Epidemiology and Microbial Mechanisms of Pathogenicity
Epidemiology: Definitions and Methodology
Definition of Epidemiology: The science that studies when and where diseases occur and how they are transmitted in populations.
Roles and Objectives of Epidemiologists:
* Determining the etiology (cause) of a disease.
* Identifying other critical factors concerning the spread of disease within a population.
* Developing methods and strategies for controlling a disease.
* Assembling data and generating graphs to outline the incidence of a disease.
Descriptive Epidemiology: This branch entails the collection of all data that describe the occurrence of the disease under study. This approach is historically associated with Snow.
* Retrospective Studies: Backtracks to identify the cause and the source of the disease after an outbreak has occurred.
* Prospective Studies: A disease-free population is selected, and their subsequent disease experiences are recorded over time.
Analytical Epidemiology: This involves the analysis of a particular disease to determine its probable cause. This approach is historically associated with Nightingale.
Experimental Epidemiology: This branch begins with a hypothesis about a specific disease, followed by experiments performed to test that hypothesis. This approach is historically associated with Semmelweis.
* Effectiveness of Drugs: An example involves treating one group of people with a drug and another with a placebo to compare outcomes.
* Clinical Trials: These involve a test group (receiving the treatment) and a control group (receiving a placebo or standard care).
Case Reporting: An important procedure used to "measure" the incidence of diseases and to establish the chain of transmission. It involves the formal report of specified diseases to local, state, and governmental agencies.
Microbial Mechanisms of Pathogenicity: Toxins
Toxins Overview: Toxins are poisonous substances produced by certain microorganisms. They are a primary factor in how bacterial pathogens damage host cells.
Exotoxins:
* Production: These are proteins produced inside pathogenic bacteria, most commonly gram-positive bacteria, as part of their growth and metabolism.
* Release: They are secreted into the surrounding medium during the log phase of bacterial growth or released following bacterial lysis.
* Chemistry: They are proteins, and many function as enzymes. The genes for the majority of exotoxins are carried by plasmids or phages.
* Lethality: Exotoxins are among the most lethal substances known. For example, of botulinum exotoxin is sufficient to kill guinea pigs.
* Mechanism: They destroy certain parts of host cells or inhibit specific metabolic functions. In the case of botulism, the disease is caused by the exotoxin itself, not the bacterial infection by Clostridium botulinum.
* Toxoids: These are inactivated exotoxins used for vaccination (e.g., diphtheria and tetanus vaccines).
* Naming Conventions: Often named based on the type of cell they attack (e.g., cardiotoxin) or the species that produces them (e.g., botulinum toxin).
Endotoxins:
* Source: These are the lipid portions of lipopolysaccharides (LPS) that are part of the outer membrane of the cell wall of gram-negative bacteria.
* Specific Component: The toxic element is specifically called Lipid A.
* Release: Endotoxins are liberated when the bacteria die and the cell wall lyses (breaks apart), or during bacterial multiplication.
* Nature of Symptoms: All endotoxins produce the same general symptoms regardless of the bacterial species: chills, fever, weakness, generalized aches, and in some cases, shock (septic shock) and death.
Types and Mechanisms of Exotoxins
1. A-B Toxins: * These consist of two polypeptide components: A and B.
* A Component: The active enzyme component that alters host cell function.
* B Component: The binding component that attaches the toxin to the host cell receptor. * Mechanism of Action:
1. The bacterium produces and releases the A-B toxin.
2. The B component attaches to a specific receptor on the host cell plasma membrane.
3. The plasma membrane invaginates (folds inward) where the toxin and receptor meet.
4. The toxin enters the cell via receptor-mediated endocytosis.
5. The toxin is enclosed in a pinched-off portion of the membrane (pinocytosis).
6. The A and B components separate.
7. The A component typically inhibits protein synthesis or otherwise alters cell function.
8. The B component is released from the cell, and the receptor is recycled back to the plasma membrane.
2. Membrane-Disrupting Toxins:
*These cause cell lysis by disrupting the host cell's plasma membrane.
* Methods of Lysis: Some form protein channels in the membrane (e.g., Staphylococcus aureus), while others disrupt the phospholipid structure of the membrane (e.g., Clostridium perfringens).
* Function: They help bacteria escape from phagosomes by destroying the phagosomal membrane.
* Leukocidins: Toxins that destroy phagocytic white blood cells (produced by staphylococci and streptococci).
* Hemolysins: Toxins that destroy red blood cells by forming protein channels. Streptolysins are hemolysins produced by streptococci that can destroy both red and white blood cells.
3. Superantigens:
* Antigens that provoke an exceptionally intense immune response.
* Mechanism: They stimulate the massive proliferation of T cells, which then release high levels of cytokines.
* Symptoms: High cytokine levels cause fever, nausea, vomiting, diarrhea, and sometimes shock.
* Examples: Staphylococcus toxins responsible for food poisoning and toxic shock syndrome (TSS).
4. Genotoxins:
* Produced by certain gram-negative bacteria such as Haemophilus and Helicobacter.
* Mechanism: These toxins specifically damage host DNA.
* Consequences: Damage results in mutations, disruption of cell division, and potentially cancer (e.g., stomach cancer).
