Identification and diversity of Actinomyces species in a clinical microbiology laboratory in the MALDI-TOF MS era

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has emerged as a reliable tool for bacterial identification. This study compared the Bruker MALDI-TOF BioTyper MS (MBT) and 16S rRNA gene sequencing for the identification of Actinomyces and Actinotignum spp. The MBT identified 68/77 (88.3%) of Actinomyces isolates to the genus-level and 44/77 (57.1%) of Actinomyces isolates to the species-level using the manufacturer's identification criteria. The MBT did not yield reliable identification for only 1/77 (1.3%) and generated no identification for 8/77 (10.4%) of the isolates. No misidentifications were found. Discordance at the species level was observed for eight isolates. Overall, the MBT demonstrated good concordance with the 16S rRNA gene sequencing with the exception of the closely related species A. naeslundii, A. viscosus and A. oris. A variety of Actinomyces spp. were isolated from orocervicofacial/dental specimens, but only a limited number of species were isolated from urine or intra-abdominal specimens. This study confirms the utility of MBT in the identification of Actinomyces spp. and describes the diversity and anatomic niche of species in human clinical specimens from various body sites.