USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 28-year-old man comes to the emergency department because of a painful wound on his arm for the past two days. He fell onto a decaying log while jogging on a forest trail, and sustained a deep laceration on his arm. He says it has grown progressively more painful since that time and seems to be spreading. His temperature is 38.1°C (100.6°F), pulse is 88/min, respirations are 16/min, and blood pressure is 138/82 mm Hg. Physical examination shows a reddish-purple necrotic lesion on the arm with gaseous distention and crepitus of the surrounding tissue. A wound culture on blood agar shows a double-zone of both alpha and beta hemolysis. Which of the following is the most likely cause of this condition?
Clostridium perfringens is from the family of Clostridia, and as a family, they’re obligate anaerobes, meaning they don’t require oxygen to thrive, in fact, they’re better off without it.
Anaerobes, clostridia included, tend to lack the enzymes catalase or superoxide dismutase, so oxygen is actually toxic to them.
In nature, they thrive in deep, compact soil, and when they feel the stress of fresh oxygenated air, they often produce spores, which are extremely resilient to the environment, and can even survive cooking.
When conditions improve, for example, when food is slowly cooled or stored, the spores can sprout into full-fledged Clostridia.
In fact, at an optimum temperature, Clostridium perfringens has one of the fastest growing rates of any bacterium! In the lab, when doing a gram stain, Clostridium perfringens is Gram-positive, or purple when Gram stained, and look like big cylinders or rods, also called bacilli.
That’s because it typically infects food that’s prepared in large quantities, and then kept warm for prolonged periods, such as in cafeterias or buffets.
Clostridium perfringens are found in the environment and they can accidentally contaminate food when it’s been left out for a while.
If a person eats food contaminated with C. perfringens, the bacteria will soon colonize the gut.
Within 24 hours, the bacteria starts to make clostridium perfringens enterotoxin, or CPE.
The CPE specifically targets the tight junctions that connect epithelial cells lining the intestines to one another.
When the tight junctions get destroyed, it causes inflammation and compromises the structural integrity of the intestinal wall.
Fortunately, CPE is heat labile, so prolonged cooking at 72°C or above will inactivate it.
And that’s one reason why freshly cooked food is best to eat - the other reason is that it’s simply tastier that way!
But uncooked food - like salads, or reheated food - like leftovers, can be a good home to Clostridium perfringens.
Fortunately, most of these improve over the course of a day, as the bacteria makes its way through the intestines.
In terms of treatment, antibiotics aren’t needed, and instead the goal is to keep the individual well hydrated.
Now, instead of getting into food, Clostridium perfringens can also sometimes get into a wound.
And if there’s a deep wound, it can cause clostridial myonecrosis.