Detailed List: Diseases Caused by Exotoxins
Botulism: Caused by Clostridium botulinum (A-B toxin). Mechanism: Neurotoxin prevents transmission of nerve impulses, resulting in flaccid paralysis.
Tetanus: Caused by Clostridium tetani (A-B toxin). Mechanism: Neurotoxin blocks nerve impulses to the muscle relaxation pathway, resulting in uncontrollable muscle contractions.
Diphtheria: Caused by Corynebacterium diphtheriae (A-B toxin). Mechanism: Cytotoxin inhibits protein synthesis, particularly in nerve, heart, and kidney cells.
Scalded Skin Syndrome: Caused by Staphylococcus aureus (A-B toxin). Mechanism: One exotoxin causes skin layers to separate and slough off.
Cholera: Caused by Vibrio cholerae (A-B toxin). Mechanism: Enterotoxin causes secretion of large amounts of fluids and electrolytes, resulting in severe diarrhea.
Traveler's Diarrhea: Caused by Enterotoxigenic Escherichia coli and Shigella spp. (A-B toxin). Mechanism: Enterotoxin causes fluid and electrolyte secretion leading to diarrhea.
Anthrax: Caused by Bacillus anthracis (A-B toxin). Mechanism: Two A components enter the cell via the same B component. The A proteins cause shock and reduce the immune response.
Gas Gangrene and Food Poisoning: Caused by Clostridium perfringens and other Clostridium species (Membrane-disrupting toxins). Mechanism: One exotoxin (cytotoxin) causes massive red blood cell destruction (hemolysis); another (enterotoxin) causes diarrhea.
Antibiotic-Associated Diarrhea: Caused by Clostridium difficile (Membrane-disrupting toxin). Mechanism: Enterotoxin causes fluid/electrolyte secretion (diarrhea); cytotoxin disrupts the host cytoskeleton.
Food Poisoning: Caused by Staphylococcus aureus (Superantigen). Mechanism: Enterotoxin causes secretion of fluids and electrolytes resulting in diarrhea.
Toxic Shock Syndrome (TSS): Caused by Staphylococcus aureus (Superantigen). Mechanism: Toxin causes secretion of fluids and electrolytes from capillaries, decreasing blood volume and lowering blood pressure.
Stomach Cancer: Caused by Helicobacter spp. (Genotoxin). Mechanism: Toxin causes breaks in eukaryotic DNA.
Detection of Endotoxins
Limulus Amebocyte Lysate (LAL) Test: Used to detect endotoxins in drugs and medical devices.
Source: The Atlantic horseshoe crab (Limulus polyphemus).
Mechanism: The blood of the horseshoe crab contains white blood cells called amebocytes, which have large amounts of a protein that causes clotting.
Positive Test: In the presence of even trace amounts of endotoxin, the amebocytes lyse and release the protein, which reacts to form a gel-clot (precipitate).
Comparative Summary: Exotoxins vs. Endotoxins
Bacterial Source:
* Exotoxins: Mostly from gram-positive bacteria.
* Endotoxins: From gram-negative bacteria.
Relation to Microorganism:
* Exotoxins: Metabolic product of a growing cell.
* Endotoxins: Present in LPS of the outer membrane; released during cell destruction or division.
Chemistry:
* Exotoxins: Proteins, usually with two parts (A-B).
* Endotoxins: Lipid portion (Lipid A) of LPS.
Pharmacology (Effect on Body):
* Exotoxins: Specific for a particular cell structure or function (mainly affects nerves, GI tract, or specific cell functions).
* Endotoxins: General effects including fever, weakness, aches, and shock; symptoms are uniform across different bacteria.
Heat Stability:
* Exotoxins: Unstable; usually destroyed at (Staphylococcal enterotoxin is an exception).
* Endotoxins: Stable; can withstand autoclaving at for .
Toxicity:
* Exotoxins: High (high ability to cause disease).
* Endotoxins: Low.
Fever-Producing:
* Exotoxins: No.
* Endotoxins: Yes.
Immunology (Antibody Relation):
* Exotoxins: Can be converted to toxoids; neutralized by antitoxins.
* Endotoxins: Not easily neutralized by antitoxins; effective toxoids cannot be made.
Lethal Dose:
* Exotoxins: Small.
* Endotoxins: Considerably larger.
Representative Diseases:
* Exotoxins: Gas gangrene, tetanus, botulism, diphtheria, scarlet fever.
* Endotoxins: Typhoid fever, urinary tract infections, meningococcal meningitis.
The Process of Pathogenicity
Fundamental Principle: Disease results when the balance between host and microbe tips in favor of the microbe.
Portals of Entry:
* Mucous Membranes: Respiratory tract, gastrointestinal tract, genitourinary tract, conjunctiva.
* Skin.
* Parenteral Route: Direct deposition into tissues beneath the skin or mucous membranes.
Requirements for Disease Production:
1. Entering the host through a portal of entry.
2. Adherence to host tissue.
3. Penetrating or evading host defenses (via capsules, cell wall components, enzymes, antigenic variation, invasins, or intracellular growth).
4. Damaging host tissues (via siderophores, direct damage, or toxins like exotoxins and endotoxins).
Lysogenic Conversion: A process that can alter the pathogenicity of a microbe.
Cytopathic Effects: Visible effects of viral infection on host cells.
Portals of Exit: Generally the same as the portals of entry (Mucous membranes, skin, parenteral route).