What is pathogenicity?

Definition

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, there were
several types of bacteria that developed resistance to all known antibiotics.

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 1970’s. Now, it
is 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

Dieckmann, Ulf, et al., eds. Adaptive Dynamics of Infectious Diseases: In Pursuit of Virulence Management. New York: Cambridge University Press, 2005. An introductory text for infectious disease researchers.

Drlica, Karl, and David S. Perlin. Antibiotic Resistance: Understanding and Responding to an Emerging Crisis. Upper Saddle River, N.J.: FT Press, 2011. Details the development of antibiotic resistance, which critically depends on mutations of pathogens.

Kuijper, E. J., et al. “ Clostridium difficile: Changing Epidemiology and New Treatment Options.” Current Opinions in Infectious Disease 20 (2007): 376-383. Chronicles the increased prevalence of drug-resistant Clostridium difficile and the increasing threat posed by the pathogen in hospital-acquired infections.

Miller, A. A., and P. F. Miller. Emerging Trends in Antibacterial Discovery: Answering the Call to Arms. Norwich, England: Caister Academic Press, 2011. Describes the development of compounds that kill bacteria, including bacterial pathogens that have arisen due to mutations.

Schnayerson, Michael, and Mark J. Plotkin. The Killers Within: The Deadly Rise of Drug-Resistant Bacteria. Boston: Back Bay Books, 2003. Clearly describes how the overuse of antibiotics in agriculture and medicine has spawned the development of drug-resistant bacteria.

Science 321, no. 5887 (July 18, 2008). A special issue devoted to antibiotic resistance, highlighting some particularly difficult infections and discussing issues pertaining to the genetics of antibiotic resistance.

Zabay, Geoffrey. Agents of Bioterrorism: Pathogens and Their Weaponization. New York: Columbia University Press, 2008. Discusses how pathogenicity has been harnessed for warfare and terrorism.