THE HUMAN HOST’S PERSPECTIVE
Because microorganisms are found everywhere, human encounters are inevitable, but the means of encounter vary widely. Which microbial population a human is exposed to and the mechanism of exposure are often direct consequences of a person’s activity or behaviors. Certain activities carry different risks for an encounter, and there is a wide spectrum of activities or situations over which a person may or may not have absolute control. For example, acquiring salmonellosis because one fails to cook the holiday turkey thoroughly is avoid able, whereas contracting tuberculosis as a consequence of living in conditions of extreme poverty and overcrowding may be unavoidable. The role that human activities play in the encounter between humans and microorganisms cannot be overstated, because most of the crises associated with infectious disease could be avoided or greatly reduced if human behavior and living conditions could be altered.
Microbial Reservoirs and Transmission
Humans encounter microorganisms when they enter or are exposed to the same environment in which the microbial agents live or when the infectious agents are brought to the human host by indirect means. The environment, or place of origin, of the infecting agent is referred to as the reservoir. As shown in Figure 1, microbial reservoirs include humans, animals, water, food, air, and soil. The human host may acquire microbial agents by various means referred to as the modes of transmission. The mode of transmission is direct when the host directly contacts the microbial reservoir and is indirect when the host encounters the microorganism by an intervening agent of transmission.
Fig1. Summary of microbial reservoirs and modes of transmission to humans.
The agents of transmission that bring the microorganism from the reservoir to the host may be a living entity, such as an insect, in which case they are called vectors, or they may be a nonliving entity, referred to as a vehicle or fomite. Additionally, some microorganisms may have a single mode of transmission, whereas others may spread by various methods. From a diagnostic microbiology perspective, knowledge about an infectious agent’s mode of transmission is often important for determining optimum specimens for isolation of the organism and for implementing precautions that minimize the risk of laboratory-acquired infections.
Human and Microbe Interactions
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 a microorganism 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 through blood transfusions; gonorrhea, syphilis, and acquired immunodeficiency syndrome through sexual contact; tuberculosis through 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 by means of contaminated medical devices helps disseminate infections in hospitals. Hospital acquired, health care−, or long-term care−associated infections are referred to as nosocomial infections.
Animals as Microbial Reservoirs
Infectious agents from animal reservoirs can be transmit ted directly to humans through an animal bite (e.g., rabies) or indirectly 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 acquiring or depositing them in water and food supplies. For example, beavers are often heavily colonized with parasites that cause infection of the human gastrointestinal tract. These para sites may be encountered and subsequently acquired when stream water becomes contaminated by the beaver and is used by the vacationing camper. Alternatively, animals used for human food carry numerous bacteria (e.g., Salmonella and Campylobacter) that, if not destroyed through appropriate cooking during preparation, can cause severe gastrointestinal illness.
Many other infectious diseases are encountered through direct or indirect animal contact, and information regarding a patient’s exposure to animals is often a key component in the diagnosis of these infections. Some microorganisms primarily infect animal populations and on occasion accidentally encounter and infect humans. When a human infection results from such an encounter, it is referred to as a zoonotic infection.
Insects as Vectors
The most common role of insects (arthropods) in the transmission of infectious disease is as vectors rather than as reservoirs. A wide variety of arthropods transmit viral, parasitic, and bacterial disease from animals to humans, whereas others transmit microorganisms between human hosts without an intermediate animal reservoir. Malaria, a deadly disease, is a prime example of an infectious disease maintained in the human population by the feeding and survival of an insect vector, the mosquito. Still other arthropods may themselves be agents of disease. These include organisms such as lice and scabies, which are spread directly between humans and cause skin irritations but do not penetrate the body. Because they are able to survive on the skin of the host without gaining access to internal tissues, they are referred to as ectoparasites. In addition, nonfungal infections (e.g., tetanus) may result when microbial agents in the environment, such as endo spores, are mechanically introduced by the vector as a result of a bite, scratch, or other penetrating wound.
The Environment as a Microbial Reservoir
The soil and natural environmental debris are reservoirs for countless types of microorganisms. Therefore, it is not surprising that these also serve as reservoirs for microorganisms that can cause infection in humans. Many of the fungal agents (see Part V: Mycology) are acquired by inhalation of soil and dust particles containing microorganisms (e.g., San Joaquin Valley fever). Other, nonfungal infections (e.g., tetanus endospores) may result when microbial agents in the environment are introduced into the human body as a result of a penetrating wound.
THE MICROORGANISM’S PERSPECTIVE
Clearly, numerous activities can result in human encounters with many microorganisms. Because humans are engaged in all of life’s complex activities, the tendency is to perceive the microorganism as having a passive role in the encounter process. However, this assumption is a gross oversimplification.
Microorganisms are also driven by survival, and the environment of the reservoirs they occupy must allow their metabolic and genetic needs to be fulfilled. Reservoirs maybe inhabited by hundreds or thousands of different species of microorganisms. Yet human encounters with the reservoirs, either directly or indirectly do not result in all species establishing an association with the human host. Although some species have evolved strategies that do not involve the human host to ensure survival, others have included humans to a lesser or greater extent as part of their survival tactics. Therefore, the latter type of organism often has mechanisms that enhance its chances for human encounter.
Depending on factors associated with both the human host and the microorganism involved, the encounter may have a beneficial, disastrous, or inconsequential impact on each of the participants.
