This era may come to be remembered as one in which infectious diseases made a dramatic worldwide resurgence, owing to the rise of antibiotic resistance and the dwindling and startlingly scarce number of candidate antibiotics in the drug development pipeline. Our current arsenal of antibiotics is comprised of variations on the single theme of cell killing, i.e. drugs that target processes essential to bacterial viability. Targeting bacterial virulence is an alternative and attractive approach to the development of new antimicrobials that can be used to disarm pathogens in the host, increase the number of available therapeutics, and protect the effectiveness of current antibiotics. The overall strategy is to inhibit specific mechanisms that promote infection and are essential to persistence in a pathogenic cascade, though not required for cell viability per se.
A biofilm is an accumulation of microorganisms (bacteria, fungi, and/or protozoa, with associated bacteriophages and other viruses) embedded in a polysaccharide matrix and adherent to solid biological or non-biotic surfaces. Biofilms are medically important, accounting for over 80 percent of hospital-acquired microbial infections in the body. Examples include infections of the: oral soft tissues, teeth and dental implants; middle ear; gastrointestinal tract; urogenital tract; airway/lung tissue; eye; urinary tract prostheses; peritoneal membrane and peritoneal dialysis catheters, indwelling catheters for hemodialysis and for chronic administration of chemotherapeutic agents (Hickman catheters); cardiac implants such as pacemakers, prosthetic heart valves, ventricular assist devices, and synthetic vascular grafts and stents; prostheses, internal fixation devices, percutaneous sutures; and tracheal and ventilator tubing. The microorganisms tend to be far more resistant to antimicrobial agents and to be particularly difficult for the host immune system to render an appropriate response.
Biofilms are remarkably difficult to treat with antimicrobials. Antimicrobials may be readily inactivated or fail to penetrate into the biofilm. In addition, bacteria within biofilms have increased (up to 1000-fold higher) resistance to antimicrobial compounds, even though these same bacteria are sensitive to these agents if grown under planktonic conditions.
Biofilms play a significant role in the transmission and persistence of human disease and have emerged as virulence hallmarks of serious and persistent infectious diseases, including cystic fibrosis pneumonia, infective endocarditis, urinary tract infection (UTI), periodontitis, chronic infections of the middle ear, and infections of medical devices such as intravenous catheters and artificial joints. Currently available antibiotics often fail to eradicate biofilm-associated bacteria, necessitating multiple and intense antibiotic treatment regimens that drive the evolution of resistant pathogens and the exhaustion of last-resort antibiotics. As a consequence, biofilm-associated infections are the cause of significant morbidity and mortality in the clinic.
UTIs, which include infections of the bladder (cystitis) and kidney (pyelonephritis), are among the most common bacterial infections. Nearly 13 million women per year suffer from UTIs in the United States, and more than half of all women will experience a UTI during their lifetimes. A woman treated for an uncomplicated UTI has a 25-50% chance of developing a recurrent infection within one year of the primary infection and most are caused by the same bacterial strain as the initial infection. Furthermore, approximately one fourth of the yearly $4 billion cost attributed to nosocomial infections is a consequence of UTI, most of which are catheter-associated UTIs. Unfortunately, limited treatment options are available for patients with chronic and recurrent UTIs. These patients are typically given prolonged courses of antibiotics, which radically disrupt the symbiotic host microbiota and may be accompanied by the evolution of drug-resistant organisms in the urinary tract. In addition, UTIs have a strong causal correlation with systemic infection and sepsis if antibiotic therapy is ineffective.
Uropathogenic Escherichia coli (UPEC) are the most common etiologic agents, responsible for 80 to 85% of community-acquired UTIs. UPEC engage in a coordinated and regulated genetic and molecular pathogenic cascade that involves several distinct phases as examined in the mouse cystitis model and human UTIs. UPEC are thought to emerge primarily from the distal genitourinary tract and ascend the urethra into the bladder. UPEC bind to and invade the superficial umbrella cells that line the bladder lumen, where they rapidly replicate to form a biofilm-like intracellular bacterial community (IBC). In the IBC, bacteria find a safe haven where they are resistant to antibiotics, and subvert clearance by innate host responses. Even after acute infection is resolved, bacteria can persist within the bladder for many days to weeks, regardless of standard antibiotic treatments, and can be source of recurrent urinary tract infections. In the pathogenesis of catheter-associated UTI, UPEC form robust biofilms on urinary catheters and can serve as the pioneer pathogens to initiate the infectious cascade as well as more serious sequelae including bacteremia. Whether in the intracellular niche or on catheter surfaces, targeting UPEC biofilm formation has emerged as a ripe candidate for the development of anti-virulence therapeutics.
Several frank bacterial pathogens have been shown to associate with, and in some cases, grow in biofilms, including Legionella pneumophila, S. aureus, Listeria monocytogenes, Campylobacter spp., E. coli O157:H7, Salmonella typhimurium, Vibrio cholerae, and Helicobacter pylori. Although all these organisms have the ability to attach to surfaces and existing biofilms, most if not all appear incapable of extensive growth in the biofilm. This may be because of their fastidious growth requirements or because of their inability to compete with indigenous organisms. Survival and growth of pathogenic organisms within biofilms might also be enhanced by the association and metabolic interactions with indigenous organisms.
Bacteria embedded within biofilms are resistant to both immunological and non-specific defense mechanisms of the body. Contact with a solid surface triggers the expression of a panel of bacterial enzymes, which catalyze the formation of sticky polysaccharides that promote colonization and protection. The structure of biofilms is such that immune responses may be directed only at those antigens found on the outer surface of the biofilm, and antibodies and other serum or salivary proteins often fail to penetrate into the biofilm. In addition, phagocytes are unable to effectively engulf a bacterium growing within a complex polysaccharide matrix attached to a solid surface. This causes the phagocyte to release large amounts of pro-inflammatory enzymes and cytokines, leading to inflammation and destruction of nearby tissues.
In view of the importance of biofilms for microbial infection, methods of understanding and manipulating biofilm dissolution are of great interest, and are addressed herein.
Publications
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