Cutibacterium acnes in Elective Spine Surgery: A 24-Year Characterization of Treated and Untreated Culture-Positive...

Cutibacterium acnes is a frequent culture from spinal surgical-site infection (SSI), but as a ubiquitous skin commensal a positive culture is of uncertain significance, and no accepted definition of low-grade spinal implant infection exists.

To characterize all C. acnes-positive spinal SSI episodes over a 24-year period and to describe how episodes the treating team elected to treat differ from those left untreated.

Retrospective single-center cohort of consecutive elective spinal procedures at a tertiary spine center, 2002-2025.

All 164 C. acnes-positive spinal SSI episodes identified by structured manual review of an institutional database (92 treated, 72 untreated). Primary spinal infections were excluded.

Polymicrobial co-isolation, time-to-positivity (TTP), semiquantitative bacterial load, number of C. acnes-positive specimens, peak C-reactive protein (CRP) and white blood cell count (WBC), wound complications, antimicrobial susceptibility and subsequent same-region surgery.

Groups were compared by the Mann-Whitney U and Fisher exact tests. A logistic-regression model with the treatment decision as the outcome, evaluated by leave-one-out cross-validation, described factors associated with treatment.

Treated episodes were more often polymicrobial (41.3% versus 6.9%, p<0.001), grew faster (TTP 6 versus 9 days, p<0.001), more often yielded two or more positive specimens (58% versus 14%, p<0.001) and grew at higher load. Peak CRP was higher in treated episodes (90 versus 52 mg/L, p=0.012) and wound complications more frequent (40% versus 17%, p=0.001). WBC did not differ. Treated episodes were sampled more intensively (3.5 versus 1 cultures, p<0.001). Susceptibility to amoxicillin, imipenem, linezolid and vancomycin exceeded 99%. Clindamycin resistance was 3.8%.

Treated and untreated episodes differed across every culture-derived measure and in CRP and wound complications, whereas demographics and onset category were similar. Because the treating team's decision drove both the extent of sampling and the treatment itself, these features describe that decision rather than a proven infection status. Polymicrobial growth, a short TTP, more positive specimens and a higher CRP were each independently associated with the decision and remained so when sampling intensity was accounted for. They are candidate features for classifying a C. acnes-positive culture and require prospective validation against an accepted definition of low-grade spinal infection.