Pseudomonas aeruginosa is a ubiquitous Gram-negative microorganism that thrives in many environments, from soil and water to animals and people. It is an opportunistic pathogen that can cause respiratory infections, urinary tract infections, gastrointestinal infections, keratitis, otitis media, and bacteremia. P. aeruginosa is the fourth most common nosocomial pathogen, accounting for approximately 10% of hospital-acquired infections (www.cdc.gov). Immunocompromised patients, such as those undergoing cancer treatment or those infected with AIDS, burn patients, or cystic fibrosis (CF) patients, are susceptible to P. aeruginosa infections. These infections are difficult to treat by using conventional antibiotic therapies, and hence result in significant morbidity and mortality in such patients. The recalcitrant nature of P. aeruginosa infections is thought to be due to the organism's intrinsic antibiotic resistance mechanisms and its ability to form communities of bacteria encased in an exopolysaccharide matrix; such communities are known as biofilms.
P. aeruginosa possesses an impressive arsenal of virulence factors to initiate infection and persist in the host. These include secreted factors, such as elastase, proteases, phospholipase C, hydrogen cyanide, exotoxin A, and exoenzyme S, as well as cell-associated factors, such as lipopolysaccharide (LPS), flagella, and pili. The expression of these factors is tightly regulated. Many factors are expressed in a cell-density-dependent manner known as quorum sensing (QS). Quorum sensing, or cell-to-cell communication, is a means by which bacteria can monitor cell density and coordinate population behavior. The behavior was first identified in Vibrio fischeri as a mechanism to induce bioluminescence (20).