Microbial antagonism is the method of using established cultures of microorganisms to prevent the intrusion of foreign strains. When introduced to an already-colonized environment, an invasive strain of bacteria tends not to thrive and may go completely extinct.
An example of microbial antagonism in the human body is the resistance of established mouth bacteria to new strains that can be introduced via mouth-to-mouth contact. After a kiss, for example, new bacteria are introduced into the hostile environment of a foreign mouth. Once there, the invasive bacteria's growth is inhibited by antimicrobial compounds secreted by the native flora, as well as by the tight competition for resources.