Pseudomonas aeruginosa

Microbiology

Pseudomonas aeruginosa is a gram-negative bacillus found widely in nature, soil, and water. Classified as an opportunistic pathogen, P. aeruginosa causes disease infrequently in normal hosts but is a major cause of infection in patients with underlying or immunocompromising conditions. The genome of P. aeruginosa , which is especially large for a prokaryote, has provided an understanding of the metabolic and pathogenic mechanisms that underlie the success of this versatile pathogen, and it has become a model for understanding microbial genomic variation and evolution in chronic disease. P. aeruginosa has few nutritional requirements and can adapt to conditions not tolerated by other organisms. It does not ferment lactose or other carbohydrates but oxidizes glucose and xylose. Organisms grow aerobically or anaerobically if nitrate is available as an inorganic electron acceptor, as is the case in the lungs of patients with cystic fibrosis (CF). P. aeruginosa gene expression responds to environmental conditions with discrete patterns typical of environmental isolates: motility, piliation, and expression of numerous exoproducts. In subacute and chronic infections, the accumulation of intracellular dinucleotides (cyclic diguanidine monophosphate) favors a biofilm mode of growth with the formation of an extracellular polysaccharide matrix, thus enabling the organisms to avoid innate immune clearance mechanisms and persist in human airways.

The organism produces the fluorescent siderophores pyoverdin and pyochelin, which function to scavenge iron. Redox-active phenazines such as pyocyanin, the pigment that gives P. aeruginosa its characteristic blue color, play an important role in electron transport especially under microaerophilic conditions, increase the bioavailability of iron, and enhance virulence through oxidative stress. ^, P. aeruginosa can be identified biochemically as having indophenol oxidase-positive, citrate-positive, and l -arginine dehydrolase-positive activity. Differentiation of P. aeruginosa from other pseudomonads or organisms such as Burkholderia species, Stenotrophomonas maltophilia, and Achromobacter spp. occasionally can require testing for DNAse activity, growth at 42°C, and differential carbohydrate metabolism or using molecular methods.

Virulence Factors and Pathogenesis

Multiple virulence factors and their purported roles in pathogenesis are shown in Table 155.1 . P. aeruginosa thrives under conditions that would be adverse to many other bacteria with only a minimal carbon source and a moist environment. P. aeruginosa is intrinsically resistant to many classes of antimicrobial agents and can acquire additional resistance genes from other organisms, thus flourishing under selective pressures that eliminate competing flora. Depending on specific environmental conditions (e.g., availability of nutrients, especially iron) and immune responses, specific virulence factors are expressed in a highly regulated manner. Hosts without underlying conditions occasionally experience serious P. aeruginosa infections if a break in normal barrier defenses occurs through puncture wounds, skin trauma, or burns. More commonly, P. aeruginosa acts as an opportunist, infecting patients who have defective mucosal immunity, impaired pulmonary clearance as in CF, or disease-induced or iatrogenic neutropenia. ^,

P. aeruginosa expresses numerous virulence factors that stimulate both airway epithelial cells and professional immune cells to produce proinflammatory cytokines and chemokines, such as interleukin-8 (IL-8), IL-17A/F, granulocyte-macrophage colony-stimulating factor, and mucin. Secreted exoenzymes interact with specific eukaryotic targets to affect cytoskeletal components and thereby disrupt the integrity of the epithelial tight junctions and induce inflammation. Products of type III secretion systems (T3SS) are expressed commonly in acute infection, often in the setting of hospital-acquired pneumonia, by causing cytotoxicity and facilitating invasion. The T3SS effector proteins include exoenzyme U (ExoU), a potent phospholipase A ₂ that requires host superoxide dismutase for activation and often is associated with severe pneumonia ; ExoS or ExoT, closely related toxins with both adenosine diphosphate (ADP) ribosyltransferase activity and Rho GTPase activating protein activity, are expressed commonly in clinical isolates from acute infections. The T3SS virulence factors have been associated with severe P. aeruginosa pneumonia in hospitalized patients, and they have been targeted as potential antigens in Pseudomonas vaccines and immunotherapy.

