Final answer:
The most common organism causing bovine pyelonephritis is the Gram-negative bacterium Escherichia coli. It is identified through urinalysis testing for leukocyte esterase and nitrite levels, indicating active infection.
Step-by-step explanation:
The most common organism isolated from bovine cases of pyelonephritis is the Gram-negative bacterium Escherichia coli (E. coli). This bacterium is often responsible for urinary tract infections (UTIs) in both humans and animals and can lead to pyelonephritis when it ascends from the lower urinary tract to the kidneys. Diagnosis of pyelonephritis typically involves urinalysis to detect the presence of leukocyte esterase and nitrite levels, which suggests active bacterial infection. E. coli is known to produce nitrate reductase, which converts nitrate to nitrite, thus the presence of nitrite may signal an E. coli infection. Other organisms such as Proteus vulgaris and Klebsiella pneumoniae are also Gram-negative bacteria that can cause UTIs and pyelonephritis, but Escherichia coli remains the most commonly isolated in bovine cases.