiation

• Sun exposure: Facilitates vitamin D synthesis

• Perspiration: Acidic pH, salt inhibits microbes

• Lysozyme: Enzyme in sweat/tears that breaks down bacterial cell walls

• Sebum: Oily, moisturizes and protects

• Hand washing/bathing: Removes microbes

Portals of Entry in Humans

Pathogens enter the body through specific portals, including the skin, respiratory tract, gastrointestinal tract, urogenital tract, and conjunctiva.

Skin Lesions and Rashes: Terminology

• Vesicle/Vesicular rash: Small, fluid-filled elevated lesion (e.g., chickenpox)

• Macule: Flat, discolored area (e.g., freckles)

• Papule/Papular rash: Raised, solid lesion (not fluid-filled), less than 0.5 cm diameter

• Pustule: Raised lesion with pus (e.g., acne)

• Cyst: Closed fluid-filled sac under the skin

• Maculopapular rash: Slightly raised papules overlying a macule (e.g., rubella)

• Folliculitis: Swelling/redness around hair follicle due to infection

Disease Distinctions

• Chickenpox & Shingles: Both caused by Varicella-zoster virus; shingles is a reactivation of latent virus.

• Measles (Rubeola): Highly contagious, causes fever, cough, Koplik spots, and a spreading maculopapular rash.

• Roseola: Sudden high fever followed by rash, common in infants.

• Rubella: Mild fever and rash, dangerous in pregnancy due to risk of congenital rubella syndrome.