To adapt to environmental change, bacteria use signal transduction systems to sense and respond to extracellular stimuli. We study the molecular mechanism of signal transduction pathways in two different enteric bacteria. In Escherichia coli, the mechanism by which the histidine kinase, EnvZ, senses osmolarity signals is studied. The second bacterium, Xenorhabdus nematophilus represents a facscinating example of an organism that lives as a symbiont in one host and functions as a pathogen in a different host. The bacterium exists as a symbiont in the intestine of specific soil nematodes. When the nematodes ambush soil dwelling insect larvae, the bacteria are released into the insect's hemocoel and function as pathogenic agents. The bacteria multiply and produce conditions suitable for nematode production in the insect cadaver. After several cycles of sexual reproduction, the dauer juveniles emigrate from the insect in search of a new host. This nematode-bacterium complex is being used in biocontrol against soil-borne insect pests. The role of EnvZ and the global transcription regulator, OmpR, in the life cycle of Xenorhabdus is currently being investigated in my laboratory.