From what you hear on the news or read in the newspaper; you might have concluded that most bacteria are pathogens that cause diseases. However, this generalization is wrong. In fact, the vast majority of bacteria around us are either essential or harmless to the life of other organisms (including ourselves). Humans are covered with bacteria (both inside and out). You may find this fact unsettling, but right now there are approximately 400 different species of bacteria living in your gastrointestinal system.
Only a relatively small percentage of the bacterial species that colonize us (and all other multicellular organisms) are pathogenic. The vast majority of bacteria are ecologically significant and beneficial. In most instances, bacteria colonize on and live harmlessly with other species.
The state of any two species having an extended and intimate association is termed symbiosis .
The interacting organisms (symbionts) have varying relationships with one another. The host (the larger of two species) and the microsymbiont (the smaller of the two species) may derive mutual benefit from the association, in which case the symbiosis is termed mutualism . At the other end of the symbiotic spectrum is a relationship whereby one member of the symbiosis derives benefit at the expense of the other, which is termed parasitism . In the middle of this spectrum are associations which appear benign (are neither of benefit nor harm) to either or both species; this form of symbiosis is termed commensalism . These are not fixed relationships, and the association between the symbionts is not constant and can change, depending on conditions. For example, a relationship may be commensalistic (or even mutualistic) most of the time, but under certain conditions the association can turn parasitic.
We can summarize these three basic types of symbiosis using a "+" to indicate a benefit from the relationship, and a "-" to indicate a harm from the relationship:
Symbiosis is not a static process. Commensalistic or mutualistic relationships can quickly change to parasitic relationships depending on environmental conditions. For instance, the bacteria in your intestines play an important role in processing food into absorbable nutrients. This symbiotic relationship is mutualistic, but a punctured intestine can release the bacteria into other parts of the body where they can quickly induce sepsis, an often fatal condition. This sort of event is known as an opportunistic infection , where a normally commensalistic bacteria becomes pathogenic in response to a changed environment (usually a weakened immune system).
The following video (from Stated Simply) is just for fun - if you are interested in the symbiotic relationships that can occur between bacteria and animals - watch on!