Understanding the potential of probiotics in oral cancer prevention: a short review on microbial modulation

Abstract

Oral squamous cell carcinoma (OSCC) remains a pressing public health challenge, with stagnant survival rates and limited progress in preventive strategies. In addition to classical risk factors such as tobacco and alcohol consumption, recent evidence highlights the role of microbial dysbiosis, particularly the overrepresentation of Fusobacterium nucleatum and Porphyromonas gingivalis, in promoting tumor initiation and progression through inflammation, epithelial transformation, and immune escape. This narrative review aims to summarize and discuss the current evidence linking the oral microbiota to the development of OSCC, with a particular focus on the potential role of probiotics as modulators of microbial balance, inflammation, and epithelial homeostasis. Probiotics have emerged as potential modulators of these carcinogenic processes. Emerging data indicate that specific probiotic strains, including Lactobacillus plantarum, Lactobacillus salivarius, Streptococcus salivarius, and Lactobacillus acidophilus, show antiproliferative and immunomodulatory effects in preclinical models. These include the inhibition of oncogenic bacteria, restoration of microbial equilibrium, downregulation of pro-inflammatory pathways, and enhancement of anti-tumor immunity. Engineered probiotics producing guided antimicrobial peptides (gAMPs), synthetic molecules specifically designed to target tumor-promoting microbes, further exemplify the potential of precision microbial modulation. However, most available evidence derives from experimental or small-scale human studies, with limited data directly addressing oral cancer prevention. Probiotics represent a promising, biologically plausible approach to modulating the oral microenvironment in ways that may reduce carcinogenic risk. While preliminary evidence supports their mechanistic and translational potential, robust clinical trials are needed to validate their safety and efficacy as adjuncts in oral cancer prevention.

Introduction

Oral squamous cell carcinoma (OSCC) accounts for most oral malignancies and remains a global health challenge with a persistently low five-year survival rate, often below 50%, particularly in low-resource settings. This poor prognosis reflects not only delayed diagnosis and limited treatment accessibility but also a limited understanding of the complex etiopathogenesis of the disease [1, 2].

While traditional risk factors such as tobacco use, alcohol consumption, and infection with high-risk human papillomavirus (HPV) are well-established contributors to oral carcinogenesis, recent evidence implicates the oral microbiome as a key modulator of tumor initiation and progression [3, 4]. The oral cavity harbors over 700 microbial species, organized into highly structured communities that play critical roles in immune calibration, epithelial barrier maintenance, and pathogen exclusion. Perturbations to this ecological balance, or microbial dysbiosis, are increasingly linked to precancerous and malignant conditions such as leukoplakia and OSCC [5, 6].

Under physiological conditions, the oral microbiota maintains a state of eubiosis, defined as a balanced coexistence between microbes and the host that preserves mucosal and systemic health. When this equilibrium is disrupted by factors such as poor hygiene, diet, or smoking, dysbiosis occurs, favoring proinflammatory and potentially oncogenic species with implications for oral and systemic disease [7].

Among the taxa enriched in OSCC, Fusobacterium nucleatum and Porphyromonas gingivalis are of particular concern due to their capacity to induce chronic inflammation, remodel the epithelial microenvironment, activate oncogenic signaling pathways, and suppress anti-tumor immunity [6, 8]. Experimental models have shown that oral co-infection with these pathobionts accelerates tumor development in carcinogen-exposed mucosa, suggesting a causal synergy between microbial dysbiosis and malignant transformation.

This emerging paradigm has opened new avenues for microbiome-based prevention and intervention strategies. In this context, probiotics, defined as live microorganisms that confer health benefits when administered in adequate amounts, have gained attention for their potential to restore oral microbial homeostasis, inhibit oncogenic bacteria, modulate immune responses, and even directly affect tumor cell behavior [9, 10]. Certain strains of Lactobacillus, Bifidobacterium, and Streptococcus have shown preclinical potential in reducing tumor burden and enhancing antitumor immunity.

Microbial dysbiosis has emerged as a hallmark of oral carcinogenesis. By restoring eubiosis and dampening chronic inflammation, probiotics may indirectly influence early tumor-promoting events such as epithelial–mesenchymal transition and immune evasion. Therefore, this narrative review aims to provide an updated and integrative overview of the current evidence linking the oral microbiota to the development of OSCC, with particular emphasis on the potential role of probiotics as modulators of microbial balance, inflammation, and epithelial homeostasis.

