Overview of Microbial Pathogenesis

Infection versus Disease

Pathogenicity and virulence are terms that refer to an organism's ability to cause disease. Pathogenicity is the ability of a microbe to cause disease and inflict damage upon its host, whereas virulence is the degree of pathogenicity within a group or species of microbes as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism, that is its ability to cause disease, is determined by its virulence factors.

As learned earlier under Bacterial Genetics, most of the virulence factors that enable bacteria to colonize the body and/or harm the body are the products of quorum sensing genes. Many bacteria use quorum sensing to sense their own population density, communicate with each other by way of secreted chemical factors, and behave as a population rather than as individual bacteria. This plays an important role in pathogenicity and survival for many bacteria.

The genomes of pathogenic bacteria, when compared with those of similar nonpathogenic species or strains, often show extra genes coding for virulence factors, that is, molecules expressed and secreted by the bacterium that enable them to colonize the host, evade or inhibit the immune responses of the host, enter into or out of a host cell, and/or obtain nutrition from the host. These include virulence factors such as capsules, adhesins, type 3 secretion systems, invasins, and toxins.

We also learned that most genes coding for virulence factors in bacteria are located in pathogenicity islands or PAIs and are usually acquired by horizontal gene transfer. These PAIs may be located in the bacterial chromosome, in plasmids, or even in bacteriophage genomes that have entered the bacterium. The genomes of most pathogenic bacteria typically contain multiple PAIs that can account for up to 10 - 20% of the bacterium's genome. PAIs carry genes such as transpoases, integrases, or insertion sequences that enable them to insert into host bacterial DNA. Transfer RNA (tRNA) genes are often the target site for integration of PAIs. Conjugative plasmids are the most frequent means of transfer of PAIs from one bacterium to another and the transfer of PAIs can then confer virulence to a previously nonpathogenic bacterium.

An infection is when a microorganism has established itself in a host - has colonized that host - whether not it causing harm or imparting damage. A disease, on the other hand, is where there is impairment to host function as a result of damage or injury. For example, the microbes that constitute the body's normal flora or microbiota have infected the body, but they seldom cause disease unless they invade a part of the body where they do not normally reside and/or the host becomes immunocompromised. In medicine, the term etiology refers to the causes of diseases or pathologies. In terms of infectious disease, the etiologic agent is the microorganism causing that disease.

The terms signs and symptoms are often used when diagnosing disease. A sign is an objective indication of some medical fact or characteristic that may be detected by a health care professional during a physical examination. They include such objective indications as blood pressure, respiration, rate, pulse, and temperature. A symptom is a condition experienced and reported by the patient.

To cause disease, a microorganism must maintain a reservoir before and after infection

The reservoir of an infectious agent is the habitat in which that microbe normally lives, grows, and multiplies. Reservoirs can include humans, animals, and the environment. Many common human infectious diseases have human reservoirs and are transferred person-to-person without intermediaries. Examples include sexually transmitted diseases, measles, most respiratory pathogens, and strep throat. Some infections are transmitted from an animal to a human in which case the infection is called a zoonosis. Examples include rabies, plague, and much salmonellosis. Plants, soil, and water in the environment are also reservoirs for some infectious agents such as histoplasmosis, coccidioidomycosis, and Legionnaires disease.

To cause disease, a microorganism must leave the reservoir and gain access to the new host

The microorganism must leave its reservoir or host through what is called a portal of exit and be transmitted to a new host. For example, the portal of exit for respiratory infections is typically the mouth or nose; for gastrointestinal infections, the feces. Modes of transmission include:

1. Direct contact, as through skin-to-skin contact, kissing, and sexual intercourse. Examples include some Staphylococcus aureus infections, infectious mononucleosis, and gonorrhea.
2. Direct droplet contact, as in the case of aerosols produced by sneezing and coughing. Examples include meningococcal infections and pertussis (whooping cough).
3. Indirect transmission of an infectious agent from a reservoir to a host by suspended air particles, inanimate objects, or vectors.
4. Airborne transmission occurs when infectious agents are carried by dust or droplets suspended in air. Some respiratory infections can be transmitted this way although most are transmitted by contact with infectious mucus.
5. Inanimate objects include water, food, blood, and fomites (inanimate objects such as toys, handkerchiefs, bedding, or clothing). Examples include cholera, salmonellosis, listeriosis, viral hepatitis).
6. Vectors such as ticks, mosquitoes, and fleas. Examples include Lyme's disease, malaria, and typhus fever.

