Roles of the second messenger c-di-GMP in bacteria: Focusing on the topics of flagellar regulation and Vibrio spp.

Typical second messengers include cyclic AMP (cAMP), cyclic GMP (cGMP), and inositol phosphate. In bacteria, cyclic diguanylate (c-di-GMP), which is not used in animals, is widely used as a second messenger for environmental responses. Initially found as a regulator of cellulose synthesis, this small molecule is known to be widely present in bacteria. A wide variety of synthesis and degradation enzymes for c-di-GMP exist, and the activities of effector proteins are regulated by changing the cellular c-di-GMP concentration in response to the environment. It has been shown well that c-di-GMP plays an essential role in pathogenic cycle and is involved in flagellar motility in Vibrio cholerae. In this review, we aim to explain the direct or indirect regulatory mechanisms of c-di-GMP in bacteria, focusing on the study of c-di-GMP in Vibrio spp. and in flagella, which are our research subjects.