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Interactions between humans and microorganisms are exceedingly complex and are far from being completely understood. What is known about the interactions between these two living entities plays an important role both in the practice of diagnostic microbiology and in the management of infectious diseases. Understanding these interactions is necessary for establishing methods to reliably isolate specific microorganisms from patient specimens and for developing effective treatment strategies. This chapter provides the framework for understanding the various aspects of host-microorganism interactions.
Humans and microorganisms inhabit the same planet, and their paths cross in many and varied ways so that interactions are inevitable. The most important point regarding host-microorganism interactions is that the relationships are always bidirectional in nature. The bias is to believe that humans dominate the relationship because humans use microorganisms in various settings, including the food and fermentation industry, as biologic insecticides for agriculture; to genetically engineer a multitude of products; and even for biodegrading our industrial wastes. However, as one realizes that microbial populations share the common goal of survival with humans, and that they have been successful at achieving that goal, then exactly which participant in the relationship is the user and which one is the used becomes much less clear. This is especially true when considering the microorganisms most closely associated with humans and human disease.
The complex relationships between human hosts and medically relevant microorganisms are best understood by considering the sequential steps in the development of microbial-host associations and subsequent development of infection and disease. The stages of interaction are shown in Figure 3-1 and include physical encounter between host and microorganism, colonization or survival of the microorganism on an internal (gastrointestinal, respiratory, or genitourinary tract) or external (skin) surface of the host, microbial entry, invasion, and dissemination to deeper tissues and organs of the human body, and resolution or outcome. Each stage is discussed from two perspectives: the human host's and the microorganism's.
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