Which pathogen is most commonly associated with pyelonephritis?

Escherichia coli is the pathogen most commonly associated with pyelonephritis. This is because E. coli, a type of gram-negative bacterium, is part of the normal flora found in the gastrointestinal tract but can cause urinary tract infections (UTIs) when it ascends to the kidneys, leading to pyelonephritis. This bacterium is responsible for a significant majority of UTI cases, and when left untreated, these infections can escalate to involve the kidneys, resulting in inflammation and infection of the renal parenchyma.

Because of its prevalence and virulence factors, such as fimbriae that allow it to adhere to the uroepithelium, E. coli is often the first pathogen considered in cases of pyelonephritis. Other pathogens can cause urinary infections, but they are less frequently the causative agents of pyelonephritis compared to E. coli, making it the most identified organism in clinical settings related to this condition.