Colon adenocarcinoma is one of the most common malignancies, and it is highly lethal. Chemotherapy plays an important role in the treatment of colon cancer at various stages of the disease. The gut microbiome has emerged as a key player in colon cancer development and progression, and it can also alter the therapeutic agent’s efficacy and toxicities. Antibiotics can directly and/or indirectly affect the balance of the gut microbiome and, therefore, the clinical outcomes. In this article, we provided an overview of the composition of the gut microbiome under homeostasis and the mechanistic links between gut microbiota and colon cancer. The relationship between the use of oral antibiotics and colon cancer, as well as the impact of the gut microbiome on the efficacy and toxicities of chemotherapy in colon cancer, are discussed. Potential interventions to modulate microbiota and improve chemotherapy outcomes are discussed. Further studies are indicated to address these key gaps in the field and provide a scientific basis for the design of novel microbiota-based approaches for prevention/use as adjuvant therapeutics for patients with colon cancer.

Use of adjuvant chemotherapy has improved survival for many patients with breast cancer. Unfortunately, such treatment can come at a price, in particular, malignancies. We present a case of a 36-year-old woman with heterozygous mutations in the ataxia telangiectasia mutated (ATM) gene who was admitted to the hospital for fatigue and diffusely scattered bruises. She was diagnosed with invasive ductal carcinoma of the left breast and had bilateral mastectomy with axillary node clearance followed by adjuvant chemotherapy 3 years prior. Her vitals were stable. Lab tests revealed thrombocytopenia, leukocytosis, and anemia. Peripheral blood smear and bone marrow biopsy revealed numerous myeloblasts. After flow cytometry and FISH analysis were performed, a diagnosis of therapy-related acute myeloid leukemia (t-AML) was made. The patient was treated with induction chemotherapy and a bone marrow biopsy revealed residual disease. Re-induction therapy was given and a bone marrow biopsy revealed complete remission. She subsequently received an allogenic stem cell transplant and was cured. Her treatment course was uncomplicated. We raise the question as to whether certain chemotherapy agents should be avoided in patients with mutations in DNA repair genes. Furthermore, it is essential for physicians to educate patients on the risk of secondary malignancies arising from chemotherapeutic treatment.