There are two types of amensalism, competition and antagonism (or antibiosis). Resistance can be seen as antixenosis, tolerance and antibiosis. Resistance may be conferred by antibiosis, antixenosis, or tolerance. Three main types of plant resistance to pests are recognized as nonpreference, antibiosis, and tolerance. The various mechanisms include antibiosis, parasitism, inducing host-plant resistance, and competition. These observations of antibiosis between microorganisms led to the discovery of natural antibacterials.