Intradiscal Cutibacterium acnes Prevalence Immediately Prior to Fusion or Total Disc Replacement Device Implantation: A Prospective Registry Study of Cervical and Lumbar Discs

Abstract

Background

Cutibacterium acnes has been identified in excised intervertebral disc tissue in multiple studies; however, contamination remains a persistent limitation, and all prior investigations have relied on excised tissue sampling. No study has evaluated swab-based sampling directly from the disc space. The primary aim of this study was to determine the prevalence of C acnes in intervertebral disc spaces following discectomy but prior to fusion cage or total disc replacement implantation using anaerobic culture and quantitative polymerase chain reaction (qPCR) and to assess concordance between detection methods.

Methods

This prospective cohort registry study included 29 patients (53 discs: 28 cervical, 25 lumbar) undergoing discectomy. Prior to discectomy, deep tissue was irrigated with 0.05% chlorhexidine gluconate (Irrisept) to suppress dermal-origin organisms. Following discectomy and before device implantation, a sterile swab was inserted into the disc space. Swabs were divided for 21-day anaerobic culture with semiquantitative grading and 16S rRNA qPCR (positivity threshold >1.04 copies/reaction). Cohen kappa, McNemar test, and Clopper-Pearson exact 95% confidence intervals were calculated.

Results

Overall disc-level prevalence was 43.4% (23/53; 95% CI: 29.8%–57.7%) by culture and 54.7% (29/53; 95% CI: 40.4%–68.4%) by qPCR. Cervical discs demonstrated significantly higher culture positivity than lumbar discs (71.4% vs 12.0%; P < 0.001), while qPCR positivity was similar (50.0% vs 60.0%). When either method was positive, prevalence was 85.7% in cervical and 64.0% in lumbar discs. Inter-method agreement was poor overall (κ = −0.044; 95% CI: −0.31 to 0.22). In lumbar discs, qPCR identified significantly more positive discs than culture (P = 0.003).

Conclusions

Cutibacterium acnes is detectable in intervertebral disc spaces at high rates using swab-based sampling after deep-tissue chlorhexidine gluconate irrigation, particularly in cervical discs. Poor concordance between culture and qPCR suggests each method identifies distinct subsets of positive discs, particularly in lumbar discs.

Clinical Relevance

This is the first report of disc-space swab analysis for intradiscal C acnes detection at the time of cervical and lumbar surgery. Detection prior to fusion cage or total disc replacement implantation may help characterize the microbiological environment at the implant interface and supports further investigation into strategies to reduce disc-space C acnes burden prior to implantation, particularly in total disc replacement.

Level of Evidence

3.

Introduction

Cutibacterium acnes (formerly Propionibacterium acnes) is a slow-growing, anaerobic, gram-positive rod that colonizes human skin and sebaceous glands.^1,2 Over the past 2 decades, accumulating evidence has implicated C acnes as a pathobiont within intervertebral disc tissue, with reported prevalence ranging from 0% to over 60% depending on detection methodology, anatomic region, and contamination-control measures.^3–7 A 2019 systematic review and meta-analysis of 16 studies comprising 2084 surgical disc specimens reported a pooled bacterial detection rate of 25.3%, with C acnes accounting for 15.5%.⁸ However, recent reports have demonstrated greater cervical prevalence using anaerobic culture (66.4% of 79 discs in 40 patients undergoing anterior cervical discectomy and fusion [ACDF]).⁶ Recent microbiome investigations have demonstrated a biofilm-type environment conducive to C acnes viability.⁹ More recently, C acnes antigens have been identified intracellularly within nonherniated disc cells, correlating with catabolic markers including matrix metalloproteinase-3 and gasdermin D, supporting a biologically active rather than contaminant role.¹⁰ In addition, various genetic strands have been isolated from the biofilm of orthopedic implant infections that may play a synergistic role in the degree of pathological processes present.¹¹

A fundamental challenge in intradiscal C acnes research is distinguishing endogenous colonization from surgical contamination. Because C acnes is the dominant bacterium of the pilosebaceous unit, any tissue specimen retrieved through an open surgical wound is susceptible to contamination from the skin incision and subcutaneous tissues. We have conceptualized this problem using a 2-reservoir framework: reservoir 1 comprises C acnes residing in the dermal and subcutaneous tissues traversed during surgical approach, while reservoir 2 comprises organisms endogenous to the disc space itself. Without attenuation of reservoir 1, any positive result from a disc specimen is potentially confounded by contamination during retrieval.

