We have stressed on more than one occasion during the course of this book that microorganisms in nature do not exist as pure cultures but alongside numerous other organisms, microbial or otherwise, with which they may have to compete in the never-ending struggle for survival. In a number of cases, this coexistence may extend beyond merely sharing the same environmental niche; some microorganisms form a close physical association with another type of organism, from which special benefits may accrue for one or both parties. Such associations are termed collectively symbiosis ('living together') (Table 15.1). Three general forms of symbiotic relationship may be defined:
• Parasitism: an association from which one partner derives some or all of its nutritional requirements by living either in or on the other (the host), which usually suffers harm as a result.
• Mutualism: an association from which both participants derive benefit. The relationship is frequently obligatory, that is, both are dependent upon the other for survival. Non-obligatory mutualism is sometimes called protocooperation.
• Commensalism: an association from which one participant (the commensal) derives benefit, and the other is neither benefited nor harmed. The relationship is not usually obligatory.
Microorganisms may be associated with plants, animals or other types of microorganism in any of these types of symbiosis (Tables 15.2-15.4).
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