The genus Pseudomonas is made up of Gram-negative, rodshaped bacteria that inhabit many niches. Pseudomonas species are common inhabitants of the soil, water, and vegetation. The genus is particularly noteworthy because of the tendency of several species to cause infections in people who are already ill, or whose immune systems are not operating properly. Such infections are termed opportunistic infections.
Pseudomonas rarely causes infections in those whose immune systems are fully functional. The disease-causing members of the genus are therefore prevalent where illness abounds. Pseudomonas are one of the major causes of nosocomial (hospital acquired) infections.
Bacteria in this genus not only cause infections in man, but also cause infections in plants and animals (e.g., horses). For example, Pseudomonas mallei causes ganders disease in horses.
The species that comprise the genus Pseudomonas are part of the wider family of bacteria that are classified as Pseudomonadaceae. There are more than 140 species in the genus. The species that are associated with opportunistic infections include Pseudomonas aeruginosa, Pseudomonas maltophilia, Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas cepacia, Pseudomonas stutzeri, and Pseudomonas putrefaciens. Pseudomonas aeruginosa is probably the most well-known member of the genus.
Pseudomonas are hardy microorganisms , and can grow on almost any available surface where enough moisture and nutrients are present. Members of the genus are prone to form the adherent bacterial populations that are termed biofilms. Moreover, Pseudomonas aeruginosa specifically change their genetic behavior when on a surface, such that they produce much more of the glycocalyx material than they produce when floating in solution. The glycocalyx-enmeshed bacteria become extremely resistant to antibacterial agents and immune responses such as phagocytosis .
In the hospital setting Pseudomonas aeruginosa can cause very serious infections in people who have cancer, cystic fibrosis, and burns. Other infections in numerous sites in the body, can be caused by Pseudomonas spp. Infections can be site-specific, such as in the urinary tract or the respiratory system. More widely disseminated infections (termed systemic infections) can occur, particularly in burn victims and those whose immune systems are immunosuppressed.
For those afflicted with cystic fibrosis, the long-lasting lung infection caused by Pseudomonas aeruginosa can ultimately prove to be fatal. The bacteria have a surface that is altered from their counterparts growing in natural environments. One such alteration is the production of a glycocalyx around the bacteria. The bacteria become very hard for the immune system to eradicate. The immune response eventually damages the epithelial cells of the lung. So much so, sometimes, that lung function is severely compromised or ceases.
Another bacterium, Pseudomonas cepacia, is also an opportunistic cause of lung infections in those afflicted with cystic fibrosis. This species is problematic because it is resistant to more antibiotics than is Pseudomonas aeruginosa.
Glycocalyx production in some strains of Pseudomonas aeruginosa can be so prodigious that colonies growing on solid media appear slimy. Indeed, some species produce such mucoid colonies that the colonies will drip onto the lid of the agar plate when the plate is turned upside down. These slimy growths are described as mucoid colonies, and are often a hallmark of a sample that has been recovered from an infection.
Disease-causing species of Pseudomonas can possess a myriad of factors in addition to the glycocalyx that enable a bacterium to establish an infection. The appendages known as pili function in adherence to host cells. A component of the outer membrane possesses an endotoxin. Finally, a number of exotoxins and extracellular enzymes can cause damage at a distance from the bacterium. One such exotoxin, which is called toxin A, is extremely potent, and may be the prime cause of damage by the bacteria in infections.
Some species, especially Pseudomonas aeruginosa are a problem in hospitals. By virtue of their function, hospitals are a place where many immunocompromised people are found. This is an ideal environment for an opportunistic disease-causing bacterium. Moreover, Pseudomonas aeruginosa has acquired resistance to a number of commonly used antibiotics. As yet, a vaccine to the bacterium does not exist. Prevention of the spread of Pseudomonas involves the observance of proper hygiene , including handwashing.
See also Bacteria and bacterial infection; Infection and resistance; Lipopolysaccharide and its constituents