Microbes as a programmable toolbox for cancer diagnosis, monitoring, and theranostics.

Microbes have evolved molecular machinery to recognize hosts, access tissues, sense local conditions, replicate, and modify their environments. These capabilities are increasingly being repurposed for cancer detection and control. Phage and yeast display generate tumour-recognition modules; engineered bacteria sense and amplify local disease signals; reporter-expressing viruses reveal tumour infection and transgene activity; and microbial signatures provide minimally invasive biomarkers. Here, we compare these mechanistically distinct platforms through an input-processing-output framework, examine their diagnostic and theranostic functions, and assess the readout, safety, manufacturing, analytical, and regulatory requirements that determine clinical actionability. We also propose tumour-reporting viruses and theranostic viruses as functional terms extending beyond the predominantly therapeutic concept of oncolytic viruses.