Our group previously demonstrated in a randomized controlled trial that subcutaneous irrigation with 0.05% chlorhexidine gluconate (CHG; Irrisept) during primary shoulder arthroplasty reduced deep-tissue C acnes culture rates by approximately 50% compared with saline.¹² Liu et al independently confirmed the superiority of 0.05% CHG over povidone-iodine and saline for reducing bacterial contamination on implant surfaces during lumbar fusion.¹³ Building on these findings, we developed a protocol in which 0.05% CHG is irrigated into the deep surgical tissue (prevertebral fascia) prior to discectomy, with the specific intent of suppressing reservoir 1 organisms before the disc space is entered. This approach allows specimens obtained from within the disc space to more reliably reflect reservoir 2 colonization. With recent reports of devastating delayed and late total disc replacement (TDR) C acnes infections,^14,15 removals of disc-space C acnes presence prior to device implantation might be preventative. In addition, the etiology of significant cervical TDR heterotopic ossification (HO) at 10 years may be related to the hyperinflammatory state of C acnes chronic colonization in biofilm.^16,17

A notable limitation of the existing literature is that all prior studies analyzing C acnes at the disc level have used excised disc tissue—nucleus pulposus or annulus fibrosus fragments retrieved during discectomy for culture^3–7 or quantitative polymerase chain reaction (qPCR) of C acnes.^8,18 However, to our knowledge, no study has reported swab-based sampling of the disc space itself prior to implantation of a fusion device or TDR. Swab specimens offer a complementary approach: rather than culturing organisms embedded within tissue, a swab samples the disc-space environment directly, potentially capturing organisms adherent to endplates or present in the residual disc milieu following discectomy. If a significant prevalence of residual C acnes in the disc space is present, then improved removal methods might need to be employed before device implantation.

A second important gap in the literature is the absence of concordance data between culture and molecular detection methods for intradiscal C acnes. While qPCR has been used as an adjunct to culture in shoulder surgery and other implant infections,^19,20 its application to intervertebral disc specimens has not been systematically evaluated. The purpose of this prospective registry study was 3-fold: (1) to determine the prevalence of C acnes in cervical and lumbar disc spaces using swab-based sampling following deep-tissue CHG irrigation, (2) to compare 21-day anaerobic culture with 16S rRNA gene-targeted qPCR as parallel detection methods, and (3) to evaluate the concordance between these methods across spinal regions.

Methods

Study Design and Population

Institutional review board approval was obtained prior to enrollment. Inclusion criteria were age ≥18 years and planned discectomy (ACDF or lumbar discectomy/fusion). Exclusion criteria included active spinal infection, prior surgery at the index level, and antibiotic use within 14 days of surgery. The prospective cohort study enrolled near-consecutive patients undergoing cervical or lumbar discectomy at a single institution (Irvine, CA) as part of an ongoing C acnes disc registry. At consent, peripheral blood was drawn for future C acnes biomarker analysis, and the patient was identified as a registry subject. Four subjects signed the informed consent form but then refused the blood draw. These cases were considered screen failures. The surgery date was canceled after the consent was signed and blood was drawn in 3 patients. The swab specimens were not collected in 5 patients due to surgical decision-making. After excluding these cases, 29 patients (53 disc levels) were enrolled with full data, comprising 28 cervical and 25 lumbar disc levels.

