The challenges and strategies for navigating in vivo barriers to therapeutic bacterial delivery.

Bacteria have emerged as promising living therapeutics for the treatment of infections, inflammatory disorders, metabolic diseases, and neoplastic diseases. However, their clinical efficacy is often compromised by in vivo barriers that reduce their viability and impair their function after administration. Rational engineering can help them sustain their viability under a harsh microenvironment and enhance their targetability as well as retention, ultimately improving their functional performance in vivo. This review starts with a concise synopsis of current status in live bacterial therapy, with particular emphasis on the major biological barriers to their in vivo application. Engineering strategies for harnessing bacterial intrinsic characteristics and overcoming their specific barriers are discussed. Representative examples are then presented to illustrate how engineered live bacteria overcome these biological barriers and achieve therapeutic applications via oral, topical, intravenous, and inhalational administration. Finally, we identify the key challenges associated with translating laboratory advances into reliable and safe clinical therapeutics.