A Unique Case of Aeromonas salmonicida Peritonitis in a Patient Undergoing Peritoneal Dialysis in Mexico

<i>Aeromonas salmonicida</i> is a Gram-negative, oxidase-positive, facultative anaerobic bacterium primarily recognized as a pathogen in fish. Its isolation in humans is rare and scarcely reported, particularly in cases of peritonitis. We present the case of a 16-year-old female patient with end-stage chronic kidney disease, recent renal graft rejection, and undergoing continuous ambulatory peritoneal dialysis, who presented with severe abdominal pain and cloudy peritoneal effluent. Fluid analysis revealed an elevated leukocyte count (15,022 cells/μL, including 13,330 polymorphonuclear cells). Empirical treatment with cefalotin and amikacin was initiated. Following the observation of short Gram-negative bacilli, the regimen was adjusted to intraperitoneal cefepime and amikacin. After 72 hours, the VITEK® 2 automated system identified <i>A. salmonicida</i> with 98% probability. The patient showed clinical and laboratory improvement after 14 days of targeted therapy. Human infections caused by <i>A. salmonicida</i> are exceptionally rare. This case highlights the importance of considering atypical pathogens in immunocompromised patients with peritonitis. Given the challenges in its identification, the use of automated or molecular diagnostic methods is recommended. Although no clear aquatic exposure was identified, the isolation of this microorganism supports the need for broad microbiological surveillance. Antimicrobial susceptibility profiles should be evaluated on a case-by-case basis.