15.3 Virulence factors of bacterial and viral pathogens  (Page 2/17)

Bacterial exoenzymes and toxins as virulence factors

After exposure and adhesion, the next step in pathogenesis is invasion , which can involve enzymes and toxins. Many pathogens achieve invasion by entering the bloodstream, an effective means of dissemination because blood vessels pass close to every cell in the body. The downside of this mechanism of dispersal is that the blood also includes numerous elements of the immune system. Various terms ending in –emia are used to describe the presence of pathogens in the bloodstream. The presence of bacteria in blood is called bacteremia . Bacteremia involving pyogens (pus-forming bacteria) is called pyemia . When viruses are found in the blood, it is called viremia . The term toxemia describes the condition when toxins are found in the blood. If bacteria are both present and multiplying in the blood, this condition is called septicemia .

Patients with septicemia are described as septic , which can lead to shock , a life-threatening decrease in blood pressure (systolic pressure<90 mm Hg) that prevents cells and organs from receiving enough oxygen and nutrients. Some bacteria can cause shock through the release of toxins (virulence factors that can cause tissue damage) and lead to low blood pressure. Gram-negative bacteria are engulfed by immune system phagocytes, which then release tumor necrosis factor , a molecule involved in inflammation and fever. Tumor necrosis factor binds to blood capillaries to increase their permeability, allowing fluids to pass out of blood vessels and into tissues, causing swelling, or edema ( [link] ). With high concentrations of tumor necrosis factor, the inflammatory reaction is severe and enough fluid is lost from the circulatory system that blood pressure decreases to dangerously low levels. This can have dire consequences because the heart, lungs, and kidneys rely on normal blood pressure for proper function; thus, multi-organ failure, shock, and death can occur.

Exoenzymes

Some pathogens produce extracellular enzymes, or exoenzyme s , that enable them to invade host cells and deeper tissues. Exoenzymes have a wide variety of targets. Some general classes of exoenzymes and associated pathogens are listed in [link] . Each of these exoenzymes functions in the context of a particular tissue structure to facilitate invasion or support its own growth and defend against the immune system. For example, hyaluronidase S, an enzyme produced by pathogens like Staphylococcus aureus , Streptococcus pyogenes , and Clostridium perfringens , degrades the glycoside hylauronan (hyaluronic acid), which acts as an intercellular cement between adjacent cells in connective tissue ( [link] ). This allows the pathogen to pass through the tissue layers at the portal of entry and disseminate elsewhere in the body ( [link] ).