The oral microbiome in cancer development

Evidence from longitudinal sequencing studies indicates that this transition is marked by a decline in protective taxa, including Neisseria subflava and Corynebacterium durum, and a parallel increase in pathogens such as F. nucleatum and P. gingivalis [11, 12]. Notably, these microbial signatures are often detectable not only within tumoral tissues but also in normal mucosa surrounding dysplastic or malignant areas, suggesting their involvement in early steps of transformation and field cancerization [13].

Microbial dysbiosis appears to support carcinogenesis through multiple converging mechanisms. Chronic inflammation driven by persistent microbial signaling, particularly via Toll-like receptors and NF-κB activation, promotes epithelial damage, cytokine release, and angiogenesis [6, 14]. Simultaneously, species such as F. nucleatum activate β-catenin pathways through E-cadherin binding, upregulating MYC and CCND1, while also driving epithelial–mesenchymal transition (EMT) and facilitating immune evasion through TIGIT–Fap2 interactions [14, 15]. These mechanisms are not merely theoretical: in vivo experiments show that co-infection with F. nucleatum and P. gingivalis significantly accelerates tumor progression in carcinogen-exposed murine models [16].

Recent computational proteomic studies have deepened insights into host-microbe dynamics in OSCC. A key finding identified Fusobacterium nucleatum as predominant in OSCC tissues, with active participation in L-glutamate metabolism, converting host-derived glutamate into butyrate. This metabolic reprogramming was linked to the upregulation of SLC7A11 in tumor cells, enhancing glutamate export and supporting bacterial colonization. These interactions were associated with increased tumor spheroid growth, epithelial–mesenchymal transition, and Galectin-9 expression, all indicative of enhanced malignancy [17].

Meta-analytic data confirms the frequent detection of F. nucleatum in OSCC tissues and its correlation with tumor stage and worse prognosis [18]. In parallel, P. gingivalis, a key pathogen in periodontitis, has been shown to inhibit apoptosis, enhance stemness, and shift immune cell polarization toward tumor-promoting phenotypes [19–23]. These features align with the concept of a microbiome–oncogenesis axis, wherein dysbiotic consortia not only reflect disease but actively shape its trajectory.

Emerging data support a bidirectional oral–intestinal microbiota axis relevant to oncologic risk. Intestinal inflammation can propagate oral dysbiosis, while ectopic gut colonization by oral bacteria such as Fusobacterium and Klebsiella amplifies mucosal immune activation and barrier dysfunction [24, 25]. Inflammatory bowel disease illustrates this reciprocity, where oral microbes translocate to the intestine and shape metabolic and cytokine landscapes linked to disease activity [26]. These connections justify cross-ecosystem strategies that couple oral probiotic or antimicrobial interventions with gut-directed approaches to restore immune and epithelial homeostasis.

Taken together, these findings underscore the role of the oral microbiota as a dynamic and actionable component of the carcinogenic process. A concise summary of these microbial–host interactions and their contribution to oral carcinogenesis is presented in Table 1. The identification of specific microbial drivers of OSCC lays the foundation for translational strategies aimed at early detection, risk stratification, and ultimately, microbiome-based interventions in oral cancer prevention.

Table 1.

Summary of mechanisms linking oral microbial dysbiosis to oral squamous cell carcinoma pathogenesis

Mechanistic axis	Representative bacteria	Host or molecular effects	Pathogenic outcome	References
Chronic inflammation and immune evasion	Fusobacterium nucleatum, Porphyromonas gingivalis	Persistent NF-κB activation; overproduction of IL-6, IL-8, TNF-α; IL-10 imbalance	Promotes angiogenesis, immune suppression, and tumor-supportive inflammation	[6, 14]
Epithelial–mesenchymal transition and stemness	F. nucleatum, P. gingivalis	Activation of TGF-β/Snail and NOD1/KLF5/SCD1 signaling; E-cadherin degradation	Increased invasion, migration, and acquisition of stem-like properties	[21, 22]
Metabolic reprogramming and biofilm synergy	F. nucleatum	L-glutamate-to-butyrate conversion; upregulation of SLC7A11; altered redox metabolism	Bacterial persistence and metabolic support for tumor growth	[17]
Immune modulation and checkpoint interference	F. nucleatum, P. gingivalis	Activation of TIGIT–Fap2 interactions; macrophage M2 polarization; suppression of cytotoxic T-cell function	Evasion of immune surveillance and persistence of pro-tumor microenvironment	[15, 19]
Field cancerization and cross-ecosystem influence	Oral–intestinal microbial translocation (Fusobacterium, Klebsiella, Streptococcus)	Cytokine amplification; intestinal inflammation feedback to oral mucosa; epithelial barrier disruption	Systemic propagation of dysbiosis and tumor-promoting inflammatory milieu	[24–26]

