Viruses, Bacteria, and Infectious Agents: Structure, Diversity, and Antibiotics

Viruses & Bacteria

Infectious Agents and Pathogens

Infectious agents, also known as pathogens, are biological entities that can cause disease in a host organism. Understanding their diversity and mechanisms is crucial in biology and medicine.

• Disease: A condition that affects an entire body or some of its parts. Causes can be genetic, environmental, pathogenic, or a combination.

• Pathogens: Infectious agents that cause disease.

• Prions: Infectious proteins that cause proteins to misfold. They are non-living and lack genetic material.

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

• Bacteria: Single-celled prokaryotes.

• Protists: Single-celled eukaryotes.

• Fungi: Eukaryotes that can be unicellular or multicellular.

• Parasitic worms: Eukaryotes that can cause disease.

• Most pathogens are specific to certain hosts and tissues.

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

Viruses

Viruses are extremely simple infectious agents, often consisting of only a protein coat and genetic material. They are not considered living organisms because they cannot reproduce or carry out metabolism outside a host cell.

• Consist of a protein coat (capsid) and genetic material (DNA or RNA, single or double-stranded).

• Do not have organelles; sometimes contain lipids or carbohydrates.

• Neither prokaryotic nor eukaryotic.

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

Viral Replication

• Viruses force host cells to make and assemble new virions (virus particles).

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

Special Types of Viruses

• Bacteriophages: Viruses that infect specific bacteria.

• Viruses as Medicine: Used to defeat bacterial infections (phage therapy).

• 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 (e.g., making cDNA from mRNA).

Example: HIV is a retrovirus; antiretroviral drugs target its replication.

Pathogenic Viruses

• Most viruses do not infect humans. Examples of human-infecting viruses: HIV, Zika, rabies, hepatitis, influenza, 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 will not infect human cells. The ACE2 protein is the receptor for some coronaviruses.

Bacteria

Bacteria are single-celled prokaryotes (distinct from archaea) with diverse metabolic capabilities and ecological roles.

• Have a rigid cell wall, plasma membrane, ribosomes, DNA, and sometimes cilia or flagella for movement.

• Obtain energy by consuming organic molecules, breaking down inorganic molecules, or photosynthesis.

• Maintain internal conditions (homeostasis).

Bacterial Diversity

Bacteria exhibit great diversity in energy sources and carbon acquisition.

• Can reproduce extremely rapidly via asexual reproduction (binary fission).

• In ideal conditions, a single bacterium can produce over 8 billion progeny in 11 hours (doubling every 20 minutes).

Genetic Diversity in Bacteria

Despite asexual reproduction, bacteria have several mechanisms to increase genetic diversity:

• Possess small, circular DNA molecules called plasmids, which can be exchanged between bacteria, spreading traits like antibiotic resistance.

• Plasmids are separate from the main chromosome and often carry genes used in unusual circumstances.

• Viruses that infect bacteria (bacteriophages) can leave behind genetic material.

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

Helpful Bacteria

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

• Only about 0.64% of bacteria are pathogenic.

• Roles include nitrogen fixation, bioremediation, and forming part of the human microbiota.

• Make up a large proportion of the total number of cells in the human body (though much smaller in size).

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

Pathogenic Bacteria

Some bacteria cause disease by producing toxins or destructive enzymes.

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

• Pathogenic bacteria can dissolve connections between muscle cells and digest cells (e.g., bacterial gangrene).

Antibiotics

Antibiotics are chemicals that kill bacteria or inhibit their growth. They are a cornerstone of modern medicine but must be used responsibly to prevent resistance.

• Work by targeting bacterial cell walls (e.g., penicillin), DNA, or ribosomes (e.g., tetracycline).

• Generally do not affect human cells due to differences between prokaryotic and eukaryotic cells.

• Some antibiotics are broad-spectrum; others are specific to certain bacteria.

• Antibiotic resistance is a growing problem, with some bacteria (e.g., MRSA) resistant to all common antibiotics.

Mechanisms of Antibiotic Resistance

• Bacteria may modify the antibiotic target, inactivate the drug, or bypass the affected pathway.

• Resistance genes can be spread via plasmids or horizontal gene transfer.

Antibiotic Use and Misuse

• Antibiotics are only effective against bacteria, not viruses.

• Misuse includes using antibiotics for viral infections, not completing prescribed courses, and overuse in agriculture.

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

Agent	Cell Type	Living?	Genetic Material	Example Disease
Virus	Neither prokaryote nor eukaryote	No	DNA or RNA	Influenza, HIV, chicken pox
Bacteria	Prokaryote	Yes	DNA	Tuberculosis, strep throat
Fungi	Eukaryote	Yes	DNA	Ringworm, athlete's foot
Prion	None	No	None	Mad cow disease

Additional info: The table above summarizes the main differences between infectious agents discussed in the notes.