When does infection occur?
Colonization: the presence of organisms on a body surface or in a lumen,
but not producing disease
all persons have bacteria (and some fungi) on skin surfaces or in the oral cavity. The gastrointestinal tract has multiple bacterial species present, which can be beneficial in generating nutrients (vitamin K) and in suppressing growth of harmful organisms (such as Clostridium difficile). The organisms that form the 'normal flora' of a body site are non-pathogenic under normal circumstances. this complicates interpretation of culture results, because you must be
aware of the possibility of contamination of a specimen in the collection
process (e.g., did you have aseptic technique when the blood culture was drawn? Is the sputum specimen mostly oral flora?)
Invasion: organisms have moved into tissues to cause disease Virulence: the ability of an organism to cause infectious disease
some organisms, such as Staphylococcus epidermidis, diptheroids, or
Entameba nana, are unlikely to cause disease some organisms, such as Vibrio cholerae, Salmonella typhi, Mycobacterium
tuberculosis, or Yersinia pestis (plague) are highly infectious and potentially fatal
Resistance: the ability of the host to prevent infection from occurring
and infectious disease from developing. Resistance is normally aided by:
barriers to infection: intact, functional epithelial surfaces (mucociliary apparatus of respiratory tract, gastric mucosal acid production, antibacterial action of bladder mucosal secretions and saliva of oral cavity) immune system (inflammatory cells such as neutrophils and lymphocytes, as
well as immune globulins) normal anatomical structure (e.g., intact epithelium is a good barrier)
Resistance is diminished by:
debilitation from malnutrition (poor diet, alcoholism) neoplasia poorly functioning immune system (congenital or acquired) drug therapy - corticosteroids, antibiotics that alter normal flora previously damaged or abnormal anatomical structure from congenital,
circulatory, infectious, autoimmune disease, etc., particularly with obstruction
of any lumen or orifice
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