Oral dysbiosis drives OSCC through inflammation, metabolic reprogramming, and immune evasion, linking microbial imbalance to malignant transformation. Field cancerization is defined as a genetically altered epithelial area predisposed to multiple independent primary tumors. NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; IL, interleukin; TNF-α, tumor necrosis factor alpha; EMT, epithelial–mesenchymal transition; TGF-β, transforming growth factor beta; NOD1, nucleotide-binding oligomerization domain-containing protein 1; KLF5, Krüppel-like factor 5; SCD1, stearoyl-CoA desaturase 1; SLC7A11, solute carrier family 7 member 11; TIGIT, T-cell immunoreceptor with Ig and ITIM domains; Fap2, Fusobacterium adhesion protein 2

Mechanisms of probiotic action in oral cancer prevention

The growing recognition of the oncogenic potential of the dysbiotic oral microbiome has prompted increasing efforts to harness microbial modulation through probiotics for therapeutic benefit. These live microorganisms, traditionally studied in gastrointestinal contexts, are now being explored for their capacity to stabilize oral microbial communities, restore mucosal homeostasis, and attenuate tumor-promoting inflammation in OSCC.

Pathogen suppression and microbial antagonism

One of the most robustly documented mechanisms by which probiotics exert anticancer activity is through the suppression of pathogenic microbes enriched in OSCC. F. nucleatum and P. gingivalis, two key bacterial drivers of oral carcinogenesis, are susceptible to antimicrobial peptides, particularly bacteriocins, produced by commensal probiotic species. Streptococcus salivarius, for instance, synthesizes salivaricin A and B, which inhibit F. nucleatum by targeting its cell wall biosynthesis without inducing membrane disruption [27]. Engineered probiotic strains, such as Lactococcus lactis expressing guided antimicrobial peptides (gAMPs), have recently been shown to selectively eliminate F. nucleatum in colorectal models without disturbing commensal diversity, suggesting that similar precision approaches could be adapted for the oral cavity [28]. Recent technological progress now supports the rational engineering of probiotics through multi-omics integration, combining genomics, transcriptomics, proteomics, and metabolomics to predict host–microbe interactions and design strains with optimized immunomodulatory and antitumor properties. These “pro-biomics” platforms enable the metabolic rewiring of probiotic species to enhance bacteriocin production, redox balance, and cytokine modulation [29]. Moreover, integrating engineered probiotics with prebiotics and immune checkpoint therapies has shown synergistic potential to reduce inflammation and promote epithelial homeostasis in precancerous settings [30–32]. This antimicrobial action is further potentiated by ecological competition: probiotics interfere with pathogenic colonization by occupying adhesion sites and competing for metabolic substrates within the oral biofilm [33].

Reestablishment of eubiosis and metabolic modulation

Beyond pathogen control, probiotics play a crucial role in reestablishing eubiosis. Certain strains of Lactobacillus and Bifidobacterium foster the recovery of health-associated microbiota and increase microbial diversity; a factor often diminished in OSCC [4]. These restorative effects have been linked to reductions in procarcinogenic metabolites and the stabilization of microbial networks that support epithelial health [4]. Moreover, their metabolic outputs—short-chain fatty acids, lactic acid, and hydrogen peroxide—alter the local environment in ways that reduce carcinogenic metabolite production and support epithelial integrity [34]. Emerging evidence also points to the ability of probiotic metabolites to modulate intracellular signaling cascades such as MAPK and NF-κB, potentially attenuating oxidative stress and DNA damage in dysplastic epithelia [35].

Immune modulation and inflammatory control

Probiotics also engage host immunity in ways directly relevant to tumor suppression. In vitro and animal studies have shown that they enhance the activity of natural killer (NK) cells, modulate dendritic cell maturation, and upregulate cytokines such as IL-12 and IFN-γ, both pivotal for cytotoxic T cell responses [36]. In a preclinical model of oropharyngeal carcinoma, S. salivarius increased CD8⁺ T cell infiltration and potentiated the efficacy of anti–PD-1 therapy, highlighting its promise as an immunotherapeutic adjuvant [36].

