ABSTRACT
During our lives we are constantly faced with a multitude of experiences, most of which are so minor that they go unnoticed during our daily routines. These seemingly unimportant life events are used by us, most of the time unconsciously, to perceive the environment around us. For instance, as we walk into a crowded room, a social gathering for example, we immediately perceive a number of features of the environment using our senses. If you are hungry, you might smell food being cooked and move closer to the kitchen. You may also hear the voice of an unpleasant coworker and use that information to avoid him or her. You will also immediately have a visual perception of everything happening in the room and act accordingly. You will feel the temperature in the
6.1 Introduction ........................................................................................................................... 147 6.2 One-Component Signaling ................................................................................................... 148 6.3 Two-Component Signaling ................................................................................................... 150 6.4 Second Messenger Signaling ................................................................................................ 152
6.4.1 cAMP ........................................................................................................................ 152 6.4.2 cGMP ........................................................................................................................ 154 6.4.3 (p)ppGpp ................................................................................................................... 155 6.4.4 c-di-GMP .................................................................................................................. 155 6.4.5 c-di-AMP .................................................................................................................. 156 6.4.6 Remaining Questions in Second Messenger Signaling ............................................ 157
6.5 Cell-Cell Signaling ............................................................................................................... 157 6.5.1 Examples of Intraspecific Cell-Cell Signaling Systems .......................................... 158
6.5.1.1 Acyl-Homoserine Lactone Signaling in Proteobacteria ............................. 158 6.5.1.2 Oligopeptide Signaling in Gram-Positive Bacteria .................................... 160 6.5.1.3 Other Signaling Systems ............................................................................ 161
6.5.2 Examples of Interspecific Cell-Cell Signaling Systems........................................... 161 6.5.2.1 Autoinducer-2 ............................................................................................. 161 6.5.2.2 Antibiotics ................................................................................................... 162
6.5.3 Quorum Sensing in Natural Habitats ........................................................................ 162 6.6 Interrelationship of Signaling Systems ................................................................................. 163
6.6.1 Second Messenger Integration .................................................................................. 163 6.6.2 Second Messengers That Function as Extracellular Signals..................................... 164 6.6.3 Second Messenger and Quorum-Sensing Crosstalk ................................................. 165 6.6.4 The Interdependence of Bacterial Signals ................................................................ 165
6.7 Conclusion ............................................................................................................................. 166 Glossary ......................................................................................................................................... 167 References ...................................................................................................................................... 167
environment and decide whether or not you want to wear the jacket you have been holding since coming inside. Such “sensing” capabilities evolved to allow us to quickly and effectively assess the potential benefits and costs of any situation we get ourselves involved in. Ultimately, this is done so that we can adapt to the conditions in our environment in order to maximize our chances of surviving. Although with different degrees of sophistication and using different methods, every living being can sense its surroundings and use this information to adapt. In simple terms, this adaptation means avoiding danger and seeking potential benefits. Microorganisms are no different. Microbes have evolved the capacity to obtain information from the external world and translate this information into beneficial behavior. To do this, they must have mechanisms to sense external signals and transfer that information to a meaningful event, that is, a change in behavior. Most of the time, this is done through the modulation of gene expression. By choosing the sets of genes that will be expressed at any given time, microbes can shape their metabolism and behavior in accordance with the particular environmental conditions they find themselves in. The reason this is advantageous is simple: gene expression can be metabolically costly. Therefore, by carefully regulating gene expression, microbes can avoid unnecessary metabolic costs in conditions where certain genes are not useful. To do this, microbes have evolved a number of different systems that sense external information and modulate gene expression in response to that information. In general, these systems are composed of protein modules with specific functions that can coordinate their activities to sense multiple stimuli and generate the appropriate output. Some of the most commonly used systems are the one-component systems, two-component systems, intracellular chemical signals (second messenger signaling), and extracellular chemical signals (cell-cell signaling). In this chapter, we will review the main examples of microbial signaling systems, how they function at the molecular level, and how they allow microbes to sense their surroundings and adapt accordingly. We will also address how these systems might function in nature, in the natural habitats of the microbes being discussed, as well as the potential of microbial signaling systems as sources of both biotechnologically and therapeutically relevant knowledge.
