Keeping their options open: acute versus persistent infections.

Among the reasons for the growing interest in studying biofilm formation is the role of these microbial communities in chronic infections (24, 38). Such biofilm-like chronic infections include the respiratory infections caused by Pseudomonas aeruginosa in the cystic fibrosis (CF) lung (7), relapsing otitis media primarily caused by Haemophilus influenzae (41), and staphylococcal lesions in endocarditis (17). It is also important to note, however, that all of these microbes can also contribute to acute infections in human patients and, in fact, are among the most feared nosocomial pathogens (48). So, how do these organisms cause acute infections in some settings and chronic infections in others? A series of recent papers suggests that bacteria can choose which strategy they employ, either causing an acute infection, growing and spreading rapidly in the host, or, alternatively, adopting a chronic, biofilm infection strategy. While replication in the context of the chronic infection is likely less rapid, bacteria involved in these long-term infections can persist for extended periods of time and continue to shed planktonic (e.g., free-swimming) bacteria, as well as biologically active molecules, into the host during the course of the persistent infection (69). Establishing that acute and chronic infections are distinct processes requires we demonstrate that these are really two different strategies employed by microbes when interacting with a host. That is, do organisms differ in the molecular mechanisms utilized to cause acute versus chronic infections? Furthermore, a single microbe must presumably have the capability to cause both acute and persistent infections. Take P. aeruginosa as an example. In some cases, P. aeruginosa is capable of causing pneumonia, breaking down lung defenses and disseminating in the bloodstream, leading to death of a patient within hours or days. This organism has a type III secretion system (TTSS) and produces a variety of extracellular toxins that are thought to play a role in acute infections such as pneumonia (4, 22, 35). For example, the TTSS of P. aeruginosa has been shown recently to play a role in survival of P. aeruginosa in the blood and in systemic dissemination (57). Similarly, an exsA mutant of P. aeruginosa, which is defective for expression of the TTSS, and a strain lacking the TTSS effectors ExoT and ExoU were defective in causing acute corneal disease (32). The TTSS has also been characterized by its role in rapidly killing cultured cells in vitro (4). In addition to its TTSS, P. aeruginosa produces a variety of other virulence factors required for pathogenesis, as shown in acute models of burn and corneal infections in mice, in the invertebrate Caenorhabditis elegans, and in the plant model Arabidopsis thaliana, including quorum-sensing molecules (52, 53, 70), elastase (21, 35), hydrogen cyanide (20, 43, 44), type IV pili (TFP) (71), and lipopolysaccharide (42, 67). Historically, studies of pathogenesis have focused on virulence factors required for acuteinfection pathways. Again using P. aeruginosa as an example, this same organism capable of causing the acute infections described above also participates in a chronic infection of the lungs of CF patients. This chronic infection can last for decades, but P. aeruginosa rarely if ever reaches the bloodstream, indicating that the acute versus chronic infections caused by this microbe may be quite distinct. In contrast to acute infections, chronic infections have received less attention, likely due to more-complex animal models and difficulty in modeling these infections in vitro. The concept that bacterial biofilm growth may be responsible for some chronic in vivo infections has gained recent support (13, 15, 19). What evidence supports the idea that biofilms are analogous to chronic infections? At this stage, the data are intriguing but in most cases not definitive. For example, there are few well-defined markers of in vitro biofilm growth that can be correlated with putative in vivo biofilm-like infections. Chronic infection of the CF lung by P. aeruginosa has been postulated to be a biofilm-type infection. Singh and colleagues (51) reported synthesis profiles of quorum-sensing signals in CF sputum samples that were consistent with in vitro profiles of these molecules observed for biofilm rather than planktonic bacteria. Furthermore, images of polysaccharide-encased bacterial clusters in CF sputum are also consistent with the idea that P. aeruginosa exists in a biofilm or biofilm-like state in the CF lung (51). Another key trait associated with biofilms is their resistance to antibiotics (34). This resistance phenotype is shared by P. aeruginosa in the CF lung; this microbe is notorious for the fact that it cannot be eliminated from the CF lung by use of current antibiotic therapies (10). Studies of H. influenzae with otitis media model systems also support a role for a biofilm existence for these microbes, including direct microscopic visualization of bacterial communities in the middle ear and recalcitrance to antibiotic treatment (40, 41). Similarly, staphylococcal endocarditis is associated with compact bacterial cell masses surrounded by a matrix and the ability to withstand extended antibiotic treatment (38). Biofilms have been conclusively demonstrated to form on a variety of medical and surgical implants; like other biofilm * Corresponding author. Mailing address: Department of Microbiology and Immunology, Rm. 505 Vail Building, Dartmouth Medical School, Hanover, NH 03755. Phone: (603) 650-1248. Fax: (603) 6501245. E-mail: [email protected].