Surgical Protocol and Deep-Tissue CHG Irrigation

All procedures were performed by fellowship-trained spine surgeons (J.S., G.A., and J.W.). Standard surgical preparation included CHG-based skin antisepsis. All patients received standard-of-care preincisional doses of cefazolin (or, for PCN allergies, vancomycin), followed by cefazolin every 8 hours and vancomycin every 12 hours until 24 hours after the end of surgery. After incision and exposure of the anterior cervical or lumbar (or lateral) spine, and before disc entry, the deep surgical tissue (prevertebral fascia) was irrigated with 450 mL of 0.05% CHG (Irrisept; Irrimax Corporation, Lawrenceville, GA, USA) delivered via the manufacturer’s pressurized delivery system and allowed to dwell on the tissues for 1 minute. This deep-tissue irrigation step was designed to suppress reservoir 1 organisms in the wound and surrounding soft tissues so that specimens subsequently obtained from the disc space would preferentially reflect reservoir 2 (endogenous disc-space) organisms. Discectomy was then performed per standard technique.

Discectomy and Specimen Collection

Immediately following deep tissue irrigation, the discectomy was performed at each level, and a single sterile culturette swab was inserted into the evacuated disc space and rotated against the prepared endplates for approximately 10 seconds. The swab was then withdrawn, placed in transport medium, and divided at the laboratory: one portion was submitted for 21-day anaerobic culture, and the second for qPCR analysis. At multilevel procedures, independent swabs were obtained from each disc level. All specimens were labeled with level-specific identifiers to enable disc-level analysis.

21-Day Anaerobic Culture

Swab specimens were inoculated onto anaerobic blood agar and into thioglycolate (THIO) broth and incubated under strict anaerobic conditions at 37°C for 21 days. Plates were inspected at days 7, 14, and 21. Growth was reported using a semi-quantitative grading scale: no growth, rare, light, moderate, or heavy. Isolates identified as C acnes were confirmed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Cultures positive only from THIO broth enrichment were reported as “isolated from THIO.”

Swab DNA Extraction

The second swab portion was stored frozen at −20°C until further processing. At the processing event, the swabs were thawed on ice for 15 minutes, then vortexed vigorously for 2 minutes at the highest speed to elute cells in transport buffer, then 200 µL were taken from each swab sample tube and centrifuged at 10,000 g for 5 minutes at 4°C. The supernatant was carefully removed, and 100 µL 1 × TE buffer (pH 8.0) was added to each pellet and mixed well prior to enzymatic prelysis to disrupt the C acnes gram-positive cell wall. A total of 20 µL of lysozyme solution (Thermo Scientific, Cat#90082), at a working concentration of 10 mg/mL, was then added to each sample, mixed, and incubated at 37°C for 60 minutes with agitation. DNA was extracted following QIAamp Mini Kit’s instructions for gram-positive bacteria (Qiagen, Cat#51304). DNA concentration was assessed using the Qubit dsDNA High Sensitivity Assay (Thermo Scientific, Cat#Q33230). Nanodrop was also used to assess the purity of the samples and to flag the presence of polymerase chain reaction inhibitory material. All samples were normalized to 1 ng of input for qPCR assessment.

qPCR

A TaqMan-based qPCR assay targeting the 16S ribosomal RNA gene was performed with primers and probe specific for C acnes reported by previous publications.²⁰ The 15 µL reactions contained 5 µL of sample input, 500 nM each of forward and reverse primer, 250 nM of FAM/BHQ-1 dual-labeled probe, and 1 × Fast Advanced TaqMan Master Mix for qPCR (Thermo Scientific, Cat#4444556). The amplification was done on QuantStudio 5 Real-Time PCR System (Thermo Scientific) using the following cycling conditions: uracil-DNA glycosylase (UDG) carryover prevention at 50°C for 2 minutes and initial denaturation at 95°C for 2 minutes followed by 45 cycles of denaturation at 95°C for 10 seconds and combined annealing/extension with fluorescence acquisition at 60°C for 30 seconds. All patient samples were run in duplicate, with a minimum of 2 No Template Control (NTC) wells and 1 positive control well included per plate, along with a 7-point calibration curve using Quantitative Genomic DNA from C acnes (ATCC, Cat# 6919DQ).

