Which microorganism is usually NOT associated with acute pyelonephritis?

Acute pyelonephritis is a bacterial infection of the kidney that often ascends from the lower urinary tract. The most common causative agent of acute pyelonephritis is Escherichia coli, which accounts for a significant majority of cases. Other organisms such as Proteus and Pseudomonas can also be implicated, particularly in certain populations or conditions, such as patients with urinary tract obstructions or compromised immune systems.

Staphylococcus aureus, while an important pathogen in various infections, is generally not a common cause of acute pyelonephritis. This bacterium is more typically associated with systemic infections, skin and soft tissue infections, and osteomyelitis, among others. Its presence in the urinary tract does not usually lead to acute pyelonephritis in healthy individuals. In the context of pyelonephritis, the focus is predominantly on bacteria that colonize or infect the urinary system directly, which further underscores why Staphylococcus aureus is not typically linked to this specific renal condition.