Synchronized cycles of bacterial lysis for in vivo delivery

Clinically relevant bacteria have been engineered to lyse synchronously at a threshold population density and release genetically encoded therapeutics; treatment of mice with these bacteria slowed the growth of tumours. There is growing interest in using bacteria as living therapeutics, although the complications due to host responses and long-term effectiveness remain to be established. Omar Din et al. have now engineered a quorum-sensing clock into a strain of Salmonella known to target solid tumours by releasing a tumour-targeting toxin. The clock drives periodic lyses of the bacterial colony, thereby controlling the bacterial population and ensuring sustained anti-tumour toxin delivery in a mouse cancer model. Although the system as it stands does not represent an effective cure, this work indicates that synthetic biology can be harnessed to achieve dynamic and sustained delivery of therapeutics in vivo. The widespread view of bacteria as strictly pathogenic has given way to an appreciation of the prevalence of some beneficial microbes within the human body1,2,3. It is perhaps inevitable that some bacteria would evolve to preferentially grow in environments that harbour disease and thus provide a natural platform for the development of engineered therapies4,5,6. Such therapies could benefit from bacteria that are programmed to limit bacterial growth while continually producing and releasing cytotoxic agents in situ7,8,9,10. Here we engineer a clinically relevant bacterium to lyse synchronously at a threshold population density and to release genetically encoded cargo. Following quorum lysis, a small number of surviving bacteria reseed the growing population, thus leading to pulsatile delivery cycles. We used microfluidic devices to characterize the engineered lysis strain and we demonstrate its potential as a drug delivery platform via co-culture with human cancer cells in vitro. As a proof of principle, we tracked the bacterial population dynamics in ectopic syngeneic colorectal tumours in mice via a luminescent reporter. The lysis strain exhibits pulsatile population dynamics in vivo, with mean bacterial luminescence that remained two orders of magnitude lower than an unmodified strain. Finally, guided by previous findings that certain bacteria can enhance the efficacy of standard therapies11, we orally administered the lysis strain alone or in combination with a clinical chemotherapeutic to a syngeneic mouse transplantation model of hepatic colorectal metastases. We found that the combination of both circuit-engineered bacteria and chemotherapy leads to a notable reduction of tumour activity along with a marked survival benefit over either therapy alone. Our approach establishes a methodology for leveraging the tools of synthetic biology to exploit the natural propensity for certain bacteria to colonize disease sites.