Viruses & Bacteria

Infectious Agents and Pathogens

Infectious agents are biological entities that can cause disease in living organisms. Pathogens are a subset of infectious agents that specifically cause disease.

• Disease: A condition that affects an entire body or some of its parts, caused by genetic, environmental, or pathogenic factors.

• Pathogens: Infectious agents that cause disease.

• Prions: Infectious proteins that cause proteins to misfold; considered non-living.

• Viruses: Simple infectious agents that have no activity outside of a host; also non-living.

• Bacteria: Single-celled prokaryotes.

• Protists: Single-celled eukaryotes.

• Fungi: Eukaryotes.

• Parasitic worms: Eukaryotes.

• Most infectious agents are not pathogenic to humans; very few archaea are known to be pathogenic.

Example

• Leprosy: Caused by a pathogen, but only certain people are susceptible.

Viruses

Viruses are extremely simple, tiny infectious agents that require a host cell to replicate. They consist of genetic material (DNA or RNA) surrounded by a protein coat.

• Smallest infectious agents: protein coat + genetic material (single or double-stranded DNA or RNA).

• 5-8% of human DNA consists of viral DNA remnants.

• No organelles; too small for complex cell structures, but may contain lipids or carbohydrates.

• Neither prokaryote nor eukaryote.

Viral Replication

Viruses replicate by hijacking the host cell's machinery to produce new viral particles (virions).

• Associated with diseases such as cold sores (herpes), blisters, and chicken pox.

• Forces host cell to assemble and release new virions.

• Some viruses are extremely simple, with as few as three genes.

Special Types of Viruses

Some viruses have unique properties or infect specific hosts.

• Bacteriophages: Infect specific bacteria.

• Viruses can be used as medicine to defeat bacterial infections.

• Retroviruses: Use reverse transcriptase to convert viral RNA into DNA, which is then inserted into the host's genome.

• Reverse transcriptase is important in biotechnology.

• Estimated 5-8% of human DNA is viral in origin, remnants of ancient infections.

Example

• HIV (antiretrovirals): A retrovirus that uses reverse transcriptase.

Pathogenic Viruses

Most viruses do not infect humans, but some are highly pathogenic.

• Examples: HIV, ZIKA, rabies, hepatitis, influenza (flu), warts, chicken pox, Ebola.

• Viruses are often specific to hosts and tissues due to interactions between proteins and carbohydrates on cell and virus surfaces.

Example

• Bacteriophage won't infect human cells.

• ACE2 receptor is used by coronaviruses to infect human cells.

Bacteria

Structure and Characteristics

Bacteria are single-celled prokaryotes with diverse metabolic capabilities and rapid reproduction rates.

• Consist of a rigid cell wall, plasma membrane, ribosomes, DNA, and sometimes cilia/flagella for movement.

• Obtain energy and maintain internal conditions through various metabolic pathways.

Bacterial Diversity

Bacteria exhibit diversity in energy sources and carbon acquisition.

• Can consume organic molecules, break down inorganic molecules, or perform photosynthesis.

• Most pathogenic bacteria consume organic molecules.

• Reproduce rapidly via asexual reproduction; under ideal conditions, can double every 20 minutes.

• In 11 hours, a single bacterium can produce over 8 billion progeny.

Genetic Diversity in Bacteria

Despite asexual reproduction, bacteria have mechanisms to increase genetic diversity.

• Contain small, circular DNA molecules called plasmids that can be exchanged between cells.

• Plasmids often carry genes for antibiotic resistance and other specialized functions.

• Genetic material can be exchanged via transformation, transduction, or conjugation.

• Viruses infecting bacteria may leave behind genetic material.

• Bacteria can take up DNA from the environment, especially under stress.

Helpful Bacteria

The vast majority of bacteria are not pathogenic and play essential roles in ecosystems and human health.

• 99.36% of bacteria are non-pathogenic.

• Roles include nitrogen fixation and bioremediation.

• Many bacteria live on or inside humans as part of the microbiota.

• Bacteria make up a large proportion of total cell count in the human body.

• Benefits include extracting otherwise inaccessible calories/nutrients and outcompeting harmful bacteria.

Pathogenic Bacteria

Some bacteria are pathogenic and can cause serious diseases in humans.

• Examples: tetanus, cholera, tuberculosis, UTIs, pneumonia, salmonella, anthrax, leprosy, strep throat.

• Pathogenic bacteria often produce destructive enzymes or toxins to aid in spreading infection.

Example

• Bacterial gangrene: Enzymes dissolve connections between muscle cells, then digest cells.

Antibiotics

Mechanism and Use

Antibiotics are substances that kill bacteria or inhibit their growth, targeting bacterial structures and processes.

• Work against cell wall synthesis (e.g., penicillin, vancomycin), disrupt DNA, or inhibit protein synthesis (e.g., tetracycline).

• Target bacterial ribosomes, which differ from eukaryotic ribosomes.

• Many antibiotics are broad-spectrum; some are specific to certain bacteria.

• Do not affect human cells due to differences in cell structure.

Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve mechanisms to survive exposure to antibiotics.

• Experts warn of a potential post-antibiotic era due to widespread resistance.

• Antibiotic-resistant staph infections (MRSA) are common in hospitals.

Misuse and Overuse of Antibiotics

Improper use of antibiotics contributes to resistance and reduces their effectiveness.

• Using antibiotics against pathogens they cannot treat (e.g., viral infections).

• Not completing the full course of antibiotics.

• Overuse in agriculture.

• Antibiotics are only effective against bacteria.

Summary Table: Infectious Agents

Key Equations and Concepts

• Bacterial Growth Rate:

• Where is the final number of bacteria, is the initial number, is time, and is the generation time.

• Antibiotic Mechanism:

Additional info: Some context and examples were expanded for clarity and completeness. The summary table was inferred and constructed to aid comparison of infectious agents. Equations for bacterial growth and antibiotic action were added for academic completeness.