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Within deep tissue sites, extracellular bacterial pathogens often replicate in clusters that are surrounded by immune cells. Disease is modulated by interbacterial interactions as well as bacterial-host cell interactions resulting in microbial growth, phagocytic attack and secretion of host antimicrobial factors. To overcome the limited ability to manipulate these infection sites, we established a system for Yersinia pseudotuberculosis ( Yptb) growth in microfluidics-driven microdroplets that regenerates microbial social behavior in tissues. Chemical generation of nitric oxide (NO) in the absence of immune cells was sufficient to reconstruct microbial social behavior, as witnessed by expression of the NO-inactivating protein Hmp on the extreme periphery of microcolonies, mimicking spatial regulation in tissues. Similarly, activated macrophages that expressed inducible NO synthase (iNOS) drove peripheral expression of Hmp, allowing regeneration of social behavior observed in tissues. These results argue that topologically correct microbial tissue growth and associated social behavior can be reconstructed in culture., Competing Interests: SC, DT, LS, KD, EA, Tv, RI No competing interests declared, (© 2020, Clark et al.)

Macrophages are the front line of immune defense against invading microbes. Microbes, however, have evolved numerous and diverse mechanisms to thwart these host immune defenses and thrive intracellularly. Legionella pneumophila, a Gram-negative pathogen of amoebal and mammalian phagocytes, is one such microbe. In humans, it causes a potentially fatal pneumonia referred to as Legionnaires' disease. Armed with the Icm/Dot type IV secretion system, which is required for virulence, and approximately 300 translocated proteins, Legionella is able to enter host cells, direct the biogenesis of its own vacuolar compartment, and establish a replicative niche, where it grows to high levels before lysing the host cell. Efforts to understand the pathogenesis of this bacterium have focused on characterizing the molecular activities of its many effectors. In this article, we highlight recent strides that have been made in understanding how Legionella effectors mediate host-pathogen interactions.