When bacteria infect an organism, it is important that the number of bacterial cells reach a certain critical level before virulence is possible. This is because one bacteria on it’s own will not cause an infection, however millions of bacterial cells working together at the same time may cause an infection. Communication between cells allows individual bacterial to work together on a multi-cellular level (Zhao et al. 2020).
Bacteria can sense the presence of other bacteria in their environment by communicating to each other through a process called quorum sensing (also known as density sensing). This process involves chemical signals known as autoinducers, which are produced, released and detected by quorum sensing bacteria (Miller and Bassler, 2001).
When one bacteria alone release these autoinducers, they dissipate into the environment and have no effect. However, when the bacteria multiply, they produce more of these molecules in proportion to the number of bacterial cells that are present. The rise in quorum sensing molecules are detected by the individual bacterial cells, and they can sense how many other individuals are present in the environment. When the levels of these quorum sensing molecules reach a critical level, the bacteria can release virulence in synchronicity (see Figure 1). This is the mechanism by which bacteria control pathogenicity, as they work together and more successfully in a high population rather than as individuals. As well as toxin production, the quorum sensing system regulates cellular processes such as luminescence, disinfectants tolerance, spore formation, motility, drug resistance and biofilm formation (Zhao et al., 2020).
Figure 1. Bacterial communication through quorum sensing.
Intra-species and inter-species quorum sensing molecules
There are species-specific inducers, which are detected by the corresponding species-specific receptors. These molecules are intra-species, and allows bacteria to have conversations only with members of the same species. These different kinds of intra-specific molecules have a unique shape, however they all share one part of their molecular structure (Ng and Bassler, 2009) (see Figure 2).
There are also a universal inducer that is used to detect other bacteria in the environment. These molecules are inter-species, and are used by all different species of bacteria, which are detected by the same shaped receptor sites. Using this kind of communication, the bacteria can decide which species carries out which tasks depending on which species is in the minority and majority of the population (Smith and Ahmer, 2003). This universal molecule is known as autoinducer 2, or AI-2 (Day, 2001) (see Figure 2).
Figure 2. Bacterial intra-species and inter-species communication through quorum sensing.
Antibiotics targeting communication systems
Some antibiotics contain antagonist molecules that are very similar to the intra-cellular inducers. They lock into the receptors, resulting in the inability of inducer recognition. These antibiotics are disease specific anti quorum sensing molecules. There are also antibiotics that are antagonists of AI-2, which can be used to target all bacteria (broad spectrum) (Hentzer and Givskov, 2003).
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Zhao X, Yu Z, Ding T (2020) Quorum-Sensing Regulation of Antimicrobial Resistance in Bacteria. Microorganisms. 2020 8(3):425.