Host-Pathogen Interaction Lecture 7

Host-Pathogen Interaction (I)

• Lecture 7, Dr. Mohamed El-Azizi
• Course material covers Host-Pathogen Interaction
• Learning objectives include defining infection, disease and colonization.
• Recognizing the role of human microbiota
• Understanding the relationship between bacterial pathogenicity and host immunity
• Differentiating pathogenic bacteria from opportunistic bacteria
• Analyzing clinical case studies for bacterial infection diagnoses & interventions
• Applying host-pathogen relationship in patient counseling
• Differentiating nosocomial and community-acquired infections

Learning Outcomes

• Students will be able to define infection, disease, and colonization
• Students will know the role of normal flora in the human body
• Students will grasp the relation between bacterial pathogenicity and host immunity
• Students will differentiate pathogenic bacteria from opportunistic bacteria
• Students will analyze clinical cases correlating to bacterial infections, recommending ideal interventions.
• Students will apply the host-pathogen relationship in patient counseling.
• Students will distinguish nosocomial from community-acquired infections

Textbooks and Resources

• Patrick Murray et al. Medical Microbiology, 5th ed., 2007
• 10 copies available in the library
• Access to online resources with username/password (university only)

Bacteria Classification

• Bacteria are categorized as "Good", "Bad", and "Ugly"
• Good bacteria: non-pathogenic, produce antibiotics, valuable industrial products.
• Bad bacteria: cause illness with low/no mortality in healthy individuals. Many common disease-causing bacteria (e.g., Staphylococcus aureus, Shigella)
• Ugly bacteria: cause high mortality in untreated individuals; severe diseases resulting from Bacillus anthracis, Vibrio cholera, Clostridium tetani.
• Pathogenic bacteria can be further categorized as Overt Pathogen or Opportunistic
  • Overt pathogens: they cause disease in healthy hosts
  • Opportunistic pathogens: cause disease in compromised hosts
• Normal Flora (Symbiosis): coexist with the host without harm or benefit.
• Mutualistic & Commensal: types of normal flora

Disease

• Damage caused by microorganisms/their products.
• Symptoms are triggered by microorganisms presence
• Bacteria cause disease by:
  • Microorganism in the host eliciting immune/inflammatory response, causing damage.
  • Organism proliferation (damage).
  • Damage by microbial products (e.g., enzymes, toxins, acids, gases).
  • Absorption of proteins/amino acids via tissue degradation

Infection

• Presence of a microorganism, not necessarily causing disease.
  • Can occur without symptoms
• Many bacteria can cause infection but can't trigger disease.
• Infection in a patient is always indicative of disease.

Colonization

• Presence of microorganisms without disease.
• Usually, antibiotics are not needed.
• Applies to surfaces, not usually the blood.
• Differentiate from disease states (e.g. symptoms appear and infection ensues)
• Colonization vs. infection: colonization isn't always bad, disease results when bacteria are in sites they shouldn't be or are causing harm.

Factors affecting susceptibility to infection

• Bacterial infective dose (e.g., Shigella, Vibrio cholera)
• Bacterial strain (virulence levels)
• Host factors (e.g., immune status, route of entry)

Normal Flora

• Microorganisms frequently found in healthy subjects
• Can cause disease under specific conditions (unprotected sterile sites).
• Infections from normal flora are called endogenous infections.
• Infections from other bacteria are known as exogenous infections.
• Body sites colonized by normal flora (e.g., GI tract, upper respiratory tract, distal genitourinary tract, skin).
• Body sites typically sterile (e.g., blood, CSF, interstitial fluid, lymph, organs)

Normal Flora - Examples

• Skin: Staphylococcus aureus, Staphylococcus epidermidis
• Stomach: Helicobacter pylori (gastritis/ulcers)
• Large Intestine: E. coli, Bacteroides fragilis
• Genitourinary system: Anterior urethra, urinary bladder

Pathogenic Bacteria

• Ability to cause infection/disease in a host.
• Virulence factors (traits like capsule, fimbriae, glycocalyx) enhance ability to cause disease.
  • Facilitates spread, multiplication, immune system evasion, damage to tissues,transmission.

Factors affecting normal flora

• Environment (e.g., hospitalizations, replacement of bacteria)
• Host health
• Broad-spectrum antibiotics: can negatively affect the normal flora.