Standard Curve and Copy Number Quantification

Absolute copy number quantification was performed using a 7-point external standard curve prepared from genomic DNA of C acnes. A working concentration was prepared from the DNA (70 ng/µL) 1:125 in TE buffer to yield approximately 0.56 ng/µL, corresponding to approximately 1 × 10⁶ genome equivalents per 5 µL reaction input. A 7-point serial 1:10 dilution was performed in TE buffer (Std 1–7), spanning a range of approximately 1 × 10⁶ to 2 genome equivalents per reaction (log₁₀ ~6.0 to ~0.3). Low-binding microcentrifuge tubes (Thermo Scientific, Cat# 3404DLB) were used for all dilution points to prevent DNA adhesion to tube walls at low template concentrations. A complete standard curve was included on every PCR plate.

Specimens with Ct values within the quantifiable range of the standard curve were classified as qPCR-positive; results falling below the lowest standard were not extrapolated and were reported as not detected. The analytical limit of detection, defined as the lowest standard curve point yielding consistent amplification across all replicates, was 1.17 copies per reaction (STD#7, Ct 34.65). Given the low-biomass nature of the disc-space swab samples, a positivity threshold of >1.04 copies per reaction was applied to all clinical samples, defined as signal exceeding the extraction negative control result (1.04 copies per reaction; Ct 34.24), to differentiate true positive amplification from reagent background signal, originating from the extraction process rather than PCR reagents. The NTC signal showing no amplification across all plates further confirms the background signal is originating from the extraction process. This phenomenon of ubiquitous C acnes DNA detection in extraction reagents and negative controls has been previously reported in TaqMan-based qPCR assays for Cutibacterium species, supporting the use of empirically derived positivity thresholds above reagent background rather than analytical limit of detection (LOD) alone.²⁰

Statistical Analysis

Prevalence was calculated at the disc and patient level. A patient was classified as positive if any disc level was positive by the respective method. Exact 95% confidence intervals were calculated using the Clopper-Pearson method. Inter-method agreement was assessed using Cohen kappa (κ) with 95% confidence intervals. McNemar test with continuity correction was used to evaluate systematic differences in positivity rates between methods. Subgroup analyses were performed by spinal region (cervical vs lumbar). Statistical significance was set at P < 0.05.

Individual-level disc data, including culture and qPCR results for all 53 disc sample levels, are provided in the Supplementary Table 1.

Results

Demographics

Twenty-nine patients (15 women and 14 men) with a mean age of 61.2 ± 11.6 years (range, 29–83 years) were enrolled. Thirteen patients contributed 28 cervical disc levels (range, C3–C4 to C6–C7), and 16 patients contributed 25 lumbar disc levels (range, L2–L3 to L5–S1). The most frequently sampled levels were C5 to C6 (n = 10) and C6 to C7 (n = 9) in the cervical spine and L4 to L5 (n = 9) and L5 to S1 (n = 9) in the lumbar spine. Three surgeons performed the procedures (J.S., n = 18 patients; G.A., n = 6; J.W., n = 5). Patient demographics and surgical characteristics are summarized in the Table.

Table.

Patient demographics and surgical characteristics (N = 29).

ID	Sex	Age at Time of Surgery, y	No. of Levels	Surgery Type
1	M	63	2	ACDF C5–7
4	M	71	2	ACDF C5–7
5	F	39	2	TDR L4–S1
6	M	66	1	XLIF L3–4
9	F	64	2	ACDF C5–7
11	M	67	3	ACDF C3–6
13	F	57	1	ALIF L4–5/ PSF
15	M	29	1	ALIF L5–S1/ PSF
14	M	55	1	ACDF C5–6
17	F	70	1	ALIF L4–5
18	F	58	2	TDR C5–7
19	F	83	1	XLIF L2–3
20	M	72	2	XLIF L2–4
21	F	61	4	ACDF C3–7
22	F	69	4	ALIF L2–S1
25	M	67	1	ALIF L5–S1
28	F	72	1	XLIF L3–4
30	F	56	3	TDR C4–6
16	M	57	2	ALIF L4–S1
23	M	66	2	ALIF L3–5/ PSF
27	F	71	3	ACDF C4–7
31	M	53	2	ACDF C4–7
33	F	71	2	ALIF L4–S1/ PSF
29	F	45	1	TDR L4–5
43	M	52	2	TDR C4–6
37	M	44	1	ALIF L5–S1
38	M	73	2	ALIF L4–S1
42	F	61	1	ALIF L5–S1
44	F	64	1	ALIF L5–S1

Abbreviations: ACDF, anterior cervical discectomy and fusion; ALIF, anterior lumbar interbody fusion; ID, subject identification number; PSF, posterior spinal fusion; TDR, total disc replacement; XLIF, extreme lateral interbody fusion.

