Colonization, Infection, and Disease

COLONIZATION, INFECTION, AND DISEASE

Relatively few anatomic sites (eg, brain, blood, bone, heart, vas-cular system) are sterile. Bacteria found throughout the body usu-ally provide beneficial normal flora to compete with potential pathogens, to facilitate digestion, or to work in other ways sym-biotically with the host.

Colonization

The term colonization is used to describe microorganisms pres-ent without host interference or interaction. Understanding the principle of colonization facilitates interpretation of micro-biologic reports. Organisms reported in microbiology results often reflect colonization rather than infection.

Infection

Infection indicates a host interaction with an organism. A patientcolonized with S. aureus may have staphylococci on the skin with-out any skin interruption or irritation. If the patient had an inci-sion, S. aureus could enter the wound, with an immune system reaction of local inflammation and routing of white cells to the site. Clinical evidence of redness, heat, and pain and laboratory evidence of white cells on the wound specimen smear suggest infection. In this example, the host identifies the staphylococci as foreign. Infection is recognized by the host reaction and by organism identification.

Disease

It is important to recognize the difference between infection and disease. Infectious disease is the state in which the infected hostdisplays a decline in wellness due to the infection. When the host interacts immunologically with an organism but remains symp-tom free, the definition of disease has not been met. M. tubercu-losis is an example of an organism that often persists as infectionwithout producing disease. The host may become infected after exposure to the tubercle bacillus. The person is infected when bac-teria are first detected by nonspecific immunologic recognition and later as newly sensitized T cells propagate daughter lines of TB-specific protective cells. After this initial infection, the un-treated host has a low probability of actually becoming ill. About 90% of hosts infected with the M. tuberculosis do not develop TB, the disease. Figure 70-2 depicts response to bacterial infection at the cellular level and at the host level.