ABSTRACT Actinotignum schaalii is a human commensal bacterium that is well-known for causing urinary tract infections, predominantly in the elderly. However, reliable identification of A. schaalii in routine clinical microbiology has been challenging, mainly due to its fastidious growth and identification issues. The genus Actinotignum currently comprises three species with validly published names, i.e., A. schaalii , A. sanguinis , and A. urinale . The former two are difficult to distinguish by routine clinical microbiology laboratory protocols, including phenotypic profiling and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS). A total of 137 new Actinotignum isolates were collected from urine and various other human clinical samples between 2010 and 2023 and were identified by MALDI-TOF MS as A. schaalii / A. sanguinis . Whole-genome sequence-based analyses identified none of these isolates as A. schaalii or A. sanguinis and demonstrated that the strains were clustered into seven genomic groups. These seven genomic groups represented seven novel Actinotignum species, which we propose to be classified as A. lotii sp. nov. ( n = 82 isolates), A. saccati sp. nov. ( n = 38), A. inguinis sp. nov. ( n = 8), A. vesiculae sp. nov. ( n = 3), A. stranguriae sp. nov. ( n = 3), A. cystesis sp. nov. ( n = 2), and A. urematis sp. nov. ( n = 1). Therefore, MALDI-TOF MS analysis allows reliable identification only to the genus level for Actinotignum species, whereas commercial identification libraries need revision and extension to improve their diagnostic capacities. IMPORTANCE Actinotignum schaalii 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 A. schaalii in routine clinical microbiology is challenging. Today, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) has been recommended for the identification of Actinotignum bacteria but fails to discriminate between A. schaalii and A. sanguinis . The present study used whole-genome sequence analyses and demonstrated that, among 137 new human clinical Actinotignum isolates tentatively identified as A. schaalii / A. sanguinis through MALDI-TOF MS, none belonged to A. schaalii or A. sanguinis ; rather, they represent seven novel Actinotignum 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 Actinotignum species.
Winterflood et al. (Tue,) studied this question.