Antibiotic-Induced Morphological Changes Enhance Phage Predation: A Mathematical Model of Plaque Formation in Structured Environments

Abstract A distinctive manifestation of phage infection in solid media is the appearance of lysis plaques, which correspond to the circular thinning of a bacterial lawn. During plaque formation, successive cycles of phage replication generally take place from a single point of infection and spread radially in a matrix of immobilized bacterial hosts. Many different factors affect plaque size, such as the composition and the reticulation of the propagation matrix, the characteristics of the phage, but also parameters related to the physiology of the bacterial host. Since combined administration of both antibiotics and phages is a common practice during compassionate treatments, our research focuses on the effects of antibiotics on phage predation, which can be of crucial importance for the therapeutic applications of phages. It has been previously observed that the presence of antibiotics at sublethal concentrations can affect drastically bacterial physiology, allowing phages to spread more rapidly and resulting in better bacterial eradication. Previous experimental work has focused on the phage characteristics that influence plaque size. However, as plaque formation is strongly influenced by host growth dynamics, a comprehensive model integrating both the host growth and phage infection parameters is required. In this work, we suggest that plaque enlargement is linked to morphological changes of the host that have an impact on the rate of epidemic propagation and certainly on phage diffusion into the matrix. To support this hypothesis, we characterized the growth parameters of two different phages and bacteria in semi-solid media in the presence of various antibiotics. By combining these data, we have produced a mathematical model that accounts for these observations and explains the increase in plaque size when the host morphology is affected. Significance Statement This study provides new insights into the phenomenon of Phage-Antibiotic Synergy (PAS) by demonstrating that antibiotic-induced morphological changes in bacterial hosts play a critical role in enhancing phage propagation in structured environments. By linking these morphological alterations—such as cell filamentation and bloating—to increased lysis plaque size, the research underscores the importance of host dynamics in phage therapy. The development of a mathematical model integrating both host growth and phage infection parameters offers a novel framework for understanding and optimizing phage-based treatments in the presence of antibiotics, potentially improving therapeutic outcomes.