MicrobiologyBytes

Many bacteria use chemical signals to coordinate group behaviour. This process is called quorum sensing. Vibrio cholerae, the bacterium which causes cholera, is ‘bilingual’ – it uses two distinct signalling molecules to suppress its virulence. One of these signals is a molecule that many species of bacteria use for quorum sensing, but the identity of the second signal has remained a mystery. In the latest issue of Nature, Higgins et al. report the structure of this second signal. Their discovery represents a new structural class of quorum-sensing signal that may be exclusive to Vibrio bacteria, making it a possible lead for drug discovery.

The major Vibrio cholerae autoinducer and its role in virulence factor production. 2007 Nature 450: 883
Abstract: Vibrio cholerae, the causative agent of the human disease cholera, uses cell-to-cell communication to control pathogenicity and biofilm formation. This process, known as quorum sensing, relies on the secretion and detection of signalling molecules called autoinducers. At low cell density V. cholerae activates the expression of virulence factors and forms biofilms. At high cell density the accumulation of two quorum-sensing autoinducers represses these traits. