PATHOGENS

Pathogens and Human Illness

Germ Theory

• Definition: Germ theory states that microscopic particles cause certain diseases.

• Overview: It proposes that microorganisms cause diseases. This concept was developed by Louis Pasteur and led to rapid advances in understanding disease.

• Historical Context:
    - B.C. 7000: Ancient societies believed in evil spirits causing disease, practicing trepanation (drilling holes in heads).
    - A.D. 1330-1352: During the Black Death, herbal treatments and incense were used to cure those affected, linked to bacteria transmitted by flea bites.
    - 1857: Pasteur hypothesized that disease is caused by small "animals" (microorganisms).
    - B.C. 460-B.C. 377: Greek physician Hippocrates proposed the humoral theory, suggesting that diseases are caused by imbalances in bodily fluids (humors).
    - 1400-1600: Anatomy studies gained traction, exemplified by anatomical drawings from the Middle East in 1555.
    - 1865: Joseph Lister introduced antiseptic techniques, noting that cleaning surgical tools reduces infections.
    - 1883: Robert Koch established Koch's postulates, providing four conditions to prove a pathogen causes a disease.
    - 1928: Sir Alexander Fleming discovered penicillin, marking the beginning of antibiotic use.
    - 2002: First cases of SARS, affecting the respiratory system, were recorded in China.
    - 1900: Cities began treating drinking water with chlorine, leading to a reduction in cholera incidents.
    - 1955: Jonas Salk developed the polio vaccine, leading to the disease's elimination in the U.S. by 1994.
    - 2005: There were renewed polio concerns as efforts to vaccinate increased, with the virus reemerging in the U.S. in fewer than ten cases.

Common Types of Pathogens

1. Bacteria
      - Description: Small unicellular organisms found in a variety of environments including soil, water, and humans. They can be killed by antibiotics except when resistant.
      - Examples/Diseases:
        - E. coli
        - Salmonella
        - Strep throat

2. Viruses
      - Description: Genetic material (DNA/RNA) wrapped in a protein coat, requiring host cells to reproduce. Vaccination can prevent many viral diseases.
      - Examples/Diseases:
        - Cold
        - Flu (e.g., H1N1)
        - Chickenpox

3. Fungi
      - Description: Organisms composed of one or more cells, having a cell wall; treated with antifungal drugs.
      - Examples/Diseases:
        - Athlete's foot

4. Parasites
      - Description: Organisms made of many cells that live in or on a host organism and derive nutrients from it. Some can be killed, while others allow only the symptoms to be managed.
      - Examples/Diseases:
        - Fleas
        - Ticks
        - Tapeworms

Bacteria Overview

• Characteristics of Bacteria
     - Type: Prokaryotes
     - Size: Much smaller than eukaryotic cells
     - Cell Structure:
       - Lack organelles
       - Free-floating circular DNA in the cytoplasm

Prokaryotic vs. Eukaryotic Cells

Classification of Bacteria

• By Shape/Structure:
    - Cocci (spherical)
    - Bacilli (rod-shaped)
    - Spirilla (spiral-shaped)

• By Oxygen Response:
    - Aerobic (requires oxygen)
    - Anaerobic (does not require oxygen)

• By Energy Source:
    - Autotrophs (produce own food)
    - Heterotrophs (consume food)

• By Gram Staining:
    - Gram-positive (thick cell wall)
    - Gram-negative (thin cell wall)

Benefits of Bacteria

1. Nitrogen Fixation:
      - Nitrogen-fixing bacteria convert nitrogen gas (N₂) into ammonia, which plants can utilize.
      - Example: Rhizobium bacteria living in root nodules.

2. Decomposition:
      - Bacteria help recycle carbon by breaking down organic wastes, returning CO₂ to the atmosphere.

3. Human Microbiome:
      - Adults carry approximately 1.2 kg of bacteria (about 10^14 cells and 400 species).
      - Functions of normal flora include:
        - Serving as a barrier against pathogenic bacteria.
        - Producing vitamins in the intestines.
        - Stimulating the immune system for enhanced defense against disease.

Mechanisms of Pathogenic Bacteria

• How Bacteria Cause Disease:
     1. Using Cells as Food: Bacteria consume healthy cells, damaging tissues.
     2. Releasing Toxins: Bacteria produce toxins that enter the bloodstream, which can disrupt normal bodily functions and damage tissues.
     - Examples: Staphylococcus aureus and E. coli.

Viruses

• Definition: Viruses consist of a nucleic acid core (DNA or RNA) and a protein coat (capsid).

Virus Structure

1. Core of Nucleic Acid:
      - DNA or RNA, with RNA being structurally similar to DNA.

2. Capsid:
      - Protein coating surrounding the nucleic acid.

Virus Replication Process

1. Attachment:
      - Virus binds to receptor molecules on the host cell surface.

2. Penetration:
      - Virus enters the host cell and releases its nucleic acid.

3. Synthesis:
      - Viral genes take over host mechanisms to replicate viral components.

4. Assembly:
      - Newly formed virus particles are assembled.

5. Release:
      - New virus particles exit the host cell, typically destroying it in the process.

Comparing Bacteria and Viruses

Antigenic Drift and Shift in Influenza Viruses

• Definitions:
    1. Antigenic Drift: Gradual changes in surface antigens caused by point mutations during viral replication, leading to seasonal variation in influenza.
    2. Antigenic Shift: Sudden changes due to gene exchange between different viral strains infecting the same host cell, causing dramatic changes and potential pandemics.

Mechanism of Changes:

• Drift: Caused by mutations that occur when the virus replicates, leading to gradual alteration of surface antigens.

• Shift: Occurs when two viruses simultaneously infect a cell, swapping genes, resulting in rapid changes in surface antigens and creating new influenza strains.