Emergence of a novel NDM-4-producing Citrobacter freundii complex species in a complicated urinary tract infection...

Multidrug-resistant (MDR) Gram-negative infections represent a growing global health threat with increasingly limited therapeutic options. We report a case of complicated urinary tract infection in a 70-year-old woman with underlying renal cysts complicated by rupture and subsequent formation of left infrarenal and paracolic pelvic abscesses. The initial infection was caused by Escherichia coli; however, following initial prolonged broad-spectrum antimicrobial therapy, a novel New Delhi metallo-β-lactamase (NDM type carbapenemase)-producing Citrobacter species was subsequently isolated from urine cultures. Whole-genome sequencing confirmed identification as a novel Citrobacter species and identified the presence of a blaNDM-4 gene. Antimicrobial susceptibility testing demonstrated resistance to most tested agents, with susceptibility limited to nitrofurantoin, cefiderocol, aztreonam, doxycycline, and tigecycline. Repeat abscess drainage and treatment with cefiderocol resulted in clinical and microbiological resolution of infection. This case underscores the critical role of drainage and access to newer antibiotic agents in management of complex intra-abdominal infections. Genome sequencing aids the detection of novel reservoirs of antibiotic resistance genes in carbapenemase-producing Enterobacterales that may emerge during prolonged antimicrobial exposure.