Whole-genome sequence analyses reveal cryptic species-level diversity among clinical <i>Actinotignum</i> isolates misidentified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

<i>Actinotignum schaalii</i> is a human commensal bacterium that causes urinary tract infections, as well as invasive infections, predominantly in the elderly. Yet, isolation and reliable identification of <i>A. schaalii</i> in routine clinical microbiology is challenging. Today, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) has been recommended for the identification of <i>Actinotignum</i> bacteria but fails to discriminate between <i>A. schaalii</i> and <i>A. sanguinis</i>. The present study used whole-genome sequence analyses and demonstrated that, among 137 new human clinical <i>Actinotignum</i> isolates tentatively identified as <i>A. schaalii</i>/<i>A. sanguinis</i> through MALDI-TOF MS, none belonged to <i>A. schaalii</i> or <i>A. sanguinis</i>; rather, they represent seven novel <i>Actinotignum</i> species. The commercial MALDI-TOF MS identification databases need marked improvement if correct species-level identifications will be reliable. Currently, whole-genome sequence-based identification may need to be used to provide correct species identifications and to assess the genetic and clinical differences between established and newly defined <i>Actinotignum</i> species.