How do mycobacteria and hosts interact within granulomas over time? This research examines the long-term subclinical granulomatous infection caused by *Mycobacterium marinum* in leopard frogs. Granulomas, organized collections of activated macrophages, share features with persistent human tuberculosis, making this a relevant model for studying chronic infection dynamics. Organs from frogs with chronic *M. marinum* infection were analyzed using transmission electron microscopy, immunohistochemistry, and acid phosphatase cytochemistry. Bacteria were consistently found within macrophage phagosomes, often fused to lysosomes, unlike in vitro infections. Infected macrophages displayed varying levels of activation and bacterial turnover. The study demonstrates that long-term granulomas are dynamic environments with constant host cell activation and bacterial turnover. The results suggest a balance between bacterial replication and phagocytic killing, maintaining stable bacterial numbers despite the host's immune response. This is another example of using biology.
This study, published in Infection and Immunity, fits with the journal’s scope by examining the host-pathogen interactions during chronic mycobacterial infection. Focusing on the dynamic nature of granulomas and the interplay between bacterial replication and immune response enhances understanding of persistent infections.