Unexpected source of Proteus mirabilis bacteraemia.

Mycotic (infected) aneurysm involving the thoracic aorta is an exceedingly rare and life-threatening condition that is associated with high morbidity and mortality. We report an unusual source of Proteus mirabilis bacteraemia thought to be due to an infected aneurysm in the thoracic aortic arch in an elderly woman. Source of gram-negative bacteraemia is usually isolated to an intra-abdominal or a pelvic source. Proteus bacteraemia from an intrathoracic pathology is very uncommon, and in this case led to a delay in diagnosis. Although an infected aneurysm is a rare source of gram-negative bacteraemia, it must always be considered when common causes of bacteraemia have been ruled out especially in patients with vascular risk factors.