Equally significant is the capacity of probiotics to dampen tumor-permissive inflammation. Chronic oral inflammation, mediated by TNF-α, IL-6, and IL-8, is a hallmark of OSCC progression. Probiotic strains have been shown to downregulate these cytokines while enhancing anti-inflammatory mediators such as IL-10 and regulatory T cell populations, effectively rebalancing mucosal immunity [36]. This immunologic reprogramming may slow epithelial transformation and improve mucosal resilience against oncogenic stimuli.

Direct antitumor and cell-signaling effects

Some strains also demonstrate direct cytotoxicity toward neoplastic cells. Lactobacillus plantarum, for example, induces apoptosis in OSCC cell lines via PTEN upregulation and MAPK pathway suppression, pointing to probiotic-driven modulation of intracellular signaling cascades [35]. Other species, including marine-derived Lactobacillus isolates, inhibit cAMP-dependent protein kinase (PKA) activity and reduce tumor proliferation in animal models [10].

Recent mechanistic studies further suggest that probiotics can counteract pathogen-induced oncogenic signaling. P. gingivalis and F. nucleatum promote stemness and epithelial–mesenchymal transition through the NOD1/KLF5/SCD1 and TGF-β/Snail pathways, respectively; probiotics capable of modulating these axes may therefore mitigate microbial-driven tumorigenesis [6, 22]. Although preliminary, there is growing interest in whether these microbes, or their secreted metabolites, can epigenetically influence chromatin architecture and transcription in dysplastic oral epithelia [36].

Altogether, the mechanisms by which probiotics may prevent, or attenuate oral cancer span microbial antagonism, immune modulation, metabolic regulation, and direct antitumor signaling. This integrative network of host–microbe interactions positions probiotics as multifaceted modulators rather than single-pathway agents. Their influence across microbial, metabolic, and immunological levels may ultimately determine their therapeutic value. Future research should leverage multi-omics and standardized clinical frameworks to define which strains, dosages, and delivery systems provide reproducible benefits in human mucosa. Figure 1 summarizes the integrated probiotic mechanisms underlying oral cancer prevention, highlighting their antimicrobial, anti-inflammatory, and immunomodulatory effects on the oral mucosa.

Fig. 1.

Schematic representation of probiotic mechanisms in oral cancer prevention. Probiotics such as Lactobacillus and Streptococcus salivarius inhibit dysbiosis and suppress oncogenic bacteria including Fusobacterium nucleatum and Porphyromonas gingivalis, both implicated in oral squamous cell carcinoma (OSCC) pathogenesis. By restoring microbial balance, probiotics promote anti-inflammatory cytokines (IL-10, TGF-β) and reduce pro-inflammatory mediators (IL-6, TNF-α, NF-κB), thereby enhancing epithelial homeostasis and immune surveillance. These combined microbial, metabolic, and immunological effects contribute to a less tumor-permissive oral microenvironment and support oral cancer prevention

Probiotic strains with anticancer potential

The identification of probiotic strains capable of interfering with carcinogenic processes in the oral cavity has opened new avenues in microbiome-based cancer prevention. Among these, certain species of Lactobacillus and Streptococcus have emerged as promising candidates, not only for their ability to suppress oncomicrobes but also for their direct modulation of host epithelial signaling and immune surveillance mechanisms in OSCC models.

L. plantarum has garnered substantial attention for its capacity to modulate tumor-associated pathways. In vitro experiments show that this strain inhibits the proliferation of oral cancer KB cells by activating PTEN, a tumor suppressor gene, while concurrently inhibiting the MAPK pathway, a critical axis in cell growth and survival [35]. This dual modulation suggests a targeted anticancer mechanism. Further studies with the KUMS-Y8 strain, isolated from fermented dairy products, demonstrate comparable cytotoxic effects on OSCC cells, with responses paralleling those of conventional chemotherapeutic agents like doxorubicin and paclitaxel [37]. Beyond its intracellular effects, L. plantarum also contributes to microbiota remodeling. The HNU082 strain has been shown to suppress F. nucleatum both in vitro and in murine models, mitigating inflammation and restoring microbial equilibrium in the host [38].

Another species with demonstrated efficacy is Lactobacillus salivarius, especially the REN strain. In a 4NQO-induced rat model of oral carcinogenesis, the administration of this strain resulted in a dose-dependent reduction of tumor incidence [39]. Histological analyses attributed these effects to suppressed expression of COX-2 and PCNA, enhanced apoptosis, and reduced oxidative DNA damage in dysplastic epithelium, indicating that L. salivarius operates through both anti-inflammatory and antiproliferative mechanisms.

