Figwe 3-1 General stages of microbial-host interaction.

determining optimum specimens for organism isolation and for taking precautions that minimize the risk of laboratory-acquired infections (see Chapters 4 and 64 for more information regarding laboratory safety).

Humans as Microbial Reservoirs

Humans play a substantial role as microbial reservoirs. Indeed, the passage of a neonate from the sterile environment of the mother's womb through the birth canal, which is heavily colonized with various microbial agents, is a primary example of one human directly acquiring microorganisms from another human serving as the reservoir. This is the mechanism by which newborns first encounter microbial agents. Other examples in which humans serve as the microbial reservoir include acquisition of "strep" throat through touching; hepatitis by blood transfusions; gonorrhea, syphilis, and acquired immunodeficiency syndrome by sexual contact; tuberculosis by coughing; and the common cold through sneezing. Indirect transfer can occur when microorganisms from one individual contaminate a vehicle of transmission such as water (e.g., cholera) that is then ingested by another person. In the medical setting, indirect transmission of microorganisms from one human host to another via contaminated medical devices helps disseminate infections in hospitals. Hospital-acquired infections are referred to as nosocomial infections.

Animals as Microbial Reservoirs

From animal reservoirs infectious agents can be transmitted directly to humans such as through an animal bite (e.g., rabies) or indirectly such as through the bite of insect vectors that feed on both animals and humans (e.g., Lyme disease and Rocky Mountain spotted fever). Animals may also transmit infectious agents by water and food supplies. For example, beavers are often heavily colonized with parasites that cause infections of the human gastrointestinal tract. These parasites may be encountered and subsequently acquired when contaminated stream water is used by beaver and camper alike. Alternatively, animals used for human food carry numerous bacteria (e.g., Salmonella and Campylobacter) that if not destroyed by appropriate cooking during preparation, can cause severe gastrointestinal illness.

Many other infectious diseases are encountered through direct or indirect animal contact, and informa

Microorganism sources


Modes of transmission

1. Direct; transmitted by direct contact between reservoir and host

Humans Animals

Food (from plant and animal sources) Water Air Soil

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Pregnancy And Childbirth

Pregnancy And Childbirth

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