Infectious Agents and Pathogens: Viruses and Bacteria

Infectious Agents and Pathogens

Definition and Types of Pathogens

Pathogens are biological agents that cause disease in their hosts. Diseases can affect the entire body or specific parts and may be caused by genetic, environmental, or infectious factors.

• Pathogen: An infectious agent that causes disease.

• Types of Pathogens:

  • Prions: Infectious proteins that induce misfolding of normal proteins, leading to neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

  • Viruses: Non-cellular, extremely simple infectious agents that require a host cell to replicate.

  • Bacteria: Single-celled prokaryotes, some of which are pathogenic.

  • Protists: Single-celled eukaryotes, some pathogenic to humans.

  • Fungi: Eukaryotic organisms, with a small percentage pathogenic to humans.

  • Parasitic worms: Multicellular organisms, a minority of which are pathogenic.

  • Archaea: No known pathogenic archaea to date.

• Example: Bacillus anthracis (bacteria), HIV (virus), Plasmodium (protist), Candida albicans (fungus), tapeworms (parasitic worms).

Viruses

Structure and Classification

Viruses are acellular infectious agents composed of genetic material encased in a protein coat. They are not classified as prokaryotes or eukaryotes due to their lack of cellular structure and inability to carry out independent metabolism.

• Components:

  • Genetic Material: DNA or RNA (single or double stranded).

  • Protein Coat (Capsid): Protects the genetic material.

  • Envelope: Some viruses have a lipid envelope derived from the host cell membrane.

• Non-living Status: Viruses lack organelles and metabolic processes; they are considered non-living.

• Size Comparison: Viruses are much smaller than bacteria and eukaryotic cells.

• Example: Influenza virus, HIV, bacteriophage (virus that infects bacteria).

Special Types of Viruses

• Bacteriophages: Viruses that specifically infect bacteria.

• Retroviruses: Use reverse transcriptase to convert viral RNA into DNA, which can integrate into the host genome (e.g., HIV).

• Endogenous Viral DNA: Remnants of ancient viral infections are present in human DNA (estimated 5-8%).

• Biotechnology Application: Reverse transcriptase is widely used in molecular biology techniques.

Viral Replication

General Mechanism

Viruses cannot reproduce independently; they must infect a host cell and hijack its metabolic machinery to replicate.

• Attachment: Virus binds to specific receptors on the host cell surface.

• Entry: Viral genetic material enters the host cell.

• Replication: Host cell machinery is used to replicate viral genome and synthesize viral proteins.

• Assembly: New viral particles (virions) are assembled inside the host cell.

• Release: Virions exit the host cell, often destroying it in the process.

• Example: Cold sores (herpes virus), chicken pox (varicella-zoster virus).

Viral Genome Simplicity: Some viruses have as few as three genes.

Viral Host Specificity

• Viruses are often host-specific, infecting only certain species or cell types due to interactions between viral surface proteins and host cell receptors.

• Example: Bacteriophages infect bacteria, not human cells; ACE-2 receptor is targeted by coronavirus in human cells.

Table: Viruses vs. Organisms

Characteristic	Viruses	Organisms (Bacteria, Eukaryotes)
Cellular Structure	Non-cellular	Cellular (prokaryotic or eukaryotic)
Metabolism	No independent metabolism	Metabolically active
Reproduction	Requires host cell	Self-replicating
Genetic Material	DNA or RNA	DNA (in nucleus or nucleoid)
Size	Very small (20-300 nm)	Larger (bacteria: ~1-10 μm; eukaryotes: >10 μm)

Bacteria

Structure and Characteristics

Bacteria are single-celled prokaryotes with diverse metabolic capabilities. They are alive and capable of independent reproduction and metabolism.

• Cell Wall: Provides structural support and protection.

• Plasma Membrane: Regulates entry and exit of substances.

• Ribosomes: Site of protein synthesis; structurally different from eukaryotic ribosomes.

• Genetic Material: Circular DNA not enclosed in a nucleus.

• Motility Structures: Some have cilia or flagella for movement.

• Example: Escherichia coli, Streptococcus, Bacillus anthracis.

Bacterial Diversity

• Energy Sources: Bacteria may consume organic molecules, break down inorganic molecules, or perform photosynthesis.

• Reproduction: Asexual (binary fission); rapid under ideal conditions (e.g., one cell can produce billions in hours).

• Genetic Diversity:

  • Plasmids: Small, circular DNA molecules separate from the main chromosome; often carry genes for antibiotic resistance or other special functions.

  • Horizontal Gene Transfer: Exchange of genetic material via plasmids, uptake from environment, or transfer by viruses (transduction).

Helpful Bacteria

• Most bacteria are not pathogenic; many are beneficial (e.g., nitrogen fixation, bioremediation).

• Microbiota: Bacteria living on/in humans contribute to health, digestion, and immunity.

• Example: Gut bacteria aid in digestion and protect against harmful microbes.

Pathogenic Bacteria

• Some bacteria cause disease by releasing destructive enzymes or toxins.

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

• Pathogenicity can depend on bacterial strain and context.

Antibiotics

Mechanism and Use

Antibiotics are medicines that kill bacteria or inhibit their growth, targeting features unique to bacteria.

• Targets:

  • Cell wall synthesis (e.g., penicillin, vancomycin)

  • Bacterial ribosomes (e.g., tetracycline)

  • DNA replication (bacterial enzymes)

• Spectrum: Some antibiotics are broad-spectrum (effective against many types), others are narrow-spectrum.

• Example: Penicillin inhibits cell wall synthesis; tetracycline inhibits protein synthesis.

Antibiotic Resistance

Antibiotic resistance occurs when bacteria evolve mechanisms to survive exposure to antibiotics, posing a major public health threat.

• Causes:

  • Overuse and misuse of antibiotics (e.g., in agriculture, for viral infections)

  • Incomplete courses of antibiotics

  • Horizontal gene transfer (plasmids, transduction)

• Example: MRSA (methicillin-resistant Staphylococcus aureus) is resistant to many antibiotics.

Table: Antibiotic Targets in Bacteria vs. Human Cells

Target	Bacteria	Human Cells
Cell Wall	Present (peptidoglycan)	Absent
Ribosomes	70S (prokaryotic)	80S (eukaryotic)
DNA Location	Cytoplasm (nucleoid)	Nucleus

Key Equations and Concepts

• Bacterial Growth (Binary Fission):

$N = N_0 imes 2^n$ Where $N$ is the final number of bacteria, $N_0$ is the initial number, and $n$ is the number of generations.

• Antibiotic Resistance Spread: Facilitated by horizontal gene transfer mechanisms such as conjugation, transformation, and transduction.

Summary of Main Points

• Pathogens include prions, viruses, bacteria, protists, fungi, and parasitic worms.

• Viruses are non-living, require host cells for replication, and are host-specific due to surface protein interactions.

• Bacteria are living, reproduce rapidly, and can exchange genetic material to increase diversity and resistance.

• Most bacteria are harmless or beneficial; only a minority are pathogenic.

• Antibiotics target bacterial-specific structures and processes, but resistance is a growing problem due to misuse and genetic exchange.

Additional info: Some context and examples were inferred to clarify fragmented points and provide a self-contained study guide.