Bacteriophages (phages) are bacterial viruses and the most predominate life form on the planet. These bacterial predators use their tail structures to recognize cognate receptors on bacterial host cells (adsorption step), inject their nucleic acid genomes inside the host (infection step), hijack the bacteria's cellular machinery to replicate (replication step) and finally lyse the host cell to release infectious progeny (lysis step). Due to their mode of action, lytic or virulent phages have been utilized as an alternative therapy against multidrug-resistant bacterial infections. Acinetobacter baumannii has been identified as a priority critical target for the development of new therapeutic options by the WHO, CDC (CDC 2019) (Figure 1). The well-known “Patterson Case” was the first personalized use of phage therapy in the Western World to save the life of a critically ill patient suffering from a systemic, multidrug-resistant A. baumannii infection (Schooley RT et al 2017). Phages infecting Acinetobacter spp. are genetically diverse, being grouped into several subfamilies and genera (Oliveira H et al 2021), and often recognize the Acinetobacter capsule as receptors (Timoshina OY et al 2023, Oliveira H & Costa R et al 2017, Oliveira H et al 2019). As a result of capsule-dependent infection, A. baumannii cells that develop resistance towards phage infection often lose their capsules, becoming less virulent and more susceptible to antimicrobials and the immune system, thereby further extending the clinical efficacy of phage therapy (Altamirano FG et al 2021, Altamirano FG et al 2021) (Figure 2).

To mediate the adsorption and infection steps, some phages possess enzymatic structures, typically displayed on phage tail fibers or base plates, that break down the thick and highly diversified capsular polysaccharide layer surrounding the A. baumannii cell surface (Kenyon JJ & Hall RM 2013, Wyres KL et al 2020, Oliveira H & Costa R et al 2017). These enzymes are called capsular depolymerases and can now be predicted using advanced bioinformatic tools such as PhageDPO (Vieira M et al 2022). Since Acinetobacter capsules represent a major virulence factor, phage-derived depolymerases have potential as anti-virulence agents. Recombinantly expressed depolymerases have been shown to degrade the capsular layer (Figure 3), exposing A. baumannii cells to the host immune system, thereby controlling the infection (Oliveira H et al 2019, Oliveira et al 2019). Although these depolymerases do not kill the cells “per se”, they represent useful anti-virulence agents as evidenced in studies using animal models.

Besides the evident therapeutic effect of phage therapy against drug-resistant infections, there are still some limitations before this technology can broadly applied in the treatment of A. baumannii infections. The forefront limitations are associated with regulatory hurdles for that currently limit the clinical use of phage therapy to compassionate use cases. In addition, A. baumannii phages typically have a very narrow host-range, with many being highly specific for a given capsule type. This is further compounded by the large variation seen across the composition and structure of A. baumannii capsular polysaccharides, with over 200 capsule-types currently listed on the Kaptive database (Cahill SM et al 2022). Therefore, the isolation of novel phages that target diverse receptors other than the capsule, may provide broader host range phages with improved clinical utility. Phages can also be centralized in phage biobanks to allow rapid screening approaches of well-defined phages with demonstrated antimicrobial activity against specific clinical isolates. With the rapid advances in synthetic biology tools, it will be possible to engineer both recombinant phage-derived depolymerases with the ability to recognize new and multiple capsular polysaccharide substrates, and virulent phages with expanded host ranges and enhanced bacterial killing activity through the expression of antimicrobial payloads (e.g., antimicrobial peptides, anti-quorum sensing molecules) (Oliveira H et al 2022, Du J et al 2023, Meile S & Du Jiemin et al 2023). Further research into the discovery, characterization, and application of anti-A. baumannii phages will undoubtedly lead to innovative strategies for the treatment of multidrug-resistant infections which will increase/expand our modern medical “toolbox” against this critically important pathogen.

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