Promotion and Rescue of Intracellular Brucella neotomae Replication during Coinfection with Legionella pneumophila.

We established a new model system for study of type IV secretion system-dependent (T4SS) pathogenesis in the genus. Importantly, is a rodent pathogen, and unlike , , and , has not been observed to infect humans. It therefore can be handled more facilely using biosafety level 2 practices. More particularly, using a series of novel fluorescent protein and operon reporter systems to differentially label pathogens and track intracellular replication, we confirmed T4SS-dependent intracellular growth of in macrophage cell lines. Furthermore, exhibited early endosomal (LAMP-1) and late endoplasmic reticulum (calreticulin)-associated phagosome maturation. These findings recapitulate prior observations for human-pathogenic spp. In addition, during coinfection experiments with , we found that defective intracellular replication of a T4SS mutant was rescued and baseline levels of intracellular replication of wild-type were significantly stimulated by coinfection with wild-type but not T4SS mutant Using confocal microscopy, it was determined that intracellular colocalization of and was required for rescue and that colocalization came at a cost to fitness. These findings were not completely expected based on known temporal and qualitative differences in the intracellular life cycles of these two pathogens. Taken together, we have developed a new system for studying pathogenesis and found a remarkable T4SS-dependent interplay between and during macrophage coinfection.