MALDI-TOF misidentification of Sanguibacteroides justesenii: confirmation by 16S rRNA sequencing.

Sanguibacteroides justesenii is a rarely reported Gram-negative anaerobic bacillus of uncertain clinical significance. Its identification is challenging using conventional methods. We report the case of a 58-year-old woman admitted for acute heart failure who developed septic shock following colonic perforation. Blood cultures grew an obligate anaerobic Gram-negative bacillus. MALDI-TOF MS (Bruker®) identified the isolate as Gabonibacter massiliensis (score 2.19). Given the unexpected result, the strain was referred to a reference laboratory, where 16S rRNA gene sequencing identified S. justesenii. Repeated MALDI-TOF analyses in two laboratories reproduced the same misidentification, underscoring database limitations for rare species. The patient was treated with amoxicillin and metronidazole and recovered without complications at 6-month follow-up. This is the third reported case of S. justesenii in blood cultures and a rare associated with abdominal sepsis, emphasizing the need for molecular confirmation when MALDI-TOF results are uncertain.