Burkholderia spp. and Ralstonia pickettii are inhabitants of the environment and are not considered part of the normal human flora (Table 1). As such, their trans mission usually involves human contact with heavily contaminated medical devices or substances encountered in the hospital setting.
Table1. Epidemiology
B. cepacia, which is among the Burkholderia spp. found in the United States, is a complex of 10 distinct genomic species (genomovars) isolated from clinical specimens. Plants, soil, and water serve as reservoirs. These organ isms are able to survive on or in medical devices and disinfectants. Intrinsic resistance to multiple antimicrobial agents contributes to the organism’s survival in hospitals. Human acquisition of B. cepacia that results in colonization or infection usually involves direct contact with contaminated foods, devices such as respiratory equipment, or medical solutions, including disinfectants. Person-to-person transmission also has been documented.
B. pseudomallei is another environmental inhabitant of niches similar to those described for B. cepacia; however, it is geographically restricted to tropical and subtropical areas of Australia and Southeast Asia. The organism is widely disseminated in soil, streams, ponds, and rice paddies. Human acquisition occurs through inhalation of contaminated debris or by direct inoculation through damaged skin or mucous membranes.
Although B. mallei causes severe infections in horses and related animals, it has been identified in rare human localized suppurative or acute pulmonary infections. When transmission has occurred, it has been associated with close animal contact. B. gladioli is a plant pathogen that is only rarely found in the sputa of patients with cystic fibrosis or associated with chronic granulomatous disease; the mode of transmission to humans and its clinical significance are unknown.
R. pickettii is another environmental organism that is occasionally found in a variety of clinical specimens, such as blood, the sputa of patients with cystic fibrosis, and urine. The mode of transmission is uncertain, but iso lates have been found in contaminated sterile hospital fluids.
PSEUDOMONAS SPP. AND BREVUNDIMONAS SPP.
The genera Pseudomonas and Brevundimonas comprise several environmental species that rarely inhabit human skin or mucosal surfaces. In the clinical setting, P. aeruginosa is the most commonly encountered gram-negative species that is not a member of the family Enterobacteriaceae and is an uncommon member of the normal human flora. The organism survives in various environments in nature and in homes and hospitals (see Table 1). Brevundimonas spp. are environmental and are encountered primarily in nature in water, soil, and on plants, including fruits and vegetables. Because of the ubiquitous nature of P. aeruginosa and Brevundimonas spp., the transmission of to humans can occur in a variety of ways.
P. fluorescens, P. putida, and P. stutzeri are environmental inhabitants, but they are much less commonly found in clinical specimens than is P. aeruginosa. The other pseudomonads and Brevundimonas spp. listed in Table 1 are also environmental organisms. Because they are rarely encountered in patient specimens, the mode of transmission to humans remains uncertain.
