Actinotignum (formerly Actinobaculum) schaalii: a review of MALDI-TOF for identification of clinical isolates, and a proposed method for presumptive phenotypic identification

Abstract

Actinotignum schaalii is a recognised cause of urinary tract infection. Following a case of urosepsis, 20 isolates of A. schaalii were collected over an initial 18 month period from urine and blood culture. An additional 25 isolates were collected over the following 13 months. Actinotignum schaalii had likely been overlooked or dismissed as a contaminant in our laboratory prior to this study period. It grows slowly on blood agar with supplemental CO2 or anaerobically. It may not grow on MacConkey agar or chromogenic agars used for routine urine culture. Repeated attempts at identification by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) may be required as the optimal age of cultures for testing is unclear. Utilising its characteristic antibiogram may assist phenotypic identification of this organism which is otherwise difficult to distinguish from other actinomycetes.

Keywords: Actinobaculum schaalii; Australia; MALDI-TOF MS; actinomycete; antibiogram; case series; ciprofloxacin; fluoroquinolone; phenotypic identification; phenotypic screen; trimethoprim; urinary tract infection.