How Microorganisms Enter a Host

How Microorganisms Enter a Host

• Germs penetrate host defenses to damage tissues and cause disease.
• Mucous membranes are a common portal of entry for germs.

Respiratory Tract

• The most common entry point through mucous membranes.
• Microbes are inhaled through the nose or mouth.
• Influenza and colds are examples.

Gastrointestinal Tract

• Germs enter through food or water.
• Most are destroyed by stomach acid or bile in the intestine.
• Vibrio cholerae can survive these conditions.

Genitourinary Tract

• Sexually transmitted infections can occur here.

Other Mucous Membrane Considerations

• Some pathogens require a broken mucous membrane, like a cut or abrasion.
• Other organisms can penetrate unbroken membranes, such as the conjunctiva of the eye.

Skin

• Unbroken skin typically prevents microbe entry.
• Some microbes can enter through hair follicles.
• Other microorganisms need a wound to enter.
• Some fungi grow on the skin without needing a breach.

Parenteral Route

• Microbes are deposited directly into tissues when skin or membranes are broken.
• A tick bite can introduce bacteria, like in Lyme Disease.
• Intravenous injections can introduce pathogens like HIV.

Importance of Microbe Number

• If too few microbes enter the body, the immune system can fight them off and prevent disease.
• Disease likelihood increases with the number of microbes introduced.
• Virulence can be expressed numerically using infectious dose.

Infectious Dose (ID50)

• The amount of bacteria required to cause disease in 50% of the population.
• Bacillus anthracis can enter in different ways.
• Through skin cut, the ID50 is 10-20 endospores.
• Via inhalation, the ID50 is 10,000 to 20,000 endospores.
• Via ingestion, the ID50 is 250,000 to 1,000,000 endospores.

Potency of a Toxin

• Expressed as the lethal dose.
• The LD50 kills 50% of the infected population.

Adherence

• Surface molecules help a pathogen stick to the surface.
• Pathogens often adhere to specific receptors on host cells.
• Fimbriae or glycocalyx of bacteria or viral proteins can facilitate adherence.

Invasiveness

• A pathogen's ability to invade and multiply in healthy tissues.
• Two types of molecules promote invasiveness.

Extracellular Enzymes (Exoenzymes)

• Enzymes erode the surface of host cells and damage tissues.
• Hemolysin destroys red blood cells.
• Fibrinolysin degrades fibrin coats.
• Collagenase degrades connective tissue.
• Coagulase promotes blood clots around bacterial cells.
• Products of degradation serve as food and protect microbes from host defenses.

Invasins

• Surface proteins cause rearrangements of the host cell cytoskeleton.
• This forces the host cell to take in the bacterium.
• The bacterial cell is then protected from host defenses.

Bacterial Toxins

• Toxins are poisonous substances produced by microorganisms.
• Toxoids are inactive toxins.
• Toxoids can be used as vaccines (e.g., DTaP vaccine for diphtheria, pertussis, tetanus).

Exotoxins

• Toxins are secreted from the bacteria and are heat sensitive.
• They can be extremely toxic.
• Clostridium botulinum causes botulism.
• Botulism toxin: 1mg can kill 1,000,000 guinea pigs.

Categories of Exotoxins

• Neurotoxins interfere with nerve impulses.
• Botulinum toxin causes flaccid paralysis.
  • Produced by Clostridium botulinum.
  • Muscles permanently relax with a result of heart and respiratory system failure.
• Tetanus toxin causes rigid paralysis.
  • Produced by Clostridium tetani.
  • Leads to uncontrollable muscle contractions (spasms) and lockjaw.
  • Death usually results from spasms of the respiratory muscles.

Enterotoxins

• Interfere with salt absorption in the small intestine.
• This causes cells to pump out water to counteract intestine salt concentration, leading to watery diarrhea.
• Montezuma's revenge (E. coli) and Cholera (Vibrio cholera) are examples.

Cytotoxins

• Kill cells by interfering with protein synthesis.
• Diphtheria toxin can cause respiratory illness.

Endotoxins

• Part of the outer membrane of gram-negative bacteria.
• Lipopolysaccharide (LPS) is released and only causes problems when it is released from the membrane.
• Endotoxin (LPS) is released when a bacterium dies and this worsens symptoms.
• It can cause fever, hemorrhaging, and shock (sudden decrease in blood pressure).

Characteristics of Endotoxins

• Not as potent as exotoxins.
• Requires much more to cause symptoms.
• An example is salmonellosis (Salmonella enterica) which is food poisoning due to millions of dead bacteria.
• Symptoms are caused by the endotoxin.
• Antibiotic treatment may do more harm than good.

Exotoxins vs Endotoxins

• Exotoxins:
  • Proteins
  • Released outside the cell
  • Extremely toxic
  • Often lethal
• Endotoxins:
  • Lipopolysaccharides (LPS)
  • Released when the bacteria dies
  • Not as toxic
  • Usually not lethal

Viral Infections

• Mechanisms for evading defenses:
  • Viruses invade and grow inside of cells, shielded from the immune system.
  • Viruses disguise themselves to look like their hosts (e.g., enveloped viruses).

Cytopathic Effects (CPE)

• Visible effects of viral infection.
• Disruption of cell processes (e.g., herpes simplex virus stops cell division).
• Destruction of intracellular structures.
• Formation of inclusion bodies (viral parts).
• Fusing/giant cell formation caused by several infected cells.
• Infection often causes host cell death.