Microbiology Chapter 10: Host-Microbe Interactions

Microbiology: Basic and Clinical Principles - Chapter 10

• Chapter 10 covers Host-Microbe Interactions and Pathogenesis, presented by Janet Dowding, PhD, St. Petersburg College.
• The chapter uses a clinical case ("The Case of the Deadly Mistake") to explore host-microbe interactions. Students can visit a dedicated online resource to examine this case.

Host-Microbe Interactions

• These interactions are a dynamic exchange, not always harmful.
• Normal microbiota (the naturally occurring microorganisms on the body) colonize various body systems (digestive, genital, urinary, and respiratory).
• Microbiota help by producing vitamins, competing with harmful pathogens, and promoting immune system maturity.
• An imbalance in microbiota (dysbiosis) can lead to diseases. For example, a course of antibiotics can kill off normal gut microbiota, allowing Clostridioides difficile to flourish and cause illness.

Pathogens

• Pathogens are disease-causing microbes.
• They have adaptations to interact with specific host tissues, making them dangerous.
• Pathogenic species can be commensal in one host and pathogenic in another. For instance, Group B streptococci (GBS) are normal vaginal commensals in 30% of women, but associated with newborn sepsis, meningitis, and pneumonia.

Tropism

• Tropism is a pathogen's preference for a specific host or tissue within that host.
• This preference can, but doesn't always, translate to the development of a disease.
• Host factors (age, gender, overall health, etc.) influence the likelihood of disease progression even after successful invasion of the preferred tissue.
• Emerging pathogens often broaden their host range over time.

Virulence

• Pathogenicity is the ability of a microbe to cause disease.
• Virulence describes the degree or extent of disease caused by a pathogen.

Virulence Factors

• Mechanisms pathogens use to overcome host defenses include features that adhere to host cells, invade tissues, or induce disease.
• Virulence factors often are energy intensive for pathogens to produce, so they are selected for if they cause the pathogen to better survive and spread.

Toxins

• Toxins are molecules affecting host responses, including causing tissue damage or suppressing the immune system.
• Toxemia refers to toxins in the bloodstream.

Endotoxins

• These are lipid-based toxins found in the outer membranes of Gram-negative bacteria (LPS).
• They are released upon bacterial cell death or lysis.
• These toxins can induce high fever, chills, body aches, hypotension, tachycardia, increased respiratory rate, inflammation, disorientation, nausea, & vomiting.
• In large quantities, may cause septic shock and organ failure.
• They are not easily neutralized or eliminated.

Exotoxins

• These are proteins produced and released by actively growing Gram-positive or Gram-negative bacteria.
• These toxins can act on the nervous system (neurotoxins), gastrointestinal tract (enterotoxins), liver (hepatitoxins) or kidneys (nephrotoxins).
• Classified into families based on their mode of action:
  • Type I: membrane-acting extracellular toxins
  • Type II: membrane-damaging toxins
  • Type III: intracellular toxins(AB toxins)
• Many bacterial exotoxins are potent and cause severe diseases.

Five Steps to Infection

• Pathogens complete five steps to successfully infect a host:
  1. Enter the host
  2. Adhere to host tissues
  3. Invade host tissues and obtain nutrients
  4. Replicate while warding off immune system
  5. Transmit to a new host

Portals of Entry

• Mucous membranes are common entry sites.
• Other portals of entry include the skin, eyes, ears, and respiratory/urogenital/GI tracts.
• Some pathogens have multiple portals of entry.

Portals of Exit

• Portals of exit are typically the same as entry. This includes secretions, excretions, wounds, mucus, and bodily fluids (e.g., blood, urine, feces).

Reservoir

• Pathogens thrive in reservoirs; these can be environmental (water, soil) or in other organisms.

Safety and Health Care

• Healthcare facilities use biosafety levels (BSLs) to classify pathogens based on disease severity, mode of transmission, availability of prevention and/or treatment.
• BSL-1: Well-characterized, low disease potential pathogens.
• BSL-2: Infectious, not typically airborne.
• BSL-3: Serious or lethal pathogens, frequently airborne.
• BSL-4: Dangerous, with frequent lethal outcomes in humans; airborne transmission likely.
• Proper precautions, including PPE (personal protective equipment), are required. The specific PPE depends on BSL.

Infection Control Practices

• Standard precautions treat all patients as potential reservoirs for bloodborne and other infectious agents.
• Transmission precautions prevent pathogen transmission by contact, droplet, or airborne routes and often involve specialized isolation rooms.