Viruses, Microbial Growth, and Principles of Infectious Disease: Study Notes

Viruses and Viral Variation

Influenza Virus Structure and Classification

The influenza virus is classified based on two major surface proteins: Hemagglutinin (HA) and Neuraminidase (NA). These proteins are critical for viral entry and release from host cells.

• Hemagglutinin (HA): 18 known subtypes (H1–H18)

• Neuraminidase (NA): 11 known subtypes (N1–N11)

• Influenza A Naming System: Based on HA and NA combinations (e.g., H1N1, H3N2, H5N1)

• Note: Only Influenza A uses the H and N subtype system; Influenza B and C do not.

Antigenic Drift and Shift

Variation in HA and NA leads to two main types of genetic changes in influenza viruses:

• Antigenic drift: Small, gradual mutations over time; responsible for seasonal flu changes.

• Antigenic shift: Major genetic changes due to reassortment; produces new virus strains and can cause pandemics.

Impact of Viral Mutations

• Genotype: The virus’s genetic makeup.

• Phenotype: Observable traits such as infectivity or virulence.

• Major mutations (especially antigenic shift) are most likely to cause pandemics.

Antiviral Medications and Treatments

• Nucleoside Analogs: Block viral replication by mimicking normal nucleotides, causing premature termination of nucleic acid synthesis.

  • Ribavirin: Targets RNA polymerase; treats RSV and Hepatitis C.

  • Acyclovir: Inhibits DNA replication; effective against HHV-1, HHV-2, and Varicella-zoster virus.

• Nucleoside Reverse Transcriptase Inhibitors (NRTIs): Example: Azidothymidine (AZT); inhibits reverse transcriptase, used in HIV treatment.

• Interferons: Naturally produced proteins that signal neighboring cells to induce antiviral defenses; can be used as antiviral therapy.

Vaccination and Antiviral Limitations

• Few effective antiviral drugs exist.

• Vaccination is the most effective prevention method; trains the immune system to recognize viruses.

• Antibiotics do not treat viral infections.

Microbial Growth and Control

Microbial Growth and Binary Fission

Microbial growth refers to an increase in cell number, not cell size. Most bacteria reproduce by binary fission, a process where one cell divides into two identical daughter cells.

Microbial Growth Curve

Bacterial populations typically follow a four-phase growth curve in batch culture:

• Lag phase: Cells adjust to the environment; no division occurs.

• Log (exponential) phase: Rapid cell division; highest metabolic activity.

• Stationary phase: Growth rate equals death rate; nutrients become limited.

• Death phase: Cells die due to nutrient depletion and waste accumulation.

Environmental Growth Requirements

Microbial growth depends on environmental factors such as temperature, oxygen, salt, pressure, and nutrients.

Temperature Classifications

• Psychrophiles: Cold-loving; e.g., Arctic bacteria.

• Mesophiles: Moderate temperature; most pathogens (e.g., Escherichia coli).

• Thermophiles: Heat-loving; e.g., hot springs bacteria.

• Extreme thermophiles (Hyperthermophiles): Extremely high temperatures.

• Barophiles: Pressure-loving microbes; e.g., deep ocean microbes.

Salt Classifications

• Halophiles: Require salt; e.g., Halobacterium.

• Facultative halophiles: Tolerate salt but do not require it; e.g., Staphylococcus aureus.

Nutritional Classification of Microbes

Microbes are classified based on their energy and carbon sources:

Energy Source	Carbon Source	Type	Examples
Light	CO2	Photoautotroph	Cyanobacteria, higher plants, algae
Light	Organic compounds	Photoheterotroph	Purple and green bacteria
Chemical	CO2	Chemoautotroph	Nitrifying bacteria, sulfur bacteria
Chemical	Organic compounds	Chemoheterotroph	E. coli, humans, most pathogens

Chemoheterotrophs are the most medically important group, including most human pathogens.

Oxygen Requirements

• Obligate aerobe: Requires oxygen (e.g., Mycobacterium tuberculosis).

• Obligate anaerobe: Killed by oxygen (e.g., Clostridium botulinum).

• Facultative anaerobe: Can grow with or without oxygen (e.g., E. coli).

• Aerotolerant anaerobe: Tolerates oxygen but does not use it.

• Microaerophile: Requires small amounts of oxygen (e.g., Helicobacter pylori).

Heat Control Methods and Standards

• Autoclave (Sterilization): 121°C, 15 psi, 15–20 minutes; kills endospores; used for medical instruments and lab equipment.

• Boiling: 100°C, 10 minutes; kills most microbes but not endospores; disinfects but does not sterilize.

• Pasteurization: Flash: 72°C for 15 seconds; Ultra-high temperature: 140°C for 1–3 seconds; used for milk.

• Dry Heat Sterilization: 160–170°C, 2 hours; sterilizes glassware.

Control strategies: Decontamination, sterilization, disinfection, antiseptic, microbiostatic/microbiocidal methods; physical (heat, filtration, radiation) and chemical controls; germicide levels for critical, semicritical, and noncritical items; target-specific control (e.g., endospores, Clostridioides difficile).

Principles of Infectious Disease and Epidemiology

Pathogen Categories

• Bacteria

• Viruses

• Fungi

• Protozoa

• Helminths

• Prions

Opportunistic vs. True Pathogens

• True pathogen: Causes disease in healthy hosts.

• Opportunistic pathogen: Causes disease when host defenses are compromised or normal microbiota are disrupted.

Host–Microbe Relationships

• Mutualism: Both organisms benefit (e.g., gut bacteria and humans).

• Commensalism: One benefits, other unaffected.

• Parasitism: One benefits, other harmed (e.g., tapeworm).

• Amensalism: One harmed, other unaffected (e.g., Penicillium mold killing bacteria).

Normal microbiota support health by competing with pathogens, producing vitamins, and educating the immune system.

Normal Microbiota: Typical Sites

• Skin (especially moist/oily areas)

• Upper respiratory tract (nasal passages, oropharynx)

• Oral cavity (teeth, tongue, saliva)

• Gastrointestinal tract (especially colon)

• Urogenital tract: vagina (often Lactobacillus-dominant), distal urethra

• Sterile sites: blood, CSF, internal organs, most of urinary tract above distal urethra, lower respiratory tract

Reservoirs of Infection

• Animal reservoirs: e.g., rabies virus

• Human reservoirs: e.g., asymptomatic carriers

• Nonliving reservoirs: e.g., soil, water

Zoonoses

Disease transmitted from animals to humans. Transmission routes include direct contact, animal waste, eating animals, and arthropods.

Sources of Infection

• Exogenous sources: Outside the body (e.g., contaminated food)

• Endogenous sources: From normal microbiota (e.g., surgical infections)

Infection and Disease Progression

• Contamination: Microbe present

• Infection: Microbe grows

• Disease: Damage occurs

Portals of Entry

• Skin

• Upper respiratory tract

• Digestive tract

• Placenta

• Parenteral route

Key Definitions and Standards to Memorize

• Chemoheterotroph

• Photoautotroph

• Halophile

• Facultative halophile

• Barophile

• Obligate aerobe

• Obligate anaerobe

• Reservoir

• Zoonosis

• Mutualism

• Commensalism

• Parasitism

• Amensalism

Standards:

• Autoclave: 121°C, 15 psi, 15–20 min

• Pasteurization: 72°C, 15 sec

• Boiling: 100°C, 10 min

• Dry heat: 160–170°C, 2 hr