For example, Gamma Proteobacteria in pancreatic ductal adenocarcinoma (PDA) metabolise the chemotherapeutic drug gemcitabine to its inactive form by expressing the bacterial enzyme cytidine deaminase (CDD), thus inducing gemcitabine resistance and ultimately affecting the efficacy of gemcitabine [21]. Here, CDA is linked to pancreatic ductal adenocarcinoma.