The Conundrum of the Gram-Positive Rod: Are We Missing Important Pathogens in Complicated Skin and Soft-Tissue Infections? A Case Report and Review of the Literature

Abstract

Abstract

Background:

The designation “gram-positive bacillus” includes a variety of pleomorphic microorganisms, including diphtheroids, coryneform species, coccobacilli, and other small rods. Despite differing greatly in their virulence, sources, and even genus, these microscopically similar organisms are often difficult to differentiate without genetic testing.

Methods:

We present a patient with necrotizing fasciitis and a review of the literature to exemplify and assess the scope of this diagnostic conundrum. Cultures taken intra-operatively during surgical debridement grew Morganella morganii and “diphtheroids.” Given the low virulence of both organisms, the diphtheroids were reexamined microscopically and assayed for enzymatic activity. Genetic sequence analysis of 16S ribosomal ribonucleic acid (rRNA) was required for species identification.

Results:

Microscopic inspection identified small, non-spore-forming, gram-positive rods, arranged in clusters, that formed circular, smooth colonies. These were facultatively anaerobic, catalase-negative, non-hemolytic, and unable to reduce nitrates. Standard techniques and assays were unable to identify our organism to species. Ultimately, 16S rRNA gene sequencing of 833 base pairs achieved a 99.04% species match to Arcanobacterium bernardiae.

Conclusion:

At our facility, diphtheroids are generally considered non-pathogenic contaminants in skin and soft tissue infections. The finding of A. bernardiae in necrotizing fasciitis is unusual and clinically important but would have been missed using conventional methods. As the “gram-positive bacillus” comes to include an ever-increasing number of organisms, genetic sequencing will probably be required more regularly for species identification. Furthermore, given that these genera are similar, often mistaken as contaminants, and difficult to differentiate using standard assays, they may often be missed and are possibly a more-frequent cause of complicated skin and soft tissue infections than the literature would suggest.