Chapter 16 - Microbial Communication: Bacteria–Bacteria and Bacteria–Host

Abstract

Bacterial cells communicate by emitting specific chemicals known as signals that allow communication among cells through a process called “quorum sensing.” This chapter discusses the current understanding of the bacterial communication with a special focus on communication in Gram-negative bacteria via N-acyl homoserine lactones (AHLs), and communication in Gram-positive bacteria via γ-butyryl lactones and small peptide signals. The best studied diffusible signals are the AHLs in Gram-negative bacteria. A large number of diverse Gram-negative bacteria have been shown to utilize quorum sensing as a key regulatory mechanism. The quorum sensing regulation involves two proteins: an AHL synthase, encoded by an I gene and converts cellular precursors into one or more AHL signals, and the second is an AHL responsive regulatory protein, encoded by R gene and is required for the activation of specific genes. Increasing cell numbers results in increased AHL signal concentration, and on reaching a threshold level activates the gene expression through a sequence of events. While the majority of Gram-positive bacteria utilize small peptides—such as autoinducer-2 (AI-2)—others utilize γ-butyrolactones as their primary communication molecules. Some of the many possible ecological roles of quorum sensing are (1) coordination of gene expression and bacterial behavior within a single and among multiple bacterial populations, (2) avoidance of host defense responses, (3) and direct communication between the bacterium and the host organism. These communication networks represent key ecological control points that directly determine the outcome of host–microbe interactions. Understanding these communication networks may facilitate large-scale improvements in bacterial–host interactions, pathogen suppression, bioremediation, and water and waste treatment.