Biofilm formation represents a protected mode of growth that renders bacterial cells less susceptible to antimicrobials and to killing by host immune effector mechanisms and so enables the pathogens to survive in hostile environments and also to disperse and colonize new niches. Bacteria form biofilms as part of their survival mechanisms, and biofilms are thus ubiquitous in nature.

Bacterial biofilms are clusters of bacteria that are attached to a surface and/or to each other and embedded in a self-produced matrix. The biofilm matrix consists of substances like proteins (e.g., fibrin), polysaccharide (e.g., alginate), as well as DNA. In addition to the protection offered by the matrix, bacteria in biofilms can employ several survival strategies to evade the host defense systems. By staying dormant and hidden from the immune system, they may cause local tissue damage and later cause an acute infection. Within the biofilm, the bacteria adapt to environmental anoxia and nutrient limitation by exhibiting an altered metabolism, gene expression, and protein production, which can lead to a lower metabolic rate and a reduced rate of cell division. In addition, these adaptations make the bacteria more resistant to antimicrobial therapy by inactivating the antimicrobial targets or reducing the requirements for the cellular function that the antimicrobials interfere with. During a biofilm infection, simultaneous activation of both innate and acquired host immune responses may occur; neither of which are able to eliminate the biofilm pathogen, but instead accelerate collateral tissue damage. Consequently, biofilm-related diseases are typically persistent infections that develop slowly, are rarely resolved by the immune system, and respond inconsistently to antimicrobial treatments.

The primary infection in endocarditis is a biofilm composed of both bacterial and host components located on the cardiac valve. This biofilm causes disease in the following ways: The biofilm physically disrupts valve function, causing leakage when the valve is closed and turbulence as well as diminished flow when the valve is open; the biofilm provides a source for near-continuous infection of the bloodstream that are difficult to remove by antibiotic treatment; pieces of biofilm can break off and be carried to a terminal point in the circulation causing the brain, kidneys, and extremities particularly vulnerable to emboli.

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Microbiology: Current Research
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