Microbial Pathogens

Microbial Pathogens

1. Terminology

   • A PATHOGEN is a microorganism that is able to cause disease in a plant, animal or insect
   • PATHOGENICITY is the ability to produce disease in a host organism.
   • Microbes express their pathogenicity by means of their VIRULENCE, a term which refers to the DEGREE OF PATHOGENICITY of the microbe.
   • DETERMINANTS OF VIRULENCE of a pathogen are any of its GENETIC, BIOCHEMICAL or STRUCTURAL features that enable it to produce disease in a host.
   • The relationship between a host and a pathogen is dynamic, since each modifies the activities and functions of each other. The outcome of an infection depends on the VIRULENCE OF THE PATHOGEN and the relative degree of resistance or susceptibility of the host, due mainly to the effectiveness of the HOST DEFENSE MECHANISMS.

2. Underlying mechanisms of Bacterial Pathogenicity

   • THE ABILITY TO INVADE TISSUES: Invasiveness, which encompasses mechanisms for colonization (adherence and inital multiplication), ability to bypass or overcome host defense mechanisms, and the production of extracellular substances which facilitate invasion.
   • THE ABILITY TO PRODUCE TOXINS: Toxigenesis. Bacteria produce two typre of toxins called exotoxins and endotoxins.
     • Exotoxins are released from bacterial cells and may act at tissue sites removed from the site of bacterial growth
     • Endotoxins are cell associated substances that are structural components of the cell walls of Gram-negative bacteria

3. Establishment of infection

   • For human pathogens entry into the body can occur through:
     • Respiratory- tracts
     • Gastro- intestinal- tracts
     • Urinary- or Genital- tracts
     • Insect bites
     • Accidental or surgical trauma to the skin
   • Many opportunistic pathogens are carried as part of the normal human flora- acts as a ready source of infection in the compromised host.
   • For primary pathogens transmission is more complex:
     • Transmission of Bordetella pertussis requires contact with infectious material since this organism survives poorly in the environment
     • The sexually transmitted disease Neisseria gonorrhoeae direct person- to- person mucosal contact is required

4. Medically important Pathogens and their Environment

   • For many gastro- intestinal pathogens e.g. Salmonella, Shigella epecies the primary source is environmental, and infection follows ingestion of contaminated food or water
     • These (and other) pathogens hence need to remain viable in different environmental conditions
   • The environments that can be colonised by a pathogen are critical in determining its reservoirs and potential modes of transmission,
   • Importantly individual bacteria are not restricted to a single physiological state but can respond to environmental stimuli and undergo adaptive responses which confer improved capacity for survival in adverse conditions.