Pseudomonas, Burkholderia, and Stenotrophomonas species are non–lactose-fermenting gram-negative bacilli that are frequently multidrug resistant and are important pathogens, particularly in healthcare-associated infections in immunocompromised patients and in individuals with cystic fibrosis (CF). Previously, Burkholderia and Stenotrophomonas species were classified as Pseudomonas due to their similar biochemical properties. In 1992, the genus Burkholderia was proposed for 7 species. There has been a rapid increase in newly identified species to > 70 species that are very similar phenotypically, thus making taxonomic classification somewhat perplexing without the use of molecular strategies. The most commonly identified species within the Burkholderia cepacia complex are Burkholderia cenocepacia and Burkholderia multivorans. Stenotrophomonas was initially reclassified as Xanthomonas in 1983 and acquired its present classification in 1993. Stenotrophomonas maltophilia is the only species within its genus known to cause human disease.
EPIDEMIOLOGY AND PATHOGENESIS
Pseudomonas, Burkholderia, and Stenotrophomonas are ubiquitous environmental organisms found in water, soil, and plants, and are distributed worldwide. Some Burkholderia species are well-known causes of disease in plants. For example, B cepacia causes crop rot in onion fields.
Pseudomonas aeruginosa and S maltophilia are endemic in most hospital environments and can contaminate medical devices such as suction tubing, indwelling catheters, and sinks and faucets. P aeruginosa colonization of artificial nails worn by healthcare workers has been linked to outbreaks. Burkholderia species are less easily isolated from sources in the hospital environment, although B cepacia complex has a unique ability to grow in disinfectant solutions, which can result in nosocomial outbreaks. Similarly, intrinsic contamination of medications, including nasal sprays and bronchodilators, has been linked to human infections.
In individuals with CF, acquisition of environmental strains of P aeruginosa and Burkholderia species has been documented. There are well-documented instances of person-to-person transmission of both P aeruginosa and B cepacia complex.
One of the primary virulence mechanisms shared by these organisms is the ability to produce biofilms, which enable them to adhere to surfaces such as wounds and medical tubing, evade immune responses such as phagocytosis, and resist antimicrobial killing. Neutrophils are the most important defense against Pseudomonas, and thus, severe and prolonged neutropenia predisposes patients to severe infections. Similarly, patients with chronic granulomatous disease (CGD) who have impaired phagocytic killing due to defective function of NADPH oxidase are particularly susceptible to Burkholderia infections. Additional virulence factors include flagella and pili that aid these organisms in motility and facilitate binding to epithelial surfaces. In addition, they produce exotoxins that possess enzymatic activity and cause tissue damage. Mechanisms of antibiotic resistance that render these organisms quite difficult to treat include β-lactamase and carbapenemase production, multidrug efflux pumps, porin loss, and antibiotic modification via aminoglycoside-modifying enzymes.
The most clinically important Pseudomonas species is P aeruginosa. This opportunistic pathogen rarely causes disease in immunocompetent hosts. However, there are several well-described clinical ...