Synthetic biology resurrects bacterial cancer therapy

Engineered bacteria are being developed as living cancer therapeutics that can localize to selected tumor niches, produce payloads in situ, remodel the tumor microenvironment, and engage antitumor immunity. Their clinical translation has nevertheless been limited by heterogeneous colonization, attenuation-related loss of fitness, host clearance, uncertain pharmacology, and product-control challenges. To complement recent landscape reviews, this article applies a design-to-translation framework that links chassis selection, genetic control, delivery route, tumor and microbial ecology, human pharmacodynamics, and manufacturing constraints. We examine how hypoxia-, quorum-, and externally responsive circuits regulate payload expression; how bacterial activity can produce both therapeutic and unintended effects in malignant, stromal, immune, and microbial compartments; and how bacterial platforms compare with oncolytic viruses as complementary living therapeutics. Early clinical studies of Clostridium, Salmonella, Listeria, Yersinia, Bifidobacterium, and Escherichia coli are used to derive practical design rules. The evidence supports practical design rules governing route selection, quantitative colonization assessment, payload gating, treatment sequencing, rescue planning, and indication choice. Progress in bacterial cancer therapeutics will depend less on maximizing circuit complexity than on matching a measurable bacterial product to a permissive tumor ecology, a defined clinical need, and a reproducible development strategy.