Pseudomonas aeruginosa

Definitions & Key takeaways

Pseudomonas aeruginosa is an aerobic, gram-negative, rod-shaped bacterium commonly found in the environment, such as in soil, in the home, and in hospitals. It is known to opportunistically infect immunocompromised people, and for its multi-drug resistance that is hard to treat.

It is also known to form biofilms that provide it with extra protection against antibiotics on top of its preexisting multi-drug resistance. P. aeruginosa can cause a variety of infections, including urinary tract infections, wound infections, sepsis, and lung infections in people with cystic fibrosis. Treatment for P. aeruginosa infections is with antibiotics, such as aminoglycosides, carbapenems, and cephalosporins.

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Pseudomonas aeruginosa, or P. aeruginosa, is a gram-negative bacterium that is abundant in the environment.
It tends to opportunistically infect high-risk individuals, and is well known for its multi-drug resistance, making it hard to treat.
P. aeruginosa is an encapsulated, gram-negative, rod bacterium, that’s an obligate aerobe, so it uses oxygen for metabolism through cellular respiration.
It does not ferment lactose and does not make spores; but it’s catalase, citrate, and oxidase positive. It has a flagellum, kind of like a tail, at one end for motility; and has multiple hair-like appendages, called pili, all over that help with adhesion to other cells.
It also has a number of multidrug efflux pumps that efficiently pump medications out of the bacteria making it resistant to a variety of antibiotics.
In addition it’s able to make beta-lactamases that degrade beta-lactam antibiotics as well as aminoglycoside-modifying enzymes that alter aminoglycoside antibiotics - rendering them ineffective.
In short, it’s defenses are strong. P.
aeruginosa is everywhere in the environment - in soil; in the home, and in hospitals - where it’s found on improperly cleaned medical equipment and devices, various surfaces, and on the hands of health care workers.
It can survive for months on dry surfaces and inanimate objects – but particularly loves humid or wet conditions; like hot tubs, contact lens cases, catheters, and medical ventilators.
Transmission can occur when broken skin or mucous membranes come in contact with contaminated surfaces; or when aerosols containing P.
aeruginosa are inhaled after an infected person coughs or sneezes. Now, a P.
aeruginosa infection is actually not too common in the general population. It’s an opportunistic bacterium, meaning that it frequently causes infections in high-risk individuals like those with cystic fibrosis, chronic granulomatous disease, and type 2 diabetes mellitus.
It’s also common among individuals with an immunodeficiency; IV drug abusers, or those with severe burns or deep wounds.
P. aeruginosa produces several virulence factors to help it invade epithelial cells and survive an attack from the immune system.
First, there’s endotoxin, a molecule that’s made up of a lipid and polysaccharide. The endotoxin sits on the outer membrane of the bacteria, and it’s recognized by the immune system as being foreign and causes a really strong immune reaction that can cause extensive damage to the host cells.
Certain strains of P. aeruginosa also produce a mucoid exopolysaccharide capsule, and this contributes to the formation of a biofilm - a slimy, sticky, gummy material – that surrounds colonies of P.
aeruginosa and acts like a shield to protect the bacteria from immune cells as well as antibiotics. P.
aeruginosa can also produce toxins that can be injected directly into the host epithelial cells through a type III secretion system.
It’s kind of like a fine needle that pierces the host cell and forms a tunnel between the P. aeruginosa and the host epithelial cell, allowing toxins to pass through.
P. aeruginosa also releases toxins into the extracellular environment.
For example, it releases phospholipase C which is an enzyme that degrades the host cell membrane causing the cell to lyse.
Other secreted toxins have to move within the cell to act. For example, exotoxin A is released by P.
aeruginosa into the extracellular environment and then taken up by nearby host cells where it targets ADP-ribosylation of elongation factor 2, which is a ribosomal protein.
That inactivates protein synthesis and eventually leads to cell death. Pyocyanin is another example.
It’s a blue pigment with a sweet, fruity smell like grapes that’s released by P. aeruginosa and taken up by nearby host cells, where it generates reactive oxygen species that cause oxidative damage and cell death.
In the environment, pyocyanin is released by P. aeruginosa and it’s taken up by microbes that are competing for nutrients and space, and causes them to die as well.
A sweet fruity death. P.
aeruginosa also secretes pyoverdine, a green compound that is an iron-chelator, that binds to iron and transports it into the P.
aeruginosa. Symptoms of a P.
aeruginosa infection depend on the site of infection. Usually there’s a fever; an elevated respiratory and heart rate, and an increased white blood cell count.
If the infection is in the blood it can lead to septic shock. But there can also be localized infections, like an infection of hair follicles, called hot tub folliculitis, can look like a red, pimpley rash.
Burn or deep wound infections can have a blue-green color and a fruity grape-like smell, and immunocompromised individuals can develop a skin condition called ecthyma gangrenosum that begins with blisters but rapidly progresses to tissue necrosis.
People who use catheters can develop urinary tract infections and those with infections involving contact lenses can develop conjunctivitis and eye pain.
Individuals with diabetes can develop osteomyelitis, an infection of the bones - classically from a puncture wound through a moist sneaker insole.
It can also cause malignant external otitis, an infection in the ear canal - sometimes called “swimmer’s ear”. Pseudomonas can cause tricuspid valve endocarditis particularly in IV drug abusers.
And finally, individuals with cystic fibrosis can have pneumonia - particularly involving the formation of biofilm. Diagnosis usually involves a complete blood count, as well as cultures of the site of infection - like the blood, sputum, or skin.
In terms of treatment, some cases, like Pseudomonas folliculitis are typically self-limiting, so no treatment is usually necessary.
But in more serious infections, cultures and antibiotic susceptibility testing often guides treatment. Topical antibiotics can be used for superficial infections; and aerosolized antibiotics are frequently used for lung infections.
In severe cases, especially in immunodeficient individuals, a combination of anti-pseudomonal drugs is needed to overcome multi-drug resistance; like β-lactam with aminoglycoside; or carbapenems with quinolones and aminoglycosides.
So, to recap: P. aeruginosa is a gram-negative rod bacterium that is ubiquitous in the environment.
Virulence factors contribute to key features of a P. aeruginosa infection including blue-green color, fruity grape-like smell, and formation of biofilms which add extra protection to P.
aeruginosa on top of its already notable multi-drug resistance. It can cause a variety of diseases from hot tub folliculitis, to ecthyma gangrenosum, osteomyelitis and malignant external otitis, and also causes pneumonia in individuals with cystic fibrosis.
Definitive diagnosis is made by cultures, and susceptibility data help identify effective antibiotics.