Symbiosis
Symbiosis ("living together") is a close and long-term association between organisms of different species. There are different opinions as to what exactly a symbiotic relationship is, but they are usually considered to be one of three kinds. If the relationship is beneficial to both species, it is called mutualism. If one species benefits from the relationship while the other is neither harmed nor benefited, it is called commensalism. If one species benefits and the other is harmed, the relationship is known as parasitism.
An enormous variety of microorganisms live symbiotically with human beings, but the lines of demarcation between the different categories ar not clear-cut.
Evolutionary progress is measured in terms of surviving progeny. A symbiotic microorganism that destroys its host before the reproduction and dispersal of its progeny to new hosts is less likely to be successful by the evolutionary criteria than one that enjoys a long, comfortable relationship with its host.
How Microbes Cause Disease
This image is a microbe called Penicillium.
(Image credit: Richard Edelman)
The pathogenic effects of microbes are produced in a variety of ways. Viruses enter particular types of cells and often destroy them. Bacteria produce cell destruction also. Frequently, however, the effects we recognize as disease are caused not by direct action of the pathogens but by toxins, or poisons, produced by them.
Some diseases are the result of the body's reaction to the pathogen.
A single disease agent can cause a variety of diseases. Skin infections of Streptococcus pyogenes cause the disease known as impetigo. Throat infections by the same bacteria cause the familiar disease strep throat. Conversely, many agents can cause the same disease; the "common cold" is caused by any one of a large number of viruses.
Above can be seen a picture of Streptococcus pyogenes.
Prevention and Control of Infectious Disease
Although microorganisms were seen and depicted with remarkable accuracy by Antony von Leeuwenhoek in the late seventeenth century, they were not associated with disease until 100 years ago. This opened the way to control measures, among the most important of which was sterile procedures in hospitals. Even more important than the introduction of sterile medical procedures was the institution of public health. This included eradication of disease carrying insects, the disposal of sewage, the pasteurization of milk, and the filtration of water.
Many infectious diseases can be prevented by immunization. Many bacteria are susceptible to antimicrobial drugs, such as sulfa and penicillin. Penicillin, which is synthesized by the fungus Penicillium, was the first known antibiotic - by definition, a chemical that is produced by a living organism and is capable of inhibiting the growth of microorganisms. Many antibiotics are formed by bacteria and some are formed by fungi. Many, including penicillin, can now be synthesized in the laboratory. Antibiotics and other chemotherapeutic agents are effective because they interfere with some essential process of the pathogen without affecting the cells of the host.
Viruses are basically
impervious to attack by chemotherapeutic agents; drugs that will effectively
stop the viruses reproduction process have devastating effects on cellular
processes. Membrane receptors and viral capsids are beginning to
yield the structural secrets that explain their interactions. These
findings are raising the possibility of devising a molecule that will block
either the receptors or the capsids that fit into them.
