**1. Introduction**

Nowadays, at the bacterial world, it is widely accepted that there are biological processes that must be pointed by a coordinated behavior of entire population. Factors such as: virulence factors production, biofilm formation, secondary metabolite production, and bioluminescence are fruitless when undertaken by a single bacterium proceeding isolated [1]. A wide variety of bacteria are endowed with encoding genes for components of a cell-to-cell communication system termed as quorum sensing (QS). This QS system enables bacteria to regulate their behavior in a cell density fashion in order to modulate a gene set that enables the bacteria to adapt to environmental challenges [2]. Quorum sensing relays its activity in a production, releasing, and perception of small signal molecules called auto-inducers. In Gramnegative bacteria, the usual auto-inducers are small molecules, i.e., acyl lactone and short-chain fatty acids. The cognate receptor involved in perception of these autoinducer molecules is: cytoplasmic transcription factor and two components histidine sensor kinases. The complex produced by the auto-inducer and receptor leads the promotion of target genes regulated by quorum sensing [3]. Gram-positive bacteria

use mainly short peptides as auto-inducers, and its related receptors are transmembrane histidine sensor kinase. Usually, the union of auto-inducer and receptor triggers expression of encoding gene for AI (auto-inducer) synthase, which increases the extracellular AI concentration switching on the bacteria quorum sensing mode [4].

Disruption of quorum sensing communication system, which is termed quorum quenching, leads to a reduction in virulence factors expression without compromising bacterial survival [5]. Since a wide diversity of bacterial cells that use QS display a significant competitive advantage over other prokaryotes and eukaryotes with which they coexist in the same ecological niche, it is rational that the contender microorganisms have developed mechanism to disrupt the QS communication systems present in the ecological niche. Interference with the quorum sensing communication system either by natural or synthetic approaches may afford strategies for disease control, by reducing the virulence or turn the pathogens more susceptible to antibiotic therapy. The design, development, and employment of these approaches will depend in great measure upon the knowledge of mechanistic details of quorum sensing pathway such as: auto-inducer synthesis, signal perception, signal transduction, and genes under quorum sensing regulation [6].

Citrus is the most commercialized horticultural product in the world; however, farmers in the last two decades have seen production reduced by average of 65%, due to devastating bacterial diseases such as: Bacterial Citrus Canker (BCC) caused by *Xanthomonas citri* subsp. *citri* (Xcc), Citrus Variegated Chlorosis (CVC) caused by *Xylella fastidiosa*, Citrus Blast caused by *Pseudomonas syringae* pv. *Syringae*, and Citrus Greening or Huanglongbing (HLB) caused by *Candidatus liberibacter* sp. All bacteria aforementioned except the *Candidatus liberibacter* are endowed with quorum sensing systems, which are responsible for the pathogenesis and symptomatology in citrus host. The main objective of this chapter is to describe quorum sensing pathways in these phytopathogen bacteria, as well to review some successful approaches based in quorum sensing disruption in order to decrease disease severity.
