Final answer:
Helicobacter pylori, a bacterium adapted to the stomach's acidic environment, has seen a decrease in colonization rates in the US from 80% to 10% over time.
Step-by-step explanation:
The bacterium that once colonized 80% of individuals in the US and now only colonizes 10% is Helicobacter pylori (H. pylori). This gram-negative bacterium is adapted to live in the acidic environment of the human stomach and is known to cause peptic ulcers and gastric cancer. Over time, the prevalence of H. pylori has decreased significantly in developed countries due to improvements in hygiene and the widespread use of antibiotics and acid-suppressive medications.