Chemotherapy resistance is a recurring challenge in cancer treatment, with specific bacteria impairing the effectiveness of certain chemotherapies. This study reviews three bacteria and their impact on chemotherapy drugs: Mycoplasma  and gemcitabine, Fusobacterium nucleatum and oxaliplatin, bacterial β-glucuronase and irinotecan. Bacteria can have wide-ranging effects on cancer treatment; for instance, they may affect drug metabolism, alter toxin conversion, and encourage cancer growth. Whilst the presence of these bacteria was found to have a detrimental effect on the efficacy of chemotherapy treatment, we also consider wider interactions and interdependencies of the microbiota with drug treatments. Some cancer therapies depend on the delicate balance of the microbiome whilst simultaneously disrupting it by their very nature, particularly when antibiotics are introduced. Further research into the complex relationship between bacteria and the tumour micro-environment is needed. Treatments that focus on the immune-oncology microbiome axis or that explore genetic predisposition through the use of biomarkers could also support a more personalised approach.