Molecular diagnosis and characterization of a culture-negative mycotic aneurysm due to ST54 Haemophilus influenzae type b with PBP 3 alterations.

Mycotic aneurysm is a rare but life-threatening disease that warrants an integrated therapeutic approach involving surgical intervention and prolonged antibiotic use. However, the causative organisms are often unidentified because antibiotics started empirically render blood and tissue cultures negative. Molecular diagnosis has been reported to be useful in such culture-negative cases. We report a case of a culture-negative mycotic aortic aneurysm due to Haemophilus influenzae, diagnosed by direct 16S rRNA polymerase chain reaction (PCR) and sequencing of the resected aneurysm tissue. PCR for serotype revealed type b, and PCR and sequencing of the ftsI gene revealed alterations in penicillin-binding protein 3, suggesting resistance to ampicillin. Multilocus sequence typing demonstrated that the isolate belonged to sequence type 54.