Pathogenicity
Pathogenicity refers to the capacity of organisms, such as bacteria, viruses, fungi, and protozoa, to cause infection in a host, including humans. This ability hinges on the pathogen's strategies to overcome the host's defense mechanisms, which consist of two main lines: physical barriers, such as skin and mucous membranes, and the immune system, which targets and eliminates invaders. Pathogens can transmit through various means, including contaminated blood, food, water, and direct contact, or via vectors like insects. Notable examples of infectious diseases include hepatitis and malaria, which highlight the critical impacts of pathogens, especially in vulnerable populations.
A growing concern is the emergence of antibiotic resistance, where bacteria evolve to withstand medications that once effectively treated infections. This resistance is exacerbated by the misuse of antibiotics, leading to challenges in managing infections. Additionally, some pathogens can evade immune responses by entering host cells, complicating treatment efforts. Understanding pathogenicity is vital for developing effective prevention and treatment strategies, particularly as pathogens increasingly cross species barriers, raising public health concerns globally.
Pathogenicity
Pathogenicity is the ability of a disease-causing organism (pathogen) to produce an infection in another organism, including a human being.
Establishing Infection
The pathogenicity of bacteria, viruses, yeast, protozoa, and fungi involves the ability to establish an infection in the face of attempts by the host to destroy the infecting organism. Various strategies are involved in infectivity, and not all organisms exhibit all the strategies. Pathogenicity is influenced also by the damage that the infecting organism inflicts on the host. Again, there are various mechanisms of host destruction.
Normally, the defense mechanisms of the body’s immune system prevent infection. However, the immune system sometimes operates inefficiently. Inefficient immunity affects the very young, the elderly, and the already ill. Inefficient immunity also can be caused by a deliberately compromised immune system. One example of this is drug therapy for persons receiving transplanted tissues or organs (to prevent rejection of the transplant). With compromised immunity, infections are common.
Transmission of Pathogens
Pathogens can be acquired in several ways. A person can get infected through blood that is contaminated with a virus, such as hepatitis A virus or the human immunodeficiency virus (HIV). A person can get infected by ingesting contaminated food and drinking water. These waterborne viruses, bacteria, and protozoa kill millions of people each year around the world, particularly in undeveloped or developing countries. A person can get infected when a pathogen is transferred from person-to-person or by vectors (animals, insects, or birds). One prominent example of vector-to-human transmission is avian influenza. Another example is malaria, whose transfer to humans by infected mosquitoes leads to millions of infections and deaths annually.
Pathogenicity also can involve the air. Some microorganisms, particularly the very small and light spheres known as spores, which are formed by some bacteria, can be inhaled. Germination of the spores to actively growing and dividing bacteria can cause, for example, a deadly lung infection. The most prominent example is the pulmonary form of anthrax.
Breaking the Host’s Defense
Pathogenicity requires circumventing two lines of host defense. The first defense is the barrier between the inside of the body and the outside world: the skin, mucous membranes in the nose and throat, and tiny hairs in the nose that act to physically block invading organisms. Organisms can be washed from body surfaces by tears, blood, and sweat. This defense has no specificity and involves the physical exclusion or removal of the invader.
The second line of defense is specific and involves the immune system. The invading organism is recognized and destroyed. This defense can be enhanced by the process of vaccination, which aims to prime the immune system by introducing components of the target pathogen or a living version of the pathogen that has been treated so it is incapable of actually causing the disease. The goal of vaccination is twofold: to protect a person from an existing infection (but one that has not yet affected the vaccinated person) and, for some vaccines, to provide protection against infection that persists for years and even for a lifetime.
Resistance
An increasingly significant method of pathogenicity is bacterial resistance to antibiotics that were once capable of killing the cells. Antibiotic resistance is a major problem worldwide. In 2011, several types of bacteria developed resistance to all known antibiotics. Since then, physicians have exercised caution when prescribing antibiotics to reduce the chances of additional strains of antibiotic resistant bacteria developing.
Antibiotic resistance is caused, in part, by the widespread and sometimes inappropriate use of antibiotics (for example, using antibiotics for viral illnesses and using antibiotics in cattle feed). Bacteria can become antibiotic-resistant by growing as an adherent layer on living and nonliving surfaces. This layer, called a biofilm, was considered a curiosity in the 1970s but was later recognized as a crucial source of pathogenicity. For example, it has been shown that the chronic and often ultimately lethal lung infection that occurs in people with cystic fibrosis is caused by biofilms of Pseudomonas aeruginosa.
Bacteria and viruses also can evade destruction by entering host cells and tissues. Once inside the host structures, they are shielded from the immune system and from drugs.
Impact
Pathogenicity has become increasingly significant to human and animal health and disease. It is becoming even more important with the evolving ability of certain pathogens to cross species barriers.
Bibliography
"Antimicrobial Resistant Bacteria." Better Health Channel, 21 Nov. 2023, www.betterhealth.vic.gov.au/health/conditionsandtreatments/antibiotic-resistant-bacteria. Accessed 4 Feb. 2025.
Dieckmann, Ulf, et al., eds. Adaptive Dynamics of Infectious Diseases: In Pursuit of Virulence Management. New York: Cambridge University Press, 2005.
Drlica, Karl, and David S. Perlin. Antibiotic Resistance: Understanding and Responding to an Emerging Crisis. Upper Saddle River, N.J.: FT Press, 2011.
Kuijper, E. J., et al. “ Clostridium difficile: Changing Epidemiology and New Treatment Options.” Current Opinions in Infectious Disease 20 (2007): 376-383.
Miller, A. A., and P. F. Miller. Emerging Trends in Antibacterial Discovery: Answering the Call to Arms. Norwich, England: Caister Academic Press, 2011.
Schnayerson, Michael, and Mark J. Plotkin. The Killers Within: The Deadly Rise of Drug-Resistant Bacteria. Boston: Back Bay Books, 2003.
Science 321, no. 5887 (July 18, 2008).
Zabay, Geoffrey. Agents of Bioterrorism: Pathogens and Their Weaponization. New York: Columbia University Press, 2008.