USMLE® Step 1 style questions USMLE
A 23-year-old man was admitted five days ago to the burn ward after suffering severe lower extremity burns at work. On admission, the patient was started on prophylactic antibiotics. Currently, temperature is 39 ºC (102.2 ºF), pulse is 120/minute, blood pressure is 98/55, and respirations are 20/minute. Examination of the burn wounds shows blue-green purulent discharge. Microscopic and laboratory examination of the discharge show Gram-negative catalase-positive bacteria. Which of the following organisms is the most likely cause of this patient’s condition?
Pseudomonas aeruginosa exam links
Content Reviewers:Rishi Desai, MD, MPH
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.
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.