Pseudomonas are gram-negative, aerobic, actively motile, and rod-shaped bacteria. They move around with polar flagella and do not form spores. They can cause various diseases in humans.
What are pseudomonas?
Pseudomonas form a genus of bacteria that are gram-negative. This means that they have only a single-layer, thin murein envelope (cell wall). This gives the bacterium strength. The bacteria have the shape of rods, move actively with polar flagella, are aerobic and do not form spores. Pseudomonas are classified in the group of nonfermenters, i.e. they are not capable of fermenting glucose. Rather, they utilize it oxidatively. Pseudomonas are considered to be physiologically highly flexible. These bacteria are so-called opportunists, i.e. facultative pathogens. Thus, Pseudomonas cause disease when the host immune system is weakened.
Occurrence, distribution, and characteristics
Pseudomonas are ubiquitous. In other words, they occur everywhere in the environment. These bacteria are often referred to as “puddle germs” because they live in soil, in water, on plants as well as on animals. Pseudomonas thus prefer primarily moist habitats. The bacteria do not belong to the normal flora of humans. If they are detected in water installations, this suggests the presence of hygienic problems. Pseudomonas reach a size between 0.5 to 1.0 x 1.5 to 5.0 µm. Since the bacteria are aerobic, they generally require oxygen for their energy metabolism. The majority of Pseudomonas exhibit resistance to antibiotics. With higher cell density, they also have the ability to form biofilms. With this, they are protected from antibiotics and phagocytes. From this group, Pseudomonas aeruginosa is the most frequently occurring pathogen in humans. The name is derived from the Latin “aerugo” for verdigris and refers to the coloration of purulent wound secretions. The germ was discovered in 1900. Pseudomonas auruginosa is found primarily in moist environments and is considered a widespread soil and water germ. It is about two to three micrometers in size and has tufted lophotric flagella. Adhesive frimbia allow it to attach to surfaces. As a Gram-negative bacterium, Pseudomonas aeruginosa stains red in Gram stain (diagnostic tool in medical and scientific microbiology). The pathogen is very undemanding in terms of its living conditions and – even though it prefers a moist habitat – can survive for a long time even in dry areas. Pseudomonas aeruginosa is a so-called nosocomial germ. Infections with it occur primarily in hospitals (e.g. in medications, in dialysis machines, in urine bottles, in disinfectants), which is why it is also referred to as hospital germs. Transmission from hospital staff to patients is also possible; in principle, infections only occur through direct contact with the pathogen. Some Pseudomonas species produce TTX (tetrodotoxin), a highly dangerous neurotoxin. The high pathogenicity – especially in Pseudomonas aeruginosa – is attributed to diverse virulence genes.
Diseases and symptoms
In people with an intact immune system, Pseudomonas usually cannot cause disease. However, if the immune system is weakened (for example, after surgery or in patients with HIV infection), the risk for Pseudomonas infection is significantly increased. Infections with Pseudomonas can manifest themselves in many parts of the body (e.g. skin, bones, ears, eyes, urinary tract, heart valves, subcutaneous tissue). The localization of such an infection depends on the site of entry of the pathogen. The first sign, especially in patients in hospitals, is a so-called gram-negative sepsis (blood poisoning). Pseudomonas often colonize burn wounds. In some cases, it is so massive that it results in bacteremia. Pseudomonas aeruginosa in particular also invades deep cut wounds. Purulent wound secretions then have a typical blue-green coloration and a sweetish to fruity odor. Pseudomonas also causes otitis externa (inflammation of the outer ear), which occurs mainly in tropical regions and in which purulent secretions are discharged from the ear. Malignant otitis externa usually occurs in patients with diabetes.It is markedly more severe and characterized by severe ear pain and often unilateral cranial nerve palsies. The so-called ecthyma gangraenosum represents a pathognomonic skin lesion in neutropenic patients and is characterized by centrally ulcerated, erythematous and purplish-black areas approximately one centimeter in diameter. They frequently occur in the axilla as well as in the anogenital area (area around the anus and genitals). In addition, inflamed sinuses, cellulitis (pathological changes in connective tissue) or osteomyelitis (infectious bone marrow inflammation) can occur as part of a skin and soft tissue infection. Ventilator-associated pneumonia can be caused by Pseudomonas aeruginosa. Pneumonia or sinusitis occur more frequently, especially in patients with HIV infection. If cystic fibrosis is present, Pseudomonas bronchitis may occur later in the course of the disease. In addition, Pseudomonas very often leads to urinary tract infections, especially after urologic surgery. The eyes can also become infected, often following trauma or due to contamination of contact lenses or cleaning fluid. In rare cases, acute bacterial endocarditis can occur due to Pseudomonas. This usually affects artificial heart valves after open-heart surgery or native valves in cases of drug abuse. In many cases, Pseudomonas infection also results in bacteremia. If patients are not intubated, if there is no evidence of urologic symptoms, and if, in addition, species other than Pseudomonas aeruginosa are involved in the infection, it is probably caused by contaminated infusion solutions, disinfectants, or even drugs. The pathogens are detected by preparing a culture from the secretion of the infection site. Blood or urine can also be used for this purpose. Cephalosporins from the third generation (e.g., cefepime), acylaminopenicillins (e.g., piperacillin), carbapenems, fluoroquinolones, and aminoglycosides are used to treat infections with Pseudomonas.
