How defend their turf.

Communities of bacteria that cause Legionnaires' disease repel other bacteria by secreting an acid called HGA.

A Two-Component System That Modulates Cyclic di-GMP Metabolism Promotes Legionella pneumophila Differentiation and Viability in Low-Nutrient Conditions.

During its life cycle, the environmental pathogen alternates between a replicative and transmissive cell type when cultured in broth, macrophages, or amoebae. Within a protozoan host, further differentiates into the hardy cell type known as the mature infectious form (MIF). The second messenger cyclic di-GMP coordinates lifestyle changes in many bacterial species, but its role in the life cycle is less understood.