The manner in which a pathogen enters a susceptible host is referred to as its portal of entry. For example, the portal of entry for most respiratory infections is the mouth or nose; for gastrointestinal infections, the mouth. The portal of entry must provide access to tissues with the correct physical and chemical environment (an environment with the proper oxygen content, pH, nutrients, temperature, etc.) in which the pathogen can multiply.

To cause disease, a microorganism must Adhere to cells of the skin or mucosa of its new host and colonize the body

Almost every part of the body has a mechanism for flushing microbes out of or off of the body, including the shedding of epithelial cells from the skin and mucous membranes, urination, defecation, coughing, and sneezing. Unless the microorganisms can replicate fast enough to replace those being flushed out, as in the case of much of the normal microbiota that colonize the lumen of the intestines, they need to adhere to the epithelial cells of the skin and mucous membranes. Also, this body environment must have the correct nutrients, the proper amount of oxygen or lack of oxygen, the right pH, and the right temperature to support the growth of that microorganism. Furthermore, since the body has excellent immune defense mechanisms, anything the microorganism can do to resist body defenses to some degree will also promote colonization.

To cause disease, a microorganism must Harm or damage the body

As stated above, an infection is simply when a microorganism has established itself in a host. To cause disease, that microorganism (or toxin) must inflict damage to the host.

Summary

1. Only a relatively few bacteria cause human disease.
2. The complex mutually beneficial symbiotic relationship between humans and their natural microbes is critical to good health.
3. An infection is when a microorganism has established itself in a host - has colonized that host - whether not it causing harm or imparting damage.
4. A disease is where there is impairment to host function as a result of damage or injury.
5. Etiology refers to the causes of diseases or pathologies; in terms of infectious disease, the etiologic agent is the microorganism causing that disease.
6. A sign is an objective indication of some medical fact or characteristic that may be detected by a health care professional during a physical examination; a symptom is a condition experienced and reported by the patient.
7. The reservoir of an infectious agent is the habitat in which that microbe normally lives, grows, and multiplies.
8. Transmission of microorganisms by direct contact refers to transfer by such means as skin-to-skin contact, kissing, and sexual intercourse.
9. Transmission of microorganisms by direct droplet contact refers to transfer by aerosols produced by sneezing and coughing.
10. Transmission of microorganisms by indirect contact refers to transfer by suspended air particles, inanimate objects, or vectors (ticks, mosquitoes, fleas).
11. The manner in which a pathogen enters a susceptible host is referred to as its portal of entry; the manner in which it leaves its host is its portal of exit.
12. If relatively few bacteria enter the body then the body's natural defenses against infection have a much better chance of removing them before they can colonize, multiply, and cause harm; if a large number of bacteria enter then the body's defenses may not be as successful.
13. A person with good innate and adaptive immune defenses will be much more successful in removing potentially harmful bacteria than a person that is immunocompromised.
14. Bacterial virulence factors influence a bacterium’s ability to cause infectious disease. These include virulence factors that enable bacteria to colonize the host as well as those that harm or damage the host.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

1. Define pathogenicity. (ans)
2. Define virulence. (ans)
3. Define infection. (ans)
4. Define disease. (ans)
5. Define vector. (ans)
6. Define medical sign. (ans)
7. Even though a microorganism may be considered pathogenic, it still may not be able to cause disease upon entering the body. Discuss why. (ans)
8. Multiple Choice (ans)