Silver-colored colonies of Staphylococcus argenteus

We present a case of a 74-year-old man with Staphylococcus argenteus bacteremia from a shunt used during hemodialysis. The gram-positive organism isolated from the blood culture was initially identified as S. aureus by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. However, the isolated pathogen formed silver colonies (Fig. 1A), which is atypical for S. aureus, which typically forms gold colonies (Fig. 1B). Genetic analysis revealed the isolated strain was S. argenteus, a new subspecies under S. aureus [1]. S. argenteus forms its characteristic silver colony due to the absence of the carotenoid pigment staphyloxanthin [1]. A recent study revealed S. argenteus bacteremia has a higher mortality rate than S. aureus bacteremia [2]. Physicians also should be aware that although S. argenteus may be pathogenically similar to S. aureus, its characteristic silver-colored colonies may aid in its identification because identifying S. argenteus by conventional microbiology methods is difficult as described in this case.

Fig. 1.

Silver colonies of S. argenteus (Fig. 1A) and gold colonies of S. aureus (Fig. 1B) on BBL Trypticase Soy Agar (Becton, Dickinson and Company).

CRediT authorship contribution statement

J.H. contributed to the writing as a first author and editing of the manuscript. N.M. contributed to the Conceptualization and review of the manuscript. N.A. and H.M. contributed to Supervision and Validation.

Ethical approval

The ethics committee of our institution approved the waiver in this case report, based on the Japanese ethical guidelines for clinical research to publish the case details.

Consent

Written informed consent was obtained from the patient for the publication of this report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal on request.

Conflict of interest

We declare that all authors have no Conflict of Interest (COI).