Answer to May 2024 Photo Quiz

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ANSWER TO PHOTO QUIZ

The Gram stain reveals Gram-positive cocci in chains with some areas of over-decolorization, which is compatible with Streptococcus species. Nonhemolytic white colonies grew on the sheep blood agar. Interestingly, the organism is catalase-positive and cross-reacts weakly with Lancefield group B latex reagent. There was no identification from VITEK MS (bioMérieux); however, the isolate was identified as Streptococcus halichoeri by MALDI Biotyper (Bruker Daltonics Inc.). Antibiotic susceptibilities were performed via Sensititre Streptococcus STP6F AST Plate (Thermo Scientific) and interpreted by Clinical and Laboratory Standards Institute (CLSI) guidelines for β-hemolytic group Streptococcus species (CLSI M100-ED33:2023) due to the close relationship of S. halichoeri to pyogenic streptococci (1, 2); however, prior authors have utilized the guidelines for viridans group Streptococcus species (1, 3). The isolate was susceptible to penicillin G, ceftriaxone, clindamycin, and vancomycin. The patient was treated and improved on vancomycin.

S. halichoeri was originally isolated from grey seals (Halichoerus grypus) and proposed as a distinct species in 2004 (4). It has since been identified in fish as well as terrestrial and aquatic mammals and presents as a zoonotic infection, manifesting as cellulitis, in humans (5). The taxon is closely related to more commonly encountered large-colony streptococci including S. agalactiae and S. dysgalactiae. S. halichoeri was first described as causing human infection in 2014 in a post-operative empyema (1, 3, 4). Because of the catalase positivity and nonhemolytic colonies, S. halichoeri can be misidentified as Staphylococcus species; however, the chaining arrangement of the cocci on Gram stain is not compatible with staphylococci. Lancefield group B cross-reactivity further confuses the situation as this is not S. agalactiae, although the reactivity is weak compared to S. agalactiae (1). S. halichoeri can be discriminated from S. agalactiae by pyrrolidonyl arylamidase (PYR) positivity and hippurate hydrolysis negativity. S. pseudoporcinus also cross-reacts with Lancefield group B reagents, but this species is catalase-negative and beta-hemolytic, differentiating it from S. halichoeri (6). VITEK 2 (bioMérieux) could misidentify the organism as S. suis or S. pyogenes (1, 7). Definitive identification of S. halichoeri can be achieved using contemporary matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) analysis which has been shown to reliably identify S. halichoeri in several studies using Bruker MALDI Biotyper non-clinically validated library (S. halichoeri is absent in the bioMérieux Vitek MS databases, clinically or non-clinically validated) (1, 7). 16S rRNA sequencing can be used; however, some laboratories reported variable success with 16S sequencing (4, 7). S. halichoeri is susceptible to many antibiotic classes including beta-lactams as is typical of large-colony streptococci, though some studies have shown isolates that were resistant or nonsusceptible to erythromycin, clindamycin, doxycycline, or tetracycline (1, 2, 7).