Diagnostic challenges and therapeutic approach for aortic pseudoaneurysm infection with <i>Paenibacillus pueri</i> : a case report

ABSTRACT Background Paenibacillus species are rod-shaped, gram-variable, endospore-forming, environmental bacteria. Once classified within Bacillus spp., it was reclassified based on 16S rRNA sequencing. Paenibacillus spp. has rarely been found to be pathogenic in humans; however, recent reports have been described in immunocompromised patients. Case Summary A 60-year-old male with past medical history of hypertension, hyperlipidemia, atrial fibrillation, and bicuspid aortic valve presented to an outside hospital for 3 days of chest pain without associated symptoms. He was transferred to our institution after computed tomography (CT) demonstrated fluid around the aortic root concerning for aortic pseudoaneurysm. Admission and follow-up blood cultures were negative. He was started on ceftriaxone and vancomycin and underwent aortic root repair. Intraoperative cultures grew gram-indeterminate bacilli identified as Paenibacillus pueri by 16S broad-range PCR at an outside institution. Initial pathogen identification was complicated without discernible morphological differences between Paenibacillus spp. and Bacillus spp. Final identification often relies on molecular methods. Previous cases utilized amoxicillin-clavulanate or trimethoprim-sulfamethoxazole, with resistance demonstrated to penicillins and clindamycin. In our case, doxycycline was chosen based on limited susceptibility profiles and considering antibiotic side effect profiles. Conclusion This is the third documented human case of Paenibacillus pueri infection. Paenibacillus spp. infections have varied clinical presentations, typically described in immunocompromised patients. This case demonstrates the difficulties in identifying Paenibacillus pueri . While rare, Paenibacillus spp. has been occasionally implicated in human disease. Even in immunocompetent patients, suspicion for true pathogens should remain high with repeated isolation on separate specimens.