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Bacteria possess a number of characteristics that facilitate the lifestyle of the organism representing a variety of functional activities, including transmission from host to host, attachment to host cells and tissues, invasion of host tissues, facilitation of growth in specific host environments, or avoidance of host defenses by immune evasion and intracellular growth. Occasionally, these factors may be related, either directly or indirectly, to damage and disease in humans and are then called virulence factors.

Bacterial infections are the result of complex interactions between the bacteria causing the infection and the human host in which the infection occurs. These host–bacterial interactions, known as the damage framework,1occur in a continuous dynamic and are summarized below (see also eFig. 222.1):

eFigure 222.1.

States of host–bacterial relationships involved in human infections with bacteria. The shaded states represent the host–bacterial relationships that contribute directly to human disease.

  1. 1. Exposure of the host to the bacteria is required for the initiation of bacterial disease and occurs in a variety of ways, such as skin exposure through direct cutaneous contact, respiratory tract exposure through inspiration, or intestinal exposure through ingestion.

    2. Acquisition of the bacteria occurs in the face of a direct interaction with the host, often through specific adherence mechanisms by which the organisms bind to specific receptors on human skin or mucosal surfaces.

    3. Persistence of bacteria in or on the human host is a factor of time (as short as minutes or as long as a lifetime), during which the bacteria remain attached to the host cells or tissues and host–bacterial interactive processes, such as adherence, engulfment, intoxication, or immunity, may occur. The persistence of bacteria in or on the host, with bacterial replication, is infection.2 A state of mutualism between host and bacteria with no host damage is called commensalism. Colonization represents the continuum of persistence ranging from absence of damage (commensalism) to stimulation of the host immune response leading to immune damage.

    4. Damage is the effect of the bacteria on the host and is the means by which disease occurs. The damage may result directly from bacterial factors, such as toxins, or from host immune factors stimulated by the presence of the bacteria.

    5. Disease is the clinical manifestation (ie, symptoms) of damage to the host related to the bacterial infection.

    6. Immunity is the host response to the presence of the bacteria and may involve either the innate or the adaptive immune system. Although immunity may protect the host from future infection or may contribute to clearance of the bacteria, it may also contribute to damage to the host cells or tissues.

    7. Clearance of the bacteria from the human host may occur during multiple states in the host–bacterial interaction and may result from a variety of processes, such as bacterial detachment with loss of adherence to host cells, ...

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