Overall Prevalence

No positive 21-day cultures showed “Moderate” or “Heavy” growth. “Rare” or “Light” results were not reported to the clinical team nor were antibiotic sensitivities performed. There were no wound infections in this group of patients. However, as per our “Methods” section, these “Rare” and “Light” C acnes 21-day culture results were considered statistically “positive.”

At the disc level, 23 of 53 discs (43.4%; 95% CI: 29.8%–57.7%) were positive by anaerobic culture, and 29 of 53 discs (54.7%; 95% CI: 40.4%–68.4%) were positive by qPCR. At the patient level, 12 of 29 patients (41.4%) had at least 1 culture-positive disc, and 18 of 29 patients (62.1%) had at least 1 qPCR-positive disc. When either method was considered positive, 40 of 53 discs (75.5%) harbored detectable C acnes (Figures 1 and 2). Please refer to Supplementary Table 1 for the complete disc-level data.

Figure 1.

Disc-level Cutibacterium acnes prevalence by detection method.

Figure 2.

Patient-level Cutibacterium acnes prevalence by detection method.

Cervical Disc Results

Cervical discs demonstrated the highest detection rates. Cultures were positive in 20 of 28 discs (71.4%; 95% CI: 51.3%–86.8%) and qPCR in 14 of 28 discs (50.0%; 95% CI: 30.6%–69.4%). At the patient level, 9 of 13 cervical patients (69.2%) were culture-positive, and 9 of 13 cervical patients (69.2%) were qPCR positive. Both methods were simultaneously positive in 10 of 28 discs (35.7%), while at least 1 method was positive in 24 of 28 discs (85.7%). At the patient level, both methods were positive in 7 of 13 patients (53.8%), and either method was positive in 11 of 13 (84.6%). The predominant culture grade was light growth (13/28), followed by rare (5/28) and moderate (1/28), with 1 specimen isolated from THIO broth only.

Lumbar Disc Results

Lumbar discs showed markedly lower culture positivity: 3 of 25 (12.0%; 95% CI: 2.5%–31.2%) vs 15 of 25 (60.0%; 95% CI: 38.7%–78.9%) by qPCR. At the patient level, 3 of 16 (18.8%) were culture positive and 9 of 16 (56.2%) were qPCR positive. Both methods were positive in only 2 of 25 discs (8.0%), while either method was positive in 16 of 25 (64.0%). Lumbar culture grades were light (n = 2) and rare (n = 1).

Concordance Between Culture and qPCR

Overall agreement between culture and qPCR was 47.2%, with a Cohen κ of −0.044 (95% CI, −0.31 to 0.22), indicating negligible agreement between these different detection methods. Eleven discs were culture positive but qPCR negative, and 17 discs were qPCR positive but culture negative. McNemar test was not significant overall (P = 0.345), indicating no systematic difference in overall positivity rates between methods.

In the cervical subgroup, agreement was 50.0% (κ = 0.00; 95% CI: −0.37 to 0.37; McNemar P = 0.181). In the lumbar subgroup, agreement was 44.0% (κ = 0.028; 95% CI: −0.31 to 0.37), but McNemar test was significant (P = 0.003), reflecting the substantially higher qPCR positivity rate (60.0%) compared with culture (12.0%) in the lumbar spine.

Regional Divergence

Culture positivity was significantly higher in cervical discs (71.4%) than lumbar discs (12.0%), whereas qPCR positivity was similar across regions (cervical 50.0% and lumbar 60.0%). This regional divergence in culture yield, despite comparable qPCR rates, suggests that C acnes may be present in both regions but in forms differentially recoverable by culture—potentially reflecting differences in bacterial load, viability, or biofilm state between cervical and lumbar discs.

