Often referred to as the "symbiosis". It is the situation where both organisms work together and help each other eg Bacillus bulgaricus and Streptococcus thermophilus in Yoghurt production

Means "eating at the same table" This is when two organisms co-habitate but do not help or hinder each other. Eg. Bacteria in our colon or on our skin.

In actual fact the commensals in our gut do give us a benefit because they stop less well adapted pathogenic bacteria from growing and reduce the chance of disease. Eg Clostridium botulinum can not grow in a healthy adult intestine. But it can grow in infants, which has not yet developed an adequate intestinal flora. This and other bacteria may cause some cases of "infant cot death"

Commensals on our skin also protect us from skin infections. A good reason for not sterilising your hands.

Many of the bacteria that can take advantage of a reduced flora of commensals are normally present is small numbers in our GIT or on out skin. Eg C. diphtheriae, Strep. pneumoniae, Candida, causing thrush, and Staph. aureus. These can cause up infections if our normal flora is disturbed by antibiotics etc. Such infections that come from within us are called Autoinfections

When one organism stops or inhibits the growth of another without gaining a benefit. This is often caused by toxic products of one bacteria’s metabolism eg. Propionic acid, peroxide, nitrite, H2S or alcohol.

This is the way commensals provide protection and stop pathogens from growing too much.

When one organism changes its surroundings so that another can grow there at a later time.

This happens in some food fermentations such as in sauerkraut production were one bacterial population follows another as the acidity gradually increases. Similar changes happen in cheese production.

Biophiles are organism that can only grow on live hosts and are usually parasites ie cause some damage hence they are called pathogens. However some effects can be too small to measure. Such conditions are known as silent infections.

Generally a biophile adapts to its host so as to do, as little damage as possible and the host grows resistant to the parasite until they reach equilibrium. At that stage it is difficult to say the parasite is not a commsal. With prolonged association commensals and silent infections may mutate to become dependant on each other thus becoming mutualistic. When parasites are well adapted to there host they usually become host specific ie able to grow on only one host. They also lose ability to compete with saprophytes when living on dead material.

Necrophiles only grow on dead material (dead "living things") they may cause disease by producing toxins that kill of a host and then live in the dead remains

Saprophytes are the same as necrophiles. ie. Adapted for living on dead material. Saprophytes have the opposite adaptations to parasites. i.e. they grow quickly, produce lots or enzymes and many toxins. This allows it to colonise new material faster than other organisms. Parasites adept to grow slowly produce few enzymes and toxins. This allows them to live in a living host without detection and to avoid the host’s immune system.

Many of our worst disease are caused by microorganisms that are poorly adapted to there host and still produce lots of bad products such as enzymes and toxins. These organisms may normally grow as saprophytes or commensals and produce infections when the host is damaged or compromised.

These opportunists are often "week pathogens" that are not good at establishing a disease in healthy adults. But when they get established they do more damage than the well-adapted parasites.