Streptococcus salivarius, a commensal of the upper aerodigestive tract, offers a complementary line of evidence. The strain produces lantibiotics such as salivaricin A and B, which exhibit targeted antimicrobial activity against F. nucleatum by disrupting cell wall synthesis without inducing membrane lysis [40, 41]. This targeted suppression of a microbial driver of oral cancer is particularly compelling when paired with findings from in vivo models: in a murine study of oropharyngeal carcinoma, S. salivarius administration led to reduced tumor volume and increased infiltration of CD4 + and CD8 + T cells, effects that were potentiated in combination with anti–PD-1 immune checkpoint blockade [42]. These data position S. salivarius as a viable immunobiotic with both antimicrobial and adjuvant immunotherapeutic potential.

Other strains, though less studied, have also demonstrated noteworthy effects. Lactobacillus acidophilus has been observed to suppress OSCC cell proliferation, possibly via TRAIL-mediated apoptosis, a pathway associated with selective tumor cell killing [43]. While mechanistic details remain to be elucidated, such findings warrant deeper investigation into this strain’s therapeutic role.

Innovative approaches in probiotic engineering have further expanded this field. A bioengineered probiotic expressing a guided antimicrobial peptide (gAMP) targeted specifically to F. nucleatum has shown the ability to suppress this oncomicrobe while sparing commensal bacteria, suggesting a precision-microbiome intervention strategy [28]. This approach integrates the specificity of antimicrobial therapy with the ecological benefits of probiotics, offering a highly targeted method of pathogen suppression with minimal collateral impact.

Several strains have demonstrated selective activity against microbial and immunologic drivers of oral carcinogenesis. Table 2 presents a comparative overview of these probiotic candidates, outlining their proposed mechanisms and supporting models. These findings support the development of strain-specific probiotics in oral oncology. Emerging evidence suggests that well-characterized strains, whether natural or engineered, may influence carcinogenesis through overlapping mechanisms. Future research should prioritize mechanistic validation, optimized formulation, and clinical testing to define their therapeutic value. Reported probiotic dosages varied between 10⁷ and 10⁹ colony-forming units (CFU) per milliliter in in vitro models and up to 10⁹ CFU per day in animal experiments. Administration routes included oral gavage, co-culture systems, and lozenge formulations depending on the strain and study design [37, 39, 42]. Such variability highlights the current lack of standardization across studies and the importance of defining strain-specific therapeutic windows in future clinical applications.

Table 2.

Comparative overview of probiotic strains with anticancer potential in oral cancer

Probiotic strain	Proposed anticancer mechanisms	Model	References
Lactobacillus plantarum	↑ PTEN, ↓ MAPK; apoptosis induction; suppression of F. nucleatum; anti-inflammatory activity	in vitro (KB, OSCC); murine oral model. Dosage: 10⁸–10⁹ CFU/day; oral gavage or co-culture	[35, 37, 38]
Lactobacillus salivarius REN	↓ COX-2, ↓ PCNA; ↑ apoptosis; reduction of DNA damage	Rat model (4NQO-induced oral carcinogenesis). Dosage: 10⁹ CFU/day; oral administration in drinking water	[39]
Streptococcus salivarius K12	Lantibiotics (salivaricin A/B) inhibit F. nucleatum; ↑ CD4+/CD8 + infiltration; synergy with anti–PD-1	Murine model of oropharyngeal carcinoma. Dosage: 10⁸ CFU/day; oral inoculation or lozenge formulation	[40–42]
Lactobacillus acidophilus	TRAIL-mediated apoptosis	in vitro (OSCC cell lines, KB cells). Dosage: 10⁷–10⁸ CFU/mL; co-culture	[43]
Engineered probiotic (gAMP)	Selective suppression of F. nucleatum using guided antimicrobial peptides	in vitro bioengineering platform; simulated oral model; delivery via capsule or biofilm carrier (proposed)	[28]

