HOST-MICROBE INTERACTION IN THE GASTROINTESTINAL-TRACT

In order for an infection to occur, the target organ must come in cont act with sufficient microbes, the microbe must possess specific virule nce factors, these virulence factors must be expressed, and the defens es of the organ system must be overcome. This dynamic process, which i s ongoing in all living entities, can be described by the following re lationship: Infection proportional to [Number of Microbes]x[Virulence Characteristics]/Immune Status of the Host The establishment of infect ion first occurs in a particular organ. This phenomenon is known as ti ssue trophism and the association of microbes with organ systems gover ns the practice of clinical microbiology and infectious disease. With some microbes (e.g., Giardia, Cryptosporidium) the interaction with th e particular organ is so specific that infections are almost always co nfined to one site; with others (e.g., Salmonella, enterovirus) the mi crobe has the potential to become systemic. When attempting to establi sh health risk assessment from microbes by contact with food and drink ing water, one must therefore consider that the gastrointestinal tract is a complex organ system with a variety of specific host defense mec hanisms. It is only when the microbe has particular virulence factors for sites in gastrointestinal tract, and the specific host defense mec hanisms in the gastrointestinal tract are breached, that infection of this organ system occurs. Therefore, the general terms ''immunosuppres sion'' or ''immunocompromise'' are meaningless unless the specific imm une defect is known. A description of the microbial virulence factors active against the gastrointestinal tract and the defense mechanisms o f this organ system are reviewed to provide a biological basis health risk assessment and future food and drinking water regulations.