The more recently discovered type VI secretion systems (T6SS) export toxins for interbacterial competition, but the secreted effectors also enhance invasion of host cells through activation of the PI3K/Akt pathway to cause actin rearrangement. Other bacterial products such as phenazines (e.g., pyocyanin), initially considered to be metabolic waste products, have strong effects on other micro-organisms and also are toxic to host cells through oxidative stress.

Whereas lipopolysaccharide often is considered to be the major immunostimulatory factor in gram-negative bacteria, P. aeruginosa lipopolysaccharide does not activate proinflammatory signaling as potently as does lipopolysaccharide of other organisms. P. aeruginosa flagella are important in pathogenesis signaling through toll-like receptor 5 (TLR5) and the inflammasome leading to proinflammatory responses to the organisms. Indeed, the inflammasome plays a central role in the immune response to P. aeruginosa by being activated by bacterial products including flagellins, ^, type IV pilins, T3SS components, ^, and the toxin RhsT. Inflammasome activation also appears to be dampened by several of the T3SS effectors. ^, Whether the inflammasome response to P. aeruginosa is beneficial or detrimental to the host, either by aiding to clear bacterial infection or by causing increased tissue destruction, is likely to depend on the specific clinical circumstances.

During infection of patients with CF, shedding of bacterial immunostimulants activates cytokine and chemokine production by airway epithelial cells and recruited immune cells. After the initial steps of colonization, P. aeruginosa undergoes gene regulatory changes that aid in the establishment of persistent infection. The milieu within the CF-affected airways is relatively anaerobic, a property that affects both bacterial gene expression and antimicrobial susceptibility. Once a sufficient density of organisms is resident in the airways, bacterial gene expression is regulated by a quorum-sensing system, which involves the secretion of freely diffusible homoserine lactones that act along with transcriptional activators to coordinate the gene expression of the entire population of bacteria. Quorum sensing facilitates a biofilm mode of growth by turning off the expression of the more immunostimulatory gene products such as exoenzymes and flagella while favoring the secretion of exopolysaccharides. Although few organisms are directly adherent to the epithelial surface, a high density of organisms persists in biofilm-like aggregates in the airways as a result of reduced mucociliary clearance and loss of airway surface hydration. Because organisms within biofilms are relatively less susceptible to antimicrobial agents and phagocytosis, the ability to persist in biofilms provides a major selective advantage to the species.

In chronic CF infections, P. aeruginosa strains tend to accumulate genetic changes that diminish or eliminate expression of proinflammatory bacterial products such as flagella or pili with the appearance of muc mutants with copious production of the extracellular alginate, a phenotype that is virtually pathognomonic for CF isolates. Although resistant to phagocytosis, these organisms are not invasive, but they chronically infect the airways. Together, these changes have a profound impact on host inflammation. Long-term infections also lead to high levels of phenotypic and genomic diversity that likely have an impact on the overall fitness of the infecting bacterial population. This reproducible evolutionary tendency has been studied in detail using whole genomes, ^, and it may be enhanced by higher rates of mutation in these strains. The tendency to adapt in specific ways suggests that chronic inflammation may not be beneficial for the bacterium. More recently, changes in metabolism also have been implicated in the adaptive trajectory of P. aeruginosa to the CF niche.

Epidemiology and Clinical Manifestations

P. aeruginosa is associated with a wide variety of infections, depending on the nature of the host and the severity of underlying disease, ranging from self-limited folliculitis to overwhelming septic shock. Because of its exceptional genetic plasticity, P. aeruginosa adapts readily to healthcare settings and is a major cause of healthcare-associated infections, including ventilator-associated pneumonia and infections of indwelling catheters. Genomic sequencing studies suggest that a relatively limited number of clones may be responsible for these infections, but with extensive phenotypic diversity within clones. Patients with CF generally have a phase of intermittent colonization with a variety of distinct P. aeruginosa strains, followed by chronic infection with single clones with adapted virulence profiles. Although most strains appear to be acquired from the environment, several transmissible strains (e.g., Liverpool Epidemic Strain) have been identified, ^, and recent studies suggest that there may be more sharing of strains between CF patients than previously thought. Tables 155.2 and 155.3 show the epidemiology and clinical manifestations of P. aeruginosa infection in normal and special hosts, respectively.