Trial Utilizing Metronidazole to Optimize the Microbiome of Rectal Adenocarcinoma Undergoing Neoadjuvant Therapy

* Primary Objectives Quantify the ability of a standard course of metronidazole to decrease populations of anaerobic bacteria from within rectal cancers in participants receiving neoadjuvant therapy. * Secondary Objectives To observe and record augmented anti-tumor activity of metronidazole in combination with standard of care therapies. Although the clinical benefit of \[this/these\] drug(s) has not yet been established, the intent of offering this treatment is to provide a possible therapeutic benefit, and thus the patient will be carefully monitored for tumor response in addition to safety and tolerability. As part of the therapeutic efficacy of metronidazole in combination with standard of care therapies, overall survival and recurrence free survival will be assessed among participants enrolled in the study over the two years following study completion. Quantify differential rates of pathologic response of tumors treated with metronidazole in combination with standard neoadjuvant therapy compared against historic controls. Those participants undergoing watch and wait protocols will not have the pathologic response and will not be included in the pathologic response assessment. Exploratory analysis of immune and microbial infiltrates between participant matched pre and post treatment tissues (when available). This will include bacterial taxal populations along with populations of various immune cell populations.