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Bacteria
The original definition of symbiosis by deBary (1879) did not include a judgment on whether the partners benefit or harm each other. Currently, most people use the term symbiosis to describe interactions between the symbiont (the smaller organism) and the host (the larger organism) from which both partners benefit; this is also called a mutualism. If there is negative effect on one of the partners, it is called a parasitic symbiosis and if there is no beneficial or negative effect it is a commensal symbiosis. These clear-cut definitions are not always easy to apply in nature. Take the bacterium Pseudomonas aeruginosa for example. This bacterium can be found on the skin of humans and not cause disease, perhaps we would call it a commensial, but if the person has a severe burn P. aeruginosa can cause an infection and becomes a pathogen (a medicinal term for parasitism). This type of organism is called an opportunistic pathogen.
Another e.g., of bacterial human symbiosis is Hpylor in human GIT
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