USMLE® Step 1 style questions USMLE
A group of researchers is evaluating the association between bacteria and peptic ulcer disease. A bacterial species is isolated, and its enzymatic makeup and other laboratory characteristics are studied. Which of the following bacteria features can be used in the diagnosis?
Content Reviewers:Rishi Desai, MD, MPH
In some individuals it can cause inflammation of the stomach lining; and can result in peptic ulcers.
H. pylori is a gram-negative bacteria that’s shaped like a curved rod and it has 2 to 6 flagella, kind of like multiple tails, all at one end which it uses for movement.
It’s positive for urease, oxidase and catalase; and is a microaerophile, so that means it needs oxygen to survive, but requires less than the levels typically found in the atmosphere.
Ok, now normally, the inner wall of the entire gastrointestinal tract is lined with mucosa, which consists of three cell layers.
The innermost layer is the epithelial layer and it absorbs and secretes mucus and digestive enzymes.
The middle layer is the lamina propria and it has blood and lymph vessels.
The outermost layer of the mucosa is the muscularis mucosa, and it’s a layer of smooth muscle that contracts and helps with the break down food.
The epithelial layer dips down below the surface of the stomach lining to form gastric pits.
And these pits are contiguous with gastric glands below which contain various epithelial cell types - each secreting a variety of substances.
So for example, foveolar cells, or surface mucus cells, secrete mucus, which is a mix of water and glycoproteins that coats the stomach epithelial cells.
With all of these digestive enzymes and hydrochloric acid floating around, the stomach and duodenal mucosa would get digested if not for this mucus which coats and protects the epithelial cells.
There are also chief cells that secrete pepsinogen to digest proteins. And then there are G cells which secrete gastrin, which has a number of effects, including stimulation of parietal cells.
An H. pylori infection is thought to spread through fecal-oral, gastro-oral, and perhaps oral-oral transmission, from one infected individual to another.
That is - through contamination of food and water or even directly, with fecal matter, vomitus, or saliva.
Typically, it will migrate to regions where pH is less acidic - like the antrum, which has fewer parietal cells.
It then uses adhesin proteins to stick to the surface of foveolar cells where it can release a number of virulence factors which help it both survive and thrive, and cause damage to the mucosa.
One of the most important enzymes for their survival is aurease because it converts urea in the gastric juices to carbon dioxide and ammonia.
Ammonia, which is basic, locally neutralizes the acid gastric environment and protects H. pylori from the harsh, acidic environment of the stomach.
While H. pylori itself does not typically go inside the epithelial cells, some of the virulence factors it releases do - and they cause extensive damage to epithelial cells.
For example, some strains of H. pylori produce cytotoxin-associated gene A, or cagA, which interferes with the attachments between epithelial cells that normally help protect the underlying mucosal layers.