Microorganisms and Carriers of Disease

Basics

Microorganisms that are known to cause a disease are called pathogens. These organisms are bacteria, fungi, protozoa, and viruses. Carriers of diseases are infectious particles such as endotoxins, exotoxins, and prions. A brief description of each is given below.

Viruses

Viruses are very small and can usually only be detected with an electron microscope. They contain a nucleic acid nucleus (DNA or RNA) surrounded by a cover layer of protein. They contain no cytoplasm, cell membrane or organelles. Biologically, they are not alive because they can repair or multiply on their own. This means that they are below the cell level. The cell is the basic unit of life. Medically they are called alive because they are able to cause diseases. Viruses must enter a host cell and take over the metabolic machine to multiply. Once in a host cell, they make multiple copies of their genetic material and the proteins they are packaged in. Then they break out of the cell, destroying the cell that reproduces them. Viruses are responsible for the development of certain diseases like flu, measles, mumps, smallpox, and AIDS, to name a few.

Bacteria

Bacteria are unicellular microorganisms that are hardly visible on a microscopic level. They do not contain organelles like a nucleus or mitochondria. Their DNA is loose in the cytoplasmic space. For this reason, bacteria are classified as prokaryotes, i.e. in front of a real cell nucleus. Most bacteria are not pathogenic, but there are some that can cause disease.

Some bacteria have the ability to produce spores in adverse environmental conditions. They remain inactive for a long time and are resistant to dehydration, radiation, heat, and chemicals. Once the right conditions are created, the bacteria can break out of their spores and multiply. Some bacteria do not cause disease themselves, but release toxins that can. Bacterial diseases that are produced by bacterial toxins include Staphylococcus, streptococcus, common cold, cholera, tuberculosis, sepsis, and anthrax.

Fungi

Fungi are members of a very heterogeneous group of eukaryotic unicellular or polynuclear organisms that live by decomposition and absorption of organic materials on which they grow. This group includes fungi, molds, mildew, and yeasts. Several fungi are capable of causing human disease, some of which can be serious. Some are known for their ability to infect the skin, others thrive deep within the tissues. Common forms are the ringworm, athlete’s foot, and fungal infections (Candida albicans). Fungal infections of the lungs include histoplasmosis, blastomycosis and coccidioidomycosis.

Protozoans

Protozoa are unicellular organisms that have animal-like properties and are structurally very similar to the cells in our body. Some protozoa have the ability to cause malaria, sleeping sickness and dysentery (amoebic dysentery).

Endotoxins

Endotoxins are heat-stable, toxic lipopolysaccharides that are present in the cell wall of gram-negative bacteria. These lipopolysaccharides are pyrogenic (fever causing) and have the ability to increase capillary permeability. Endotoxins are only released when a bacterium lyses. Endotoxins are less virulent than exotoxins and they do not produce toxoids. A toxoid is a chemically modified toxin that is no longer toxic but has antigenic properties. Toxoids are often used as vaccines. In addition to fever, endotoxins can cause chills, shock, leukopenia, and diarrhea, depending on which endotoxin it is.

Exotoxins

Exotoxins are heat-labile toxic proteins, which are excreted by certain bacteria. Exotoxins are known to cause fever, skin rashes, organ damage, diphtheria, botulism, and tetanus. Exotoxins are important virulence factors in pathogenic bacteria.

Prions

Prions are abnormally folded proteins that have the ability to infect a cell. When the prion comes into contact with normal cellular prions, it causes the normal protein to fold abnormally in a chain-like reaction. The pathogenic prions are produced spontaneously by genetic mutation or by ingesting infected tissue such as infested cattle or game. Prions are believed to be the cause of brain diseases such as transmissible spongiform encephalopathies (TSE) like Bovine Spongiform Encephalitis (BSE, cow), scrapie (sheep, goats), Creutzfeldt-Jakob Disease (CJD, human) and Kuru ( New Guinea, human). Prions are found in most tissues, but they are particularly abundant in brain tissue. Normal prions are found on the surface of neurons.