Francisella spp. as an overlooked cause of acute undifferentiated febrile illness in Colombia? Unexpected evidence from febrile patients negative for other common and neglected etiologies in Villeta municipality

Background: Acute undifferentiated febrile illness (AUFI) represents a major health challenge in tropical regions due to its wide range of etiologies. In Villeta, Colombia, previous studies investigated common causes such as malaria, arboviral diseases, leptospirosis and rickettsiosis, as well as several neglected bacterial agents. However, some patients remained without an identified etiology, underscoring the need for broader approaches to uncover other potential causes. Therefore, the aim of the present study was to investigate into other potential bacterial causes of AUFI through advanced molecular strategies utilizing 16S rRNA sequencing. Methods: The study analyzed AUFI patient samples previously screened for fourteen pathogens. The V3–V9 hypervariable region of the 16S rRNA gene was amplified from whole-blood DNA of unresolved cases and sequenced using the Oxford Nanopore GridION platform. Reads were filtered, quality-checked, and taxonomically classified using the SILVA database. Results: Eight samples from individuals without evidence of infection or recent exposure to previously screened pathogens were selected for 16S rRNA sequencing. DNA quality and integrity were confirmed, and enrichment produced high-quality amplicons for all samples. Sequencing generated high-quality reads overwhelmingly dominated by Francisella, representing over 93% of classified reads, followed by Coxiella and Arcobacter. Conclusions: This study provides the first molecular evidence of Francisella in whole-blood from febrile patients in Colombia. Findings highlight its potential role in AUFI, demonstrate the value of 16S rRNA barcoding, and underscore the need for expanded surveillance of highly neglected bacterial taxa.