Viruses and Bacteria: Infectious Agents, Diversity, and Pathogenicity

Viruses & Bacteria

Infectious Agents and Pathogens

Infectious agents are biological entities that can cause disease in living organisms. Diseases may result from genetic, environmental, or pathogenic factors, and pathogens are the infectious agents responsible for disease.

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

• Pathogen: An infectious agent that causes disease.

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

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

• Bacteria: Single-celled prokaryotes.

• Protists: Single-celled eukaryotes.

• Fungi: Eukaryotes.

• Parasitic worms: Eukaryotes.

• Some infectious agents are pathogenic, but only a small percentage affect humans.

• No known pathogenic archaea.

Example: Leprosy is caused by a pathogen, but only certain people are susceptible.

Viruses

Viruses are extremely simple, tiny infectious agents. They consist of a protein coat and genetic material (either DNA or RNA, single or double-stranded). Viruses lack organelles and are neither prokaryotic nor eukaryotic.

• Viruses can be associated with diseases such as cold sores (herpes), blisters, and chicken pox.

• They force host cells to assemble new virus particles (virions).

• Some viruses have as few as three genes.

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

Special Types of Viruses

• Bacteriophages: Viruses that infect specific bacteria.

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

• Retroviruses: Special viruses that use reverse transcriptase to convert viral RNA into DNA, which is then inserted into the host's DNA.

• Reverse transcriptase is important in biotechnology.

• Examples: HIV (antiretrovirals target reverse transcriptase).

Pathogenic Viruses

• Most viruses do not infect humans. Examples of human-infecting viruses include HIV, ZIKA, rabies, hepatitis, influenza (flu), warts, chicken pox, and Ebola.

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

• Example: Bacteriophage will not infect human cells; ACE2 is a receptor for coronavirus.

Bacteria

General Characteristics

Bacteria are single-celled prokaryotes (distinct from archaea). They have a rigid cell wall, plasma membrane, ribosomes, DNA, and sometimes cilia or flagella for movement.

• Bacteria obtain energy and maintain internal conditions through various metabolic processes.

Bacterial Diversity

Bacteria exhibit diversity in energy sources and carbon acquisition. They may consume organic molecules, break down inorganic molecules, or perform photosynthesis.

• Most pathogenic bacteria consume organic molecules.

• Bacteria can reproduce rapidly via asexual reproduction.

• Under ideal conditions, bacteria can double every 20 minutes; in 11 hours, a single cell can produce over 8 billion progeny.

Genetic Diversity in Bacteria

Although bacteria reproduce asexually, they have mechanisms to introduce genetic diversity.

• Some bacteria contain small, circular DNA molecules called plasmids, separate from the main chromosome.

• Plasmids often carry genes used in unusual circumstances and can be exchanged between bacteria.

• Viruses can infect bacteria and leave behind genetic material.

• Bacteria can take up DNA from the environment, especially when stressed.

Helpful Bacteria

The vast majority of bacteria are not pathogenic. They play important roles in nitrogen fixation, bioremediation, and as part of the human microbiota.

• 99.36% of bacteria are not pathogenic.

• Bacteria live on and inside humans, making up a large proportion of total cell count (though much smaller in size).

• Benefits include:

  • Extracting otherwise inaccessible calories and nutrients.

  • Outcompeting harmful bacteria.

  • Changing host gene expression.

Pathogenic Bacteria

Some bacteria are pathogenic and can cause diseases by producing destructive enzymes or toxins, sometimes to help them spread through tissues.

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

• Pathogenic bacteria often produce enzymes or toxins that damage host tissues.

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

Summary Table: Types of Infectious Agents

Agent	Cell Type	Living/Non-living	Examples
Prion	Protein (no cell)	Non-living	Mad cow disease
Virus	None	Non-living	Influenza, HIV, bacteriophage
Bacteria	Prokaryote	Living	E. coli, Salmonella
Protist	Eukaryote	Living	Amoeba, Plasmodium
Fungus	Eukaryote	Living	Candida, Aspergillus
Parasitic worm	Eukaryote	Living	Tapeworm, roundworm

Key Equations

• Bacterial Growth Rate:

$N = N_0 imes 2^{(t/T)}$

• Where $N$ is the final number of bacteria, $N_0$ is the initial number, $t$ is time elapsed, and $T$ is the doubling time.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and equations have been inferred and formatted for study purposes.