Probiotic strains display diverse and strain-specific anticancer mechanisms, including apoptosis induction, suppression of oncogenic bacteria, and modulation of inflammatory and immune pathways. Variability in strain type, dosage, delivery format, and experimental design limits direct comparison across studies and underscores the need for standardized evaluation. Abbreviations: ↑, upregulation; ↓, downregulation; PTEN, phosphatase and tensin homolog; MAPK, mitogen-activated protein kinase; F. nucleatum, Fusobacterium nucleatum; COX-2, cyclooxygenase-2; PCNA, proliferating cell nuclear antigen; CD4⁺/CD8⁺, T-cell subsets; PD-1, programmed cell death protein 1; TRAIL, tumor necrosis factor-related apoptosis-inducing ligand; gAMP, guided antimicrobial peptide; OSCC, oral squamous cell carcinoma; 4NQO, 4-nitroquinoline-1-oxide; CFU, colony-forming units

Clinical evidence for probiotics in oral cancer prevention

While preclinical data have elucidated a variety of mechanisms by which probiotics may attenuate oncogenic processes in the oral cavity, clinical evidence supporting their efficacy in oral cancer prevention remains sparse and largely indirect. Nonetheless, several studies—including randomized controlled trials (RCTs), systematic reviews, and observational analyses—have begun to delineate the therapeutic potential of probiotics in human subjects, particularly in oncology-related oral complications.

Most clinical investigations to date have focused not on primary prevention of OSCC, but rather on mitigating treatment-associated toxicities such as oral mucositis in patients receiving chemoradiotherapy for head and neck cancers. A recent systematic review and meta-analysis conducted by Lin et al.. pooled data from multiple RCTs and reported that probiotic administration significantly reduced the severity and incidence of oral mucositis, particularly in lesions classified as WHO grade 3–4 [44]. These findings were echoed by Liu et al., who analyzed a different cohort of placebo-controlled RCTs and found that probiotics not only alleviated mucosal injury but also shortened the duration of ulcerative episodes, without increasing adverse events or systemic infections [45]. Although these studies were not designed to test oncologic endpoints, they suggest that probiotic supplementation may preserve epithelial integrity and immune equilibrium under carcinogenic stress, thus supporting the biological plausibility of a preventive role.

In parallel, observational studies have provided further support for the role of probiotics in modulating oral health. Kozak and Pawlik reviewed evidence linking probiotic use to reductions in pathogenic colonization, improvements in salivary immune markers such as IgA, and modulation of cytokine expression—factors that contribute to a less permissive environment for tumorigenesis [46]. Navidifar et al.., in a systematic review of clinical trials, evaluated probiotic use across diverse oral health conditions including periodontitis, halitosis, and gingivitis. Their synthesis found that probiotic interventions consistently improved clinical indices such as plaque accumulation and gingival inflammation, while also reducing microbial load [47]. Although these endpoints are not cancer-specific, they suggest that probiotic modulation of the oral ecosystem may indirectly reduce pro-oncogenic stimuli, including chronic inflammation and microbial dysbiosis.

Despite these encouraging findings, trials directly targeting oral cancer prevention remain scarce. For instance, a registered RCT aiming to assess the impact of Lactobacillus-containing lozenges (FLORASSIST^®) on dysplasia-associated inflammation and dysbiosis (NCT05707702) was withdrawn prior to enrollment due to administrative issues [48]. Another trial evaluating Lactobacillus rhamnosus GG for radiotherapy-induced oral mucositis (NCT06390176) confirmed symptomatic improvement but did not assess long-term cancer-related endpoints [49]. Similarly, an exploratory study using ProDentis lozenges in OSCC patients (NCT04925700) failed to establish conclusive links between probiotic intervention and reduced oncogenic risk [50]. These examples underscore the limited availability of human data directly supporting probiotics in OSCC prevention.

Although clinical trials directly targeting oral cancer endpoints are lacking, a growing body of evidence supports the utility of probiotics in managing oncologic complications. Table 3 summarizes key studies that highlight clinical outcomes, their strengths, and current limitations.

Table 3.

Summary of clinical evidence supporting probiotic use in oral oncology settings

Study type	Population/inclusion criteria	Intervention	Primary outcomes	Key findings	Limitations	References
Meta-analysis	Adult HNC patients receiving radiotherapy or chemoradiotherapy (RCTs only)	Lactobacillus spp., Bifidobacterium spp. (lozenges or sachets)	↓ Grade 3–4 mucositis; ↓ symptom duration	Consistent reduction in mucositis severity across pooled studies	Not designed for cancer prevention; strain heterogeneity	[44]
Meta-analysis	Mixed oncology patients (RCTs)	Oral probiotics, multiple species	↓ Mucositis duration; ↓ inflammation; no ↑ adverse events	Confirms safety and anti-inflammatory benefit	Indirect relevance to OSCC	[45]
Meta-analysis	HNC patients under radiotherapy	Lactobacillus spp. and Bifidobacterium spp.	↓ Severe mucositis; ↓ analgesic use	Subgroup benefit in mucositis reduction	No data on OSCC prevention; possible publication bias	[51]
Systematic review + meta-analysis	Head and neck cancer patients under antineoplastic therapy	Various probiotic formulations (capsules, lozenges)	↓ Mucositis severity; improved oral tolerance	Supports probiotics as adjuvant supportive therapy	Focused on complications, not carcinogenesis	[52]
Systematic review	Adults with oral diseases (periodontitis, gingivitis, halitosis)	Lactobacillus and Streptococcus spp. (tablets or rinses)	↓ Plaque and gingival indices; ↓ microbial load	Improved oral ecology and reduced inflammation	Surrogate markers; heterogeneous design	[47]
Observational review	General population, non-oncologic	Regular probiotic intake	↑ Salivary IgA; ↓ pathogenic colonization	Supports immunomodulatory potential at mucosal level	No OSCC-specific endpoints	[46]
Mini-review (safety)	HNC and immunocompromised patients	–	–	Highlights safety considerations for irradiated or immunosuppressed patients	No quantifiable outcomes	[53]
Registered RCT (withdrawn)	Patients with low-grade oral epithelial dysplasia	Lactobacillus-containing lozenges (FLORASSIST®)	Dysbiosis and inflammation endpoints	Designed for oral cancer prevention; not completed	Withdrawn before enrollment	[48]
Registered RCT (completed)	HNC patients undergoing radiotherapy	Lactobacillus rhamnosus GG (oral)	↓ Mucositis symptoms; improved comfort	Demonstrated symptomatic benefit	Did not assess cancer-related endpoints	[49]

Clinical evidence suggests that probiotics can mitigate oral mucositis severity, modulate inflammation, and enhance microbial balance in patients undergoing oncologic therapies. However, most studies focus on symptom management rather than direct cancer prevention. Heterogeneity in probiotic strains, dosage, formulations, and endpoints limits comparability across trials. Abbreviations: ↓, reduction; ↑, increase; HNC, head and neck cancer; OSCC, oral squamous cell carcinoma; RCT, randomized controlled trial; IgA, immunoglobulin A

Importantly, no clinical studies to date have directly evaluated probiotics for primary prevention of OSCC in high-risk individuals. This gap in translational research remains critical. However, the convergence of preclinical results with indirect human evidence supports designing prospective studies targeting cancer prevention endpoints. In populations with oral potentially malignant disorders (OPMDs), long-term tobacco or alcohol exposure, or persistent dysbiosis, probiotics may represent a potentially safe and biologically plausible complement to existing preventive approaches.

In this context, probiotics should not be viewed solely as supportive care. Instead, they may act as precision modulators of the oral microenvironment—altering microbial structure, dampening pro-inflammatory cascades, and enhancing immune responses to reduce carcinogenic potential. Future RCTs, ideally integrated with mechanistic endpoints such as host transcriptomics, proteomics, microbial profiling, and immune phenotyping, are essential to validate their role in cancer prevention frameworks.

Barriers to translation: challenges and future directions in probiotic research for oral cancer prevention

While probiotics have demonstrated clinical efficacy in improving oral health parameters such as plaque and gingival inflammation [47], and in reducing mucositis associated with cancer therapy [45], evidence directly linking probiotic supplementation to reduced oral cancer risk remains insufficient. These clinical data nonetheless provide a translational bridge supporting future interventional trials. A major barrier is the heterogeneity in trial design. Studies vary widely in strain selection, dosage, duration, delivery format, and population. This variability complicates meta-analyses and impedes the development of robust clinical guidelines [47]. Furthermore, most available studies are preliminary, relying on in vitro and animal models, with limited large-scale randomized clinical trials to confirm efficacy and safety in humans [54, 55].

Mechanistic understanding also remains limited. While in vitro and animal studies support roles for probiotics in pathogen suppression, immune modulation, and gene expression, most mechanisms remain unconfirmed in human mucosa. For example, marine-derived Lactobacillus strains modulate NF-κB and MAPK signaling in vitro, but their efficacy in human oral environments is unproven [10]. Moreover, probiotic effects appear strain-specific, with some strains such as Lactiplantibacillus plantarum Y33 or Y8 showing strong pro-apoptotic effects, whereas others exhibit minimal or no benefit [54, 56].

The lack of standardized administration protocols further complicates translation. Mode of delivery, treatment timing, and duration vary greatly, affecting strain viability and host response. Even when benefits like reduced dysbiosis are observed, this inconsistency hinders comparability and consensus development [47]. There are still no standardized formulations for probiotic use in oral cancer, including the optimal bacterial species, strain combination, dosage, or duration of treatment [51]. Clinical outcomes also appear to differ depending on the type of cancer and stage, as most evidence focuses on therapy-induced oral mucositis rather than direct tumor suppression [54].

Safety concerns, especially in immunocompromised patients, are non-negligible. Though generally safe, probiotics may rarely cause bacteremia or opportunistic infections. This risk is heightened during cancer therapy, necessitating individualized risk assessment [57]. Krishna et al. emphasize caution in irradiated head and neck cancer patients due to possible microbial translocation [53]. Strain-to-strain variability may include antibiotic-resistance gene transfer, allergic reactions, and transient gastrointestinal symptoms. Certain oral strains may be cariogenic by increasing acid production, so dental status and strain selection matter [58, 59]. Furthermore, published findings are not always consistent. While some investigations describe meaningful clinical benefits, others have failed to demonstrate significant differences compared with placebo, underscoring the need for rigorously controlled and reproducible clinical trials to clarify these discrepancies [52, 55].

Nevertheless, the field is evolving. Advances in sequencing and synthetic biology enable precise profiling and custom strain design. Future trials should incorporate biomarkers—cytokine profiles, transcriptomics, and microbiota shifts—to define causality and therapeutic windows.

Interest is also growing in using probiotics as immunotherapy adjuvants. In oropharyngeal carcinoma models, S. salivarius enhanced T-cell infiltration and boosted PD-1 blockade efficacy [42]. These findings suggest a future for microbiota-based oncology therapies. Still, the translation of these preclinical benefits to human mucosa demands multi-center, longitudinal studies using standardized probiotic formulations and validated immunological endpoints.

In conclusion, while clinical use of probiotics in oral cancer prevention is nascent, the rationale is strong. Progress depends on standardized methodologies, mechanistic insight, and safety profiling. With these, probiotics may play a key role in precision oral oncology.

Concluding remarks

Mounting evidence positions the oral microbiome as a key modulator in oral carcinogenesis, shifting attention toward microbial interventions as part of cancer prevention and treatment strategies. Probiotics have demonstrated promising effects in preclinical models, including pathogen inhibition, immune modulation, and reduction of tumor burden—especially when combined with immunotherapies.

Despite these advances, clinical translation remains limited. Heterogeneity in study designs, a lack of mechanistic clarity in human models, and safety concerns in vulnerable patients underscore the need for rigorous, standardized research. Future trials must incorporate defined probiotic strains, clear outcome measures, and translational endpoints.

As microbiome research matures, probiotics may become central to precision oral oncology, if innovation is guided by robust evidence and interdisciplinary collaboration. Their therapeutic potential, though still emerging, holds promise for reshaping prevention in high-risk populations.

Abbreviations

OSCC

Oral squamous cell carcinoma

HPV

Human papillomavirus

EMT

Epithelialmesenchymal transition

gAMP

Guided antimicrobial peptide

NK

Natural killer

PD-1

Programmed cell death protein 1

RCTs

Randomized controlled trials

OPMDs

Oral potentially malignant disorders

NF-κB

Nuclear factor kappalightchainenhancer of activated B cells

SLC7A11

Solute carrier family 7 member 11

MAPK

Mitogenactivated protein kinase

PCNA

Proliferating cell nuclear antigen

PTEN

Phosphatase and tensin homolog

IL-12

Interleukin 12

IFN-γ

Interferon gamma

IL-10

Interleukin 10

TNF-α

Tumor necrosis factor alpha

IL-6

Interleukin 6

IL-8

Interleukin 8

COX-2

Cyclooxygenase2

4NQO

4-Nitroquinoline-1-oxide

TRAIL

Tumor necrosis factorrelated apoptosisinducing ligand

WHO

World Health Organization

Author contributions

C.R. was responsible for the conception, drafting, and critical revision of the manuscript.

Funding

This work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

All data generated or analyzed during this study are included in this published article. No datasets were generated or analyzed for this narrative review.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.