Discussion

This prospective registry study demonstrates that C acnes is detectable at high rates in intervertebral disc spaces using swab-based sampling following deep tissue CHG irrigation. To our knowledge, this is the first report employing disc-space swab specimens—rather than excised disc tissue—for intradiscal C acnes detection. The use of dual detection methods (21-day anaerobic culture and 16S rRNA qPCR) revealed that each modality captures a partially nonoverlapping population of colonized discs, with poor inter-method concordance across all subgroups.

Swab-Based Disc-Space Sampling

As already stated, all prior studies evaluating C acnes in intervertebral discs have analyzed excised tissue specimens—nucleus pulposus or annulus fibrosus fragments—obtained during discectomy. Capoor et al demonstrated C acnes biofilm within homogenized disc tissue using confocal microscopy,⁴ and Nüesch et al detected intracellular C acnes antigens in nonherniated disc cells.¹⁰ While tissue-based methods detect organisms embedded within the disc matrix, swab-based sampling captures organisms present on endplate surfaces and within the disc-space milieu. These approaches are complementary: tissue culture reflects organisms sequestered within the disc body, whereas a disc-space swab interrogates the broader environment exposed during discectomy. The high prevalence observed in our study (85.7% of cervical discs positive by at least 1 method) suggests that the disc space harbors a substantial C acnes burden that may be underestimated by either method alone.

Deep-Tissue CHG Irrigation and the 2 Reservoirs Framework

The contamination challenge in intradiscal bacteriology cannot be overstated. Farhan-Alanie et al concluded that C acnes cultured from disc tissues likely represents contamination,⁷ while Astur et al found near-zero intradiscal detection when stringent contamination controls were applied.^18,21 Our approach differs fundamentally from these studies. By irrigating 0.05% CHG into the deep surgical tissue before entering the disc space, we specifically targeted reservoir 1 organisms. Our group’s prior randomized trial demonstrated that subcutaneous CHG irrigation during shoulder arthroplasty reduced deep C acnes culture rates by approximately 50% (Kassam).¹² Liu et al independently showed that 0.05% CHG was superior to both povidone-iodine and saline for reducing bacterial contamination on implant surfaces during posterior lumbar fusion.¹³ Importantly, CHG irrigation does not penetrate intact disc tissue; therefore, organisms detected within the disc space after CHG irrigation more likely represent endogenous (reservoir 2) colonization rather than contamination from the surgical approach.

Poor Concordance Between Detection Methods

The near-zero κ values observed across all subgroups indicate that culture and qPCR identify largely nonoverlapping sets of positive discs. This finding has important implications for prevalence estimation and study design. Eleven discs were culture positive but qPCR negative, suggesting either sampling variability between the 2 swab portions, DNA below the qPCR detection threshold, or PCR inhibition. Conversely, 17 discs were qPCR positive but culture negative, likely reflecting the presence of nonviable or biofilm-embedded organisms whose DNA is detectable but whose growth is suppressed under standard culture conditions. Como et al similarly observed discordance between culture and qPCR for C acnes in shoulder surgery specimens, with a 12.2% qPCR-positive rate despite 0% culture positivity.¹⁹

Regional Divergence in Detection Patterns

The markedly higher culture positivity in cervical discs (71.4%) compared with lumbar discs (12.0%) is consistent with prior tissue-based observations. Farhan-Alanie et al reported cervical C acnes isolation rates of 25.7% vs 4.8% in lumbar specimens.⁷ Camacho et al reported 60.1% culture positivity in cervical disc tissue.⁵ The proximity of cervical discs to the pilosebaceous-dense skin of the head, neck, and upper trunk may explain some of this regional variation. However, the observation that qPCR positivity was nearly identical between regions (50.0% cervical and 60.0% lumbar) suggests that the cervical-lumbar disparity may be more a function of organism viability or culturability than absolute burden. Lumbar discs may harbor C acnes in a dormant or biofilm state detectable by molecular methods but not recoverable by culture. This process may be associated with genetically different subtypes that are synergistically forming biofilm, akin to reports associated with orthopedic implant chronic infections.¹¹ Last, a recent report of rat Intervertebral Disc (IVD) C acnes injections demonstrated a progressive inflammatory and degenerative process compared with normal control disc tissue, with longer infection times associated with less ability to culture the organism. This information has added more evidence for a pathological role of C acnes in IVD, along with an explanation for culture inconsistency.²²

