Bacteriophages (phages), as natural antibacterial agents, are being rediscovered because of the growing threat of multi- and pan-drug-resistant bacterial pathogens globally. However, with an estimated 10³¹ phages on the planet, finding the right phage to recognize a specific bacterial host is like looking for a needle in a trillion haystacks. The host range of a phage is primarily determined by phage tail fibers (or spikes), which initially mediate reversible and specific recognition and adsorption by susceptible bacteria. Recent significant advances at single-molecule and atomic levels have begun to unravel the structural organization of tail fibers and underlying mechanisms of phage–host interactions. Here, we discuss the molecular mechanisms and models of the tail fibers of the well-characterized T4 phage’s interaction with host surface receptors. Structure–function knowledge of tail fibers will pave the way for reprogramming phage host range and will bring future benefits through more-effective phage therapy in medicine. Furthermore, the design strategies of tail fiber engineering are briefly summarized, including machine-learning-assisted engineering inspired by the increasingly enormous amount of phage genetic information.

A CRISPR-engineered bacteriophage T4-COVID vaccine delivers multiple SARS-CoV-2 antigens for potential broader protection.

An engineered viral vector of bacteriophage T4 linked to AAV delivers large payloads of genes and proteins into human cells.

Bacteriophages (phages) are viruses that infect bacteria and use them as replication factories to assemble progeny phages. Bacteria have evolved powerful defense mechanisms to destroy the invading phages by severing their genomes soon after entry into cells.

The COVID-19 pandemic has transformed vaccinology. Rapid deployment of mRNA vaccines has saved countless lives. However, these platforms have inherent limitations including lack of durability of immune responses and mucosal immunity, high cost, and thermal instability. These and uncertainties about the nature of future pandemics underscore the need for exploring next-generation vaccine platforms. Here, we present a novel protein-based, bacteriophage T4 platform for rapid design of efficacious vaccines against bacterial and viral pathogens. Full-length antigens can be displayed at high density on a 120 × 86 nm phage capsid through nonessential capsid binding proteins Soc and Hoc. Such nanoparticles, without any adjuvant, induce robust humoral, cellular, and mucosal responses when administered intranasally and confer sterilizing immunity. Combined with structural stability and ease of manufacture, T4 phage provides an excellent needle-free, mucosal pandemic vaccine platform and allows equitable vaccine access to low- and middle-income communities across the globe.

A multivalent, rapidly deployable, mucosal vaccine platform is desperately needed to prevent acquisition and transmission of respiratory infections during epidemics and pandemics. No such approved platform currently exists and virtually all under investigation use infectious viruses that have safety concerns and are not amenable for multivalent engineering. Herein, a non‐infectious biomaterial platform is presented, the bacteriophage T4 nanoparticle endowed with unique features for modular engineering, which is exploited to design dual COVID‐Flu mucosal vaccines. By leveraging T4's natural affinity to nasal mucosa, in vivo CRISPR engineering, and in vitro SpyCatcher‐SpyTag conjugation, hundreds of antigen molecules are incorporated from SARS‐CoV‐2 and influenza viruses into one nanoparticle. These include spike and hemagglutinin trimers and M2e peptides decorating the capsid while encapsulating matrix or nucleocapsid proteins inside, thereby achieving unprecedented antigen density and diversity, a pinnacle nanoparticle design. Intranasal administration of this adjuvant‐free T4‐CoV‐Flu vaccine induces remarkable mucosal immunity against both respiratory pathogens, including high‐titer neutralizing antibodies and secretory IgA, lung‐resident CD4⁺/CD8⁺ T cells, diverse memory B cells, and complete protection against SARS‐CoV‐2 and influenza challenges. Coupled with its scalability in bacterial systems, thermostability, and adjuvant‐ and needle‐free delivery, T4 presents an extraordinary platform to design potent mucosal vaccines against pandemic threats.