Host Microbe- relationships

Host Microbe- relationships
Microorganisms display a variety of complex relationships with other microorganisms and with larger forms of life that serve as hosts for them. A host is any organism that harbors another organism.
Symbiosis
Symbiosis is an association between two (or more) species. Meaning “living together,” the term symbiosis encompasses a spectrum of relationships. These include mutualism, commensalism, and parasitism. mutualism
in which both members of the association living together benefit from the relationshiplarge numbers of Escherichia coli live in the large intestine of humans. These bac teria release useful products such as vitamin K, which we use to make certain blood-clotting factors. Although the relationship is not obligatory, E. coli does make a modest contribution toward satisfying our need for vitamin K. The bacteria, in turn, get a favorable environment in which to live and obtain nutrients 2`Many of the bacteria on human skin are mutualistic. However, most of these organisms are commensals, which indirectly benefit us by competing with harmful organisms for nutrients and preventing those organisms from finding a site to attach to and invade tissue
parasitism,
in which one organism, the parasite, benefits from the relationship, whereas the other organism, the host, is harmed by itSome parasites obtain com fortable living arrangements by causing only modest harm to their host. Other parasites kill their hosts, thereby rendering themselves homeless . The most successful parasites are those that maintain their own life processes without severely damaging their hosts
commensalism
in which two species live together in a relationship such that one benefits and the other one neither benefits nor is harmed. For example, many microorganisms live on our skin surfaces and utilize metabolic products secreted from pores in the skin. Because those products are released whether or not they are used by microorganisms, the microorganisms benefit, and ordinarily we are nei ther benefited nor harmed. The line between commensalism and mutualism is not always clear. By taking up space and utilizing nutrients, microbes that show mutualistic or commensalistic behavior may prevent colonization of the skin by other, potentially harmful, disease-causing microbes—a phenomenon known as microbial competition. Hence these symbiotic relationships confer an indirect benefit on the host. There is also a fine line between parasitism and commensalism. In healthy hosts, many microbes of the large intestine form harmless associations, simply feeding off digested food materials. But a ‘harmless’ microbe could act as a parasite if it gains access to a part of the body where it would not normally exist. The situation in which both species harm each other without either benefiting is called antagonism.
Contamination, Infection, and Disease
Contamination, infection, and disease can be viewed as a sequence of conditions in which the severity of the effects microorganisms have on their hosts increases. Contamination means that the microorganisms are present. The surfaces of skin and mucous membranes can be contaminated with a wide variety of microorganisms. Infection refers to the multiplication of any parasitic organism within or on the host’s body. If an infection disrupts the normal functioning of the host, disease occurs.
Disease is a disturbance in the state of health wherein the body cannot carry out all its normal functions . Both infection and disease result from interactions between parasites and their hosts. Sometimes an infection produces no observable effect on the host even though organisms have invaded tissues. More often an infection produces observable disturbances in the host’s state of health; that is, disease occurs. When an infection causes disease, the effects of the disease range from mild to severe
Virulence refers to the intensity of the disease produced by pathogens, and it varies among different microbial species. For example, Bacillus cereus causes mild gastroenteritis, whereas the rabies virus causes neurological damage that is nearly always fatal. Virulence also varies among members of the same species of pathogen. For example, organisms freshly discharged from an infected individual tend to be more virulent than those from a carrier, who characteristically shows no signs of disease. The virulence of a pathogen can increase by animal passage, the rapid transfer of the pathogen through animals of a species susceptible to infection by that pathogen. As one animal becomes diseased, organisms released from that animal are passed to a healthy animal, which then also gets sick. If this sequence is repeated two or three times, each newly infected animal suffers a more serious case of the disease than the one before it. Presumably the mi crobe becomes better able to damage the host with each animal passage The virulence of a pathogen can be decreased by attenuation, by repeated subculturing on laboratory media

Koch postulates.
The first direct demonstration of the role of bacteria in causing disease came from the study of anthrax by the German physician Robert Koch Four criteria that were established by Robert Koch to identify the causative agent of a particular disease, these include:
1. the microorganism or other pathogen must be present in all cases of the disease
2. the pathogen can be isolated from the diseased host and grown in pure culture
3. the pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal
4. the pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen
many organism that do not meet the criteria of posulates have been shown to cuase the diseases For examples, Mycobacterium leprae, the causative agent of leprosy, cannot be isolated in pure culture. additionally to Neisseria gonorrhoeae there is no animal model of infection even though the bacteria can readily be cultured in vitro