Their categorization as nonpathogens may change, however, because of the adaptability of bacteria and the detrimental effect of modern radiation therapy, chemotherapy, and immunotherapy on resistance mechanisms. Finally, some bacteria, such as Lactobacillus acidophilus, are considered to be nonpathogens, because they rarely or never cause human disease. They may be the agents of disease (e.g., in patients who have been predisposed to urinary tract infections with Escherichia coli by catheterization). Opportunistic pathogens are those isolated from patients whose host defense mechanisms have been compromised. When isolated from a patient, frank or primary pathogens are considered to be probable agents of disease (e.g., when the cause of diarrheal disease is identified by the laboratory isolation of Salmonella spp.
On this basis, bacteria can be organized into three major groups.
The capacity of a bacterium to cause disease reflects its relative pathogenicity. Siderophores: Siderophores are iron-binding factors that allow some bacteria to compete with the host for iron, which is bound to hemoglobin, transferrin, and lactoferrin. Major categories include cytotoxins, neurotoxins, and enterotoxins. Invasion Factors: Surface components that allow the bacterium to invade host cells can be encoded on plasmids, but more often are on the chromosome.Ĭapsules: Many bacteria are surrounded by capsules that protect them from opsonization and phagocytosis.Įndotoxins: The lipopolysaccharide endotoxins on Gram-negative bacteria cause fever, changes in blood pressure, inflammation, lethal shock, and many other toxic events.Įxotoxins: Exotoxins include several types of protein toxins and enzymes produced and/or secreted from pathogenic bacteria. The following are types of virulence factors:Īdherence Factors: Many pathogenic bacteria colonize mucosal sites by using pili (fimbriae) to adhere to cells. Virulence factors help bacteria to (1) invade the host, (2) cause disease, and (3) evade host defenses.
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