ABSTRACTAntimicrobial resistance is a danger to global public health and threatens many aspects of modern medicine. Bacterial species such as those of theBurkholderia cepaciacomplex (Bcc) cause life-threatening respiratory infections and are highly 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 is limited by its prevailing paradigm: that only obligately lytic phages should be used therapeutically. It is thought that ‘lysogenic’ phages do not lyse all bacteria and can transfer antimicrobial resistance or virulence factors to their hosts. 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 the therapeutic suitability of a phage must be evaluated on a case-by-case basis. Concordantly, we developed several novel metrics—Efficiency of Phage Activity, Growth Reduction Coefficient, and Stable Lysogenization Frequency—and used them to evaluate eight Bcc-specific phages. Although these parameters vary considerably among Bcc phages, a strong inverse correlation (R2= 0.67;P< 0.0001) exists between lysogen formation and antibacterial activity, indicating that certain LC phages with low frequency of stable lysogenization may be therapeutically efficacious. 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 ofin vitrobacterial 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 species of theBurkholderia cepaciacomplex (Bcc), which cause life-threatening respiratory infections and are notoriously resistant to antibiotics. Phage therapy is a promising alternative being explored to combat Bcc infections and antimicrobial resistance in general, but its utility 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 phages exhibit powerfulin vitroantibacterial 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.