Extracellular pathogens are microorganisms that cause disease while living and multiplying in the spaces outside of host cells. These organisms thrive in environments such as the bloodstream, lymphatic system, and on the surfaces of tissues lining organs. Their strategy of living outside cells defines their method of causing illness and dictates how the body’s immune system must respond.
Common Types of Extracellular Pathogens
Extracellular pathogens include various bacteria, fungi, and protozoa. Well-known bacteria include Streptococcus pyogenes, the cause of strep throat, and Staphylococcus aureus, responsible for skin infections and pneumonia. These bacteria flourish on mucosal surfaces or circulate within the bloodstream to cause disease.
Fungi like Candida albicans act as extracellular pathogens, causing thrush and yeast infections by growing on mucous membranes. Protozoa in this category include Giardia lamblia, which lives in the gut and causes giardiasis. Another example is Trypanosoma brucei, the parasite behind African sleeping sickness, which multiplies in the bloodstream and cerebrospinal fluid.
Mechanisms of Pathogenesis
Extracellular pathogens cause disease by producing substances and enzymes that damage host tissues. One mechanism is the secretion of exotoxins—toxic proteins released by living pathogens that disrupt cellular processes or kill host cells, allowing the infection to spread. For example, Vibrio cholerae releases an exotoxin that causes the severe diarrhea characteristic of cholera.
Pathogens also inflict damage through endotoxins, which are components of the cell wall in gram-negative bacteria. Unlike exotoxins, endotoxins are released when bacteria die, triggering an inflammatory response that can lead to fever, shock, and organ failure. Additionally, some pathogens use enzymes like hyaluronidase to break down tissue barriers, helping the infection spread from its initial site.
Immune System Response
The body’s defense against extracellular pathogens is managed by the humoral immune system. When a pathogen is detected, B-cells are activated and produce antibodies that circulate through the blood and lymph. These antibodies bind to pathogens and their toxins, which can neutralize the toxins or tag the pathogens for destruction in a process called opsonization.
The complement system, a network of proteins in the blood, assists in this defense. Complement proteins can be activated by pathogens or by antibodies attached to a pathogen’s surface. Once activated, these proteins can create pores in the pathogen’s membrane, causing it to burst. They also enhance the ability of phagocytic cells like neutrophils and macrophages to engulf and eliminate invaders.
Contrast with Intracellular Pathogens
In contrast, intracellular pathogens must invade and replicate inside host cells to survive. This group includes all viruses and certain bacteria like Listeria monocytogenes and Mycobacterium tuberculosis. Hiding within the body’s cells makes them inaccessible to the antibodies and complement proteins that circulate in the bloodstream.
This difference in lifestyle requires a different immune response. While the humoral immune system targets extracellular pathogens, the cell-mediated immune system combats intracellular ones. This branch of immunity uses T-cells to recognize and kill infected host cells. By destroying these compromised cells, the immune system eliminates the protected environment where the pathogens multiply.