Successful intracellular replication is a defining feature of many bacterial pathogens and directly influences disease outcome. For the salmonid pathogen Piscirickettsia salmonis, the intracellular environment that supports bacterial growth has remained incompletely characterized. Here, we show that P. salmonis replicates within an acidified, Lamp-1-positive vacuole and that intracellular growth is influenced by host iron availability. Infection is accompanied by activation of lysosomal pathways in host cells and coordinated induction of bacterial stress-response mechanisms, secretion systems, iron-acquisition pathways, and numerous genes of previously unknown function. Intracellular passage also alters bacterial behavior during subsequent infection cycles, suggesting a physiological adaptation associated with host-cell residence. By defining the intracellular context in which P. salmonis proliferates and situating these features within the broader landscape of intracellular bacterial strategies, this work advances understanding of host-pathogen interactions in non-mammalian systems and provides a foundation for future functional studies relevant to aquaculture and intracellular microbiology.
Aravena et al. (Mon,) studied this question.