Clinical Implications

The clinical significance of intradiscal C acnes remains an area of active investigation. Cutibacterium acnes infection has been implicated in low back pain associated with disc degeneration and more specifically, endplate inflammation and Modic changes, and treatment with antibiotics has been recommended based upon an randomized controlled trial (RCT) in 2013.²³ However, due to poor corroborating evidence, recent reviews have not recommended this,²⁴ and a 2026 RCT published in JAMA did not replicate the positive findings of the 2013 study.²⁵ However, inflammatory cascades due to low-dose C acnes colonization may lead to symptomatic osteolysis or obvious delayed infection following total disc arthroplasty.^14,15

In addition to the severe complication of late postoperative infection with TDRs, a chronic inflammatory state may also provide an opportunity for HO. This complication has been reported with increasing frequency following TDR. Interestingly, HO and C acnes in human disc tissue are both associated with identical inflammatory cytokines. These include IL-6, IL-10, MCP-, IP-10, and MIP-1a.^16,26 A recent systematic review and meta-analysis of a 10-year follow-up of TDR reported a 70% overall incidence of HO at 10 years postoperatively and 60% within the first 5 to 6 years.¹⁶ This study’s C acnes cervical disc space prevalence rate is similar. Shared inflammatory mediators and similar prevalence rates suggest that colonized C acnes might predispose patients to this complication.

In addition to collecting the swabs, disc tissue specimens were collected in parallel at every level. A standardized homogenization protocol is under development to enable uniform culture and qPCR analysis from a single tissue input, which will be reported separately, allowing for more standardized testing with both qPCR and culture from the same sample set. A search and detailed analysis of C acnes in embedded disc tissue is planned as well. Ongoing longitudinal recruitment and follow-up within this registry will evaluate associations between baseline C acnes detection and clinical outcomes, including subsidence, pseudarthrosis, adjacent segment disease, Modic changes, and HO.

Limitations

This study has several limitations. First, the sample size, while sufficient for prevalence estimation, limits power for multivariate analyses and subgroup comparisons. Second, the single-institution design may limit generalizability. Third, the swab was divided for dual testing, which may introduce sampling variability between the culture and qPCR portions. Fourth, qPCR detects DNA from both viable and nonviable organisms, and a positive qPCR result does not confirm active infection. Fifth, deep-tissue CHG irrigation reduces but may not eliminate reservoir 1 contamination; we cannot exclude the possibility that some positive results reflect residual contaminants. Further studies are suggested on pre- and postirrigation deep tissue sampling and whole-genome subtyping of qPCR-detected C acnes in both reservoirs to directly quantify the degree of reservoir 1 suppression achieved by CHG irrigation and to confirm that positive disc-space signals reflect endogenous reservoir 2 colonization.

Conclusions

Cutibacterium acnes is detectable in intervertebral disc spaces at high rates by swab-based sampling following deep-tissue CHG irrigation, with cervical discs demonstrating the highest culture positivity (71.4%). This amount of prevalence approximates reported 10-year HO incidence following TDR replacement. The poor concordance between 21-day anaerobic culture and 16S rRNA qPCR may indicate that these methods detect partially nonoverlapping populations of organisms, and dual-method protocols might be necessary for accurate lumbar prevalence estimation. This study represents, to our knowledge, the first application of disc-space swab analysis for intradiscal C acnes detection. Future studies should evaluate C acnes within human intervertebral disc tissue with special emphasis on intracellular or extracellular location, host metabolic and inflammatory response, organism viability, full genome identification, subspecies variation, and clinical outcomes. The risk of osteolysis and late infections (and potentially HO) following TDR may be heightened by the ability of this ubiquitous and symbiotic organism to adapt to a hostile environment within intervertebral disc and surrounding tissue. Due to this reality, the goal of disc space eradication of C acnes prior to TDR insertion is an important topic for future investigation.