Life as we know it would not exist without bacteria to decompose waste and dead organisms. These bacteria ensure that the cycle of chemical exchange between organisms and their environment is continuous.
The decision as to whether bacteria are friend or foe becomes more difficult when both the positive and negative aspects of the relationship between humans and bacteria are considered. Let's discuss three types of symbiotic relationships: commensalism, mutualism, and parasitism.
Commensalism is a relationship that is beneficial to the bacteria which live off of the host, but does not help or harm the host. Most of the bacteria that reside within the bodies of humans are commensalistic.
In a mutualistic relationship, both the bacteria and the host benefit. For example, there are several kinds of bacteria which live inside the mouth, nose, throat, and intestines of humans and animals. These bacteria receive a place to live and feed while keeping other harmful microbes from taking up residence.
A parasitic relationship is one in which the bacteria benefit while the host is harmed. Pathogenic parasites, which cause disease, do so by resisting the host's defenses and growing at the expense of the host. These bacteria produce poisonous substances called endotoxins and exotoxins which are responsible for the symptoms that occur with an illness.
When all of the facts are considered, bacteria are more helpful than harmful. Humans have exploited bacteria for a wide variety of uses, such as: making cheese and butter, decomposing waste in sewage plants, and developing antibiotics. Bacteria have been able to survive without us, but we could never live without them.
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