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):
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. 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
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, ...