Revealing the clinical relevance of Staphylococcus borealis

Staphylococcus borealis, previously misidentified as Staphylococcus haemolyticus, was first described as a new species in 2020. In this study, we aimed to describe the clinical relevance of S. borealis by combining clinical data, antibiotic susceptibility profiles, and biofilm formation in isolates obtained from hospitalized and non-hospitalized patients. We established a collection of 129 S. borealis isolates from 129 adult patients from seven Norwegian hospitals. We describe clinical data at the time of microbiological specimen collection. Antibiotic susceptibility and biofilm formation were tested using established methods. Of 129 isolates (37%), 48 were from patients admitted to a hospital, the remaining from outpatients. The median (IQR) age was 62 (51-78) years, and 85/129 (66%) of the isolates were from male patients. The majority (81/129, 63%) of the S. borealis isolates were isolated from urine cultures, followed by isolation from skin and soft tissue cultures (35/129, 27%), blood cultures (8/129, 6%), and two implant-associated infections (2/129, 2%). Resistance to ≥3 antibiotic classes was observed in 43/129 (33%) of the isolates. All isolates formed a biofilm under the conditions tested; 59/129 (46%) weak, 40/129 (31%) medium, and 29/129 (23%) were strong biofilm producers. S. borealis clinical samples were predominately obtained from elderly male patients, and the majority of samples were from patients with suspected urinary tract or skin and soft tissue infections. The level of multidrug resistance was comparable to other coagulase-negative staphylococcal species, but resistance toward methicillin and penicillin was lower than in clinical S. haemolyticus isolates.

This study contributes novel knowledge on the clinical relevance of Staphylococcus borealis; this is of importance when clinical microbiologists encounter S. borealis identified in patient samples. S. borealis was mainly identified in microbiological specimens from middle-aged to elderly patients, predominantly males. Hospitalized patients were also frequently immunocompromised and had other underlying conditions accompanying a suspected S. borealis infection.