A 35-year-old man presented to a neighborhood clinic with mild pain in the right buttock in August 20XX. The physician suspected infection and referred the patient to our hospital for further evaluation and treatment. Pelvic Computed Tomography (CT) revealed a large abscess involving the right iliopsoas muscle and periphery of the hip joint, and we performed ultrasound-guided percutaneous drainage. Gram staining of the aspirated specimen revealed gram-negative filamentous rods. We had started the patient on empiric antibiotic therapy with cefazolin; however, based on the finding of Gram staining, the antibiotic regimen was escalated to piperacillin/tazobactam.

Due to challenges in bacterial identification by standard culture techniques, we referred the isolate to the Osaka Metropolitan University Hospital's Infection Control and Prevention Department for further analysis. Matrix-Assisted Laser Desorption Ionization-Time Of Flight Mass Spectrometry (MALDI-TOF MS) and 16S rRNA sequencing at that department identified Aggregatibacter aphrophilus as the causative organism.

The patient gave a history of dental treatment, which was suspected as the potential source of bacteremia leading to the formation of a perihip abscess. Appropriate management of such infections requires timely abscess drainage, accurate identification of the causative organism using susceptibility testing, and targeted antimicrobial therapy.