Although normal flora bacteria are not normally pathogenic, disease may result from invasion of normal flora into normally sterile areas or if the host immune system is deficient. When bacteria that normally reside in the GI tract (such as E. coli) are introduced to the urinary tract, for example, a urinary-tract infection may result. This is considered an endogenous infection.
Exogenous infections result from invasion of noncommensal organisms (i.e., those not normally found on the human body). Transmission of exogenous bacteria may occur by various routes, including inhalation of aerosolized organisms, ingestion (e.g., contaminated food or utensils), or direct contact of a wound or mucous membrane with organisms.
When bacteria first enter the body, local inflammation may be the first sign of infection. Physical symptoms such as pain, erythema (redness), edema (swelling), or pus formation result from the response of the immune system against the invading bacteria. If the bacteria spread to the bloodstream (bacteremia), they may disseminate to and colonize at various sites in the body.
VIRULENCE FACTORS. Bacteria have developed numerous mechanisms that allow them to invade a host and colonize an otherwise inhospitable site to cause disease. Many of these mechanisms enhance their ability to cause disease in humans; such traits are called virulence factors. Some common virulence factors include:•
Bacterial growth. The byproducts of normal bacterial growth may cause tissue destruction if colonization has occurred in a normally sterile site. For example, Clostridium perfringens is a normal flora bacteria of the GI tract but may cause gas gangrene if it infects a wound or trauma site.
Release of toxins. Some pathogenic bacteria produce proteins (toxins) that are inevitably toxic to the host. An endotoxin is composed of lipopolysaccharides found in the outer membrane of gram-negative bacteria. Exotoxins are proteins produced intracellularly and secreted by either gram-negative or gram-positive bacteria.
Capsule formation. The polysaccharide layers of a capsule form a protective shield around a bacteria and help the cell to evade immune response.
Internalization. Some bacteria are able to escape intra-cellular killing when internalized by phagosomes and go on to survive in the cytoplasm (e.g., Mycobacterium tuberculosis). In this way they are protected from anti-body-mediated immune responses.
Granuloma formation. A granuloma is a lesion formed in response to infection by some intracellular pathogens. Viable bacteria are walled off in the granuloma and thus prevented from further colonization.
Antigenic mimicry. A bacterial cell may be able to trick the immune system by presenting antigens (molecules recognized by antibodies) that are similar to host antigens. Immunological cells therefore have difficulty distinguishing between the bacterium and a host cell.
Stéphanie Islane Dionne, The Gale Group Inc., Gale, Detroit,