User:Cathalgarvey/SpecialBookPages/BacterialQuorumSensing

Although examples of horizontal gene transfer are apparent in LuxI, LuxR, and LuxS phylogenies, they are relatively rare. This result is in line with the observation that quorum sensing genes tend to control the expression of a wide array of genes scattered throughout the bacterial chromosome. A recent acquisition by horizontal gene transfer would be unlikely to have integrated itself to this degree. Given that the majority of autoinducer–synthase/receptor occurs in tandem in bacterial genomes, it is also rare that they switch partners and so pairs tend to co-evolve.^[21]

The phylogeny of quorum sensing genes in Gammaproteobacteria (which includes Pseudomonas aeruginosa and Escherichia coli) is especially interesting.^{[according to whom?]} The LuxI/LuxR genes form a functional pair, with LuxI as the auto-inducer synthase and LuxR as the receptor. Gamma Proteobacteria are unique in possessing quorum sensing genes, which, although functionally similar to the LuxI/LuxR genes, have a markedly divergent sequence.^[21] This family of quorum-sensing homologs may have arisen in the gamma Proteobacteria ancestor, although the cause of their extreme sequence divergence yet maintenance of functional similarity has yet to be explained. In addition, species that employ multiple discrete quorum sensing systems are almost all members of the gamma Proteobacteria, and evidence of horizontal transfer of quorum sensing genes is most evident in this class.^[20][21]

Controversy

As quorum sensing implies a cooperative behavior, this concept has been challenged by the evolutionary implication of cooperative cheaters. This is circumvented by the concept of diffusion sensing, which has been an alternative and complementary model to quorum sensing. However, both explanations face the problems of signalling in either complex (multiple species sharing the same space) or simple (one single cell confined to a limited volume) environments where the spatial distribution of the cells can be more important for sensing than the cell population density. A new model, efficiency sensing, which takes into account both problematics, population density and spatial confinement, has been proposed as an alternative.^[22] One of the probable reasons for controversy is that current terminologies (quorum sensing, diffusion sensing, efficiency sensing) all imply an understanding of the motives and benefits of the process, and may be observed to apply under some circumstances but not others. Perhaps a sensible resolution to these controversies could be to return the terminology of the process to autoinduction, as originally described by Hastings and coworkers, as this term does not imply understanding of the intent(s) or benefit(s) of the process.

Anti-quorum sensing medical treatments

Today, about 70% of the bacteria that cause infections are resistant to at least one of the drugs most commonly used for treatment.^{[dubious – discuss]} Some organisms are resistant to all approved antibiotics and can be treated only with experimental and potentially toxic drugs. A substancial increase in resistance of bacteria that cause community-acquired infections has also been documented, especially in the staphylococci and pneumococci, which are prevalent causes of disease and mortality. In a recent study, 25% of bacterial pneumonia cases were shown to be resistant to penicillin, and an additional 25% of cases were resistant to more than one antibiotic.^{[citation needed]}

The current state of antibiotic affairs is due to the manner in which existing antibiotics work. All current antibiotics aim to kill the individual bacteria in one manner or another (by inhibiting synthesis of new bacteria, usually).^{[citation needed]} This environmental pressure activates the evolutionary mechanisms that select for resistant strains. In other words, bacteria that are not resistant to the antibiotic are killed off, leaving the resistant organisms to multiply unchecked without competition. This is why resistant strains spread so rapidly and occur so frequently.

Recent research into quorum sensing systems has produced compounds that can disrupt the bacteria's ability to communicate, thereby disabling or diminishing the bacteria's ability to become pathogenic. Therefore, the body is not compromised by cell damage, inflammation, toxicity, or other detrimental effects of the bacteria. This gives the body time to eradicate the bacteria naturally through normal immune system functions.

The advantage of the anti-quorum sensing approach to controlling infection is that there are few evolutionary forces that select for resistance—there is little in the process that would create resistant strains. Since the compounds kill none of the bacteria, any resistant mutations must compete with living, non-resistant individuals. In other words, there is no survival advantage to the resistant mutations, and natural selection does not come into play. Resistant strains will be unlikely to occur.^[23]