Background: Fusobacterium species are obligately anaerobic, gram-negative bacilli. Especially, F. nucleatum and F. necrophorum are highly relevant human pathogens. We investigated clinical differences in patients infected with Fusobacterium spp. and determined the antimicrobial susceptibility of Fusobacterium isolates.Methods: We collected clinical data of 86 patients from whom Fusobacterium spp. were isolated from clinical specimens at a tertiary-care hospital in Korea between 2003 and 2020. In total, 76 non-duplicated Fusobacterium isolates were selected for antimicrobial susceptibility testing by the agar dilution method, according to the Clinical and Laboratory Standards Institute guidelines (M11-A9).Results: F. nucleatum was most frequently isolated from blood cultures and was associated with hematologic malignancy, whereas F. necrophorum was mostly prevalent in head and neck infections. Anti-anaerobic agents were more commonly used to treat F. nucleatum and F. varium infections than to treat F. necrophorum infections. We observed no significant difference in mortality between patients infected with these species. All F. nucleatum and F. necrophorum isolates were susceptible to the antimicrobial agents tested. F. varium was resistant to clindamycin (48%) and moxifloxacin (24%), and F. mortiferum was resistant to penicillin G (22%) and ceftriaxone (67%). β-Lactamase activity was not detected.Conclusions: Despite the clinical differences among patients with clinically important Fusobacterium infections, there was no significant difference in the mortality rates. Some Fusobacterium spp. were resistant to penicillin G, ceftriaxone, clindamycin, or moxifloxacin. This study may provide clinically relevant data for implementing empirical treatment against Fusobacterium infections.

The taxonomy of bacteria in the field of clinical microbiology is in a state of constant flux. A large-scale revamping of the classification and nomenclature of anaerobic bacteria has taken place over the past few decades, mainly due to advances in molecular techniques such as 16S rRNA and whole genome sequencing (WGS). New genera and species have been added, and existing genera and species have been reclassified or renamed. A major role of the clinical microbiological laboratories (CMLs) is the accurate identification (ID) and appropriate antimicrobial susceptibility testing (AST) for clinically important bacteria, and rapid reporting and communication of the same to the clinician. Taxonomic changes in anaerobic bacteria could potentially affect the choice of appropriate antimicrobial agents and the antimicrobial breakpoints to use. Furthermore, current taxonomy is important to prevent treatment failures of emerging pathogenic anaerobes with antimicrobial resistance. Therefore, CMLs should periodically update themselves on the changes in the taxonomy of anaerobic bacteria and suitably inform clinicians of these changes for optimum patient care. This article presents an update on the taxonomy of clinically important anaerobic bacteria, together with the previous names or synonyms. This taxonomy update can help guide antimicrobial therapy for anaerobic bacterial infections and prevent treatment failure and can be a useful tool for both CMLs and clinicians.