Chapter 25: Microbial Infection and Pathogenesis

Successful infection begins with microbial adherence, where pathogens attach to host tissues through specific interactions between microbial adhesins and host cell receptors, often involving structures such as capsules, fimbriae, pili, and flagella. Once attached, pathogens colonize host tissues and may form biofilms, structured microbial communities embedded in extracellular matrices that enhance survival and resistance to antimicrobial treatments. Disease progression frequently involves invasion, in which pathogens penetrate host cells or tissues and spread locally or systemically through the bloodstream, producing conditions such as bacteremia, septicemia, or viremia. The ability of a microorganism to cause disease is known as pathogenicity, while virulence describes the relative severity of disease caused by a pathogen and is often quantified using measures such as the lethal dose fifty. Virulence is determined by numerous virulence factors encoded on chromosomes, plasmids, or pathogenicity islands, including enzymes that degrade host tissues or interfere with immune defenses. Examples include hyaluronidase and collagenase, which facilitate tissue invasion, streptokinase, which dissolves fibrin clots, and coagulase, which protects pathogens by promoting clot formation. Microbial toxins also play major roles in disease. Exotoxins are potent secreted proteins that disrupt host cellular processes and include AB toxins, cytolytic toxins that damage cell membranes, and superantigens that overstimulate the immune system. Endotoxins, in contrast, are lipopolysaccharides found in the outer membrane of gram negative bacteria that trigger inflammatory responses when released during cell lysis. The outcome of infection depends not only on microbial factors but also on host conditions, as individuals with weakened immune defenses are more susceptible to opportunistic infections and hospital acquired diseases. Together, these mechanisms illustrate how microbial pathogens interact with host systems to establish infection, evade immune defenses, and produce disease.