A Phase I/IIa, Single-Arm, Dose-Confirmation and Dose-Expansion Study Evaluating Changes in the Oral Microbiome of Patients With Oral Cavity Squamous Cell Carcinoma (OSCC) After Short-Term Ingestion of Nisin, a Naturally Occurring Food Preservative

Inclusion Criteria: 1. Participants with a suspected clinical diagnosis of oral cavity squamous cell carcinoma (OSCC) who are planning to undergo a diagnostic biopsy. Note: Subsequent documentation from the medical record (e.g., from the diagnostic biopsy pathology report) of histological or cytological confirmation of OSCC is required prior to enrollment and treatment on study. Participants who do not have a histological/cytological confirmation of OSCC, or who are unable to provide sufficient volume of biopsy tissue for research, will not be eligible to enroll in the study. 2. OSCC mass must be \>= 2cm in size. 3. Non-metastatic OSCC (no distant metastases allowed; metastasis to the neck is eligible). 4. Participants must be planning for a complete OSCC surgical resection, with or without adjuvant radiation/chemoradiation, at UCSF, as part of their routine treatment plan. Surgical resection and adjuvant treatment will be according to the participants routine or standard of care treatment plan and will not be dictated by the study (non-interventional). 5. Age \>= 18 years 6. Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 (Karnofsky ≥ 50%). 7. Participants must be dentate (retaining at least two teeth). Teeth are required for plaque sample collection for correlative studies. 8. Ability to understand and willingness to sign a written informed consent document. Exclusion Criteria: 1. Individuals with a history of a known positive human papillomavirus deoxyribonucleic acid ((HPV DNA) test (P16-positive is permitted if HPV DNA test is negative). 2. Individuals with a history of or concurrent oropharyngeal, laryngeal, or hypopharyngeal cancers (other than OSCC). Individuals with any other prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or endpoints of this study are eligible. 3. Individuals with ongoing uncorrected oral pathology, which in the opinion of the investigator could interfere with the safety or endpoints of this study or could be exacerbated during the course of study participation, if left untreated. These conditions may be discussed with the study PI to determine eligibility. * Current and ongoing long-term dental treatment requiring major surgery; * Untreated carious lesions, severe oral infections, or known defective restorations; * Other suspicious uncorrected oral pathology. 4. Individuals with a known history of hypersensitivity reactions or oral allergies to nisin, any of its excipients, or any related food preservatives. 5. Individuals with complete inability to absorb nutrients via gastrointestinal tract due to major medical disorder or history of gastrointestinal surgical removal. 6. Individuals who have received extended (\>4 weeks) antibiotic therapy within 1 month of the baseline visit or who currently require continuous antibiotic prophylaxis at the time of enrollment. 7. Individuals who require treatment with a systemic anticancer therapy prior to enrollment, with the exception of prior immunotherapy which is permitted both prior to enrollment and concurrent with study participation. 8. Individuals currently receiving any other investigational agent or using an investigational device intended as anticancer therapy, with the exception of investigational immunotherapy, which is permitted both prior to enrollment and concurrent with study participation. 9. Individuals with ongoing Grade 2 events that are not clinically stable or ongoing \>= Grade 3 events (CTCAE v5.0 grading). 10. Individuals with any condition or social circumstance that, in the opinion of the investigator, would impair the participant's ability to comply with study activities, interfere with participant safety, or study endpoints.