Acute gastroenteritis caused by Comamonas testosteroni in a patient with hepatobiliary dysfunction: a case report highlighting diagnostic challenges during the pre-monsoon season in India.

Comamonas testosteroni is a rare, environmental Gram-negative Bacillus increasingly recognised as an emerging opportunistic pathogen. It is often misidentified as a Pseudomonas species in routine laboratories due to overlapping phenotypic characteristics. Although generally of low virulence, it can cause severe infections in patients with underlying hepatobiliary or immunocompromised conditions. We report a case of an 18-year-old male from Karnataka, South India, who presented with fever, vomiting, and watery diarrhoea during the early monsoon showers of May 2025. Laboratory evaluation revealed hepatocellular dysfunction, cholelithiasis, and coagulopathy. Stool culture bluish-green colonies on thiosulfate-citrate-bile salts-sucrose (TCBS) agar, initially resembling those of Pseudomonas. Automated identification using the VITEK-2 Compact system and confirmation at ICMR-NIRBI (Indian Council of Medical Research-National Institute for Research in Bacterial Infections), Kolkata, identified the isolate as Comamonas testosteroni. The organism was sensitive to piperacillin/tazobactam, ceftriaxone, cefoperazone/sulbactam, imipenem, meropenem, amikacin, gentamicin, ciprofloxacin, and trimethoprim/sulfamethoxazole. The patient responded well to ceftriaxone and supportive care. This case underscores the diagnostic challenge of detecting C. testosteroni, particularly during overlapping outbreaks. The inclusion of differential media and careful interpretation of culture results is crucial for accurate identification. Diligent culture reporting helps in strengthening public health surveillance, infection control, and outbreak preparedness in resource-limited settings.