Synergistic interactions among Burkholderia cepacia complex (Bcc)-targeting phages reveal a novel therapeutic role for lysogenization-capable (LC) phages

Antimicrobial resistance is an imminent danger to global public health and threatens virtually all aspects of modern medicine. Particularly concerning are the species of the Burkholderia cepacia complex (Bcc), which cause life-threatening respiratory infections among patients who are immunocompromised or afflicted with cystic fibrosis, and are notoriously resistant to antibiotics. One promising alternative being explored to combat Bcc infections is phage therapy (PT) - the use of phages to treat bacterial infections. Unfortunately, the utility of PT against many pathogenic species, including the Bcc, is limited by the prevailing paradigm of PT: that only obligately lytic phages, which are rare, should be used therapeutically - due to the conviction that so-called lysogenic phages do not reliably clear bacteria and instead form lysogens to which they may transfer antimicrobial resistance or virulence factors. In this study, we argue that the tendency of a lysogenization-capable (LC) phage to form stable lysogens is not predicated exclusively on its ability to do so, and that this property, along with the therapeutic suitability of the phage, must be evaluated on a case-by-case basis. Concordantly we developed several novel metrics - Efficiency of Phage Activity (EPA), Growth Reduction Coefficient (GRC), and Lysogenization Frequency (f(lys)) and used them to evaluate eight phages targeting members of the Bcc. We found that while these parameters vary considerably among Bcc phages, a strong inverse correlation exists between lysogen-formation and antibacterial activity, indicating that certain LC phages may be highly efficacious on their own. Moreover, we show that many LC Bcc phages interact synergistically with other phages in the first reported instance of mathematically defined polyphage synergy, and that these interactions result in the eradication of in-vitro bacterial growth. Together, these findings reveal a novel therapeutic role for LC phages, and challenge the current paradigm of PT. IMPORTANCEThe spread of antimicrobial resistance is an imminent threat to public health around the world. Particularly concerning are the species of the Burkholderia cepacia complex (Bcc), which cause life-threatening respiratory infections and are notoriously resistant to antibiotics. Phage therapy (PT) is a promising alternative being explored to combat Bcc infections and antimicrobial resistance in general, but the utility of PT against many pathogenic species, including the Bcc, is restricted by the currently prevailing paradigm of exclusively using rare obligately lytic phages - due to the perception that lysogenic phages are therapeutically unsuitable. Our findings show that many lysogenization-capable (LC) phages exhibit powerful in vitro antibacterial activity both alone and through mathematically defined synergistic interactions with other phages, demonstrating a novel therapeutic role for LC phages and therefore challenging the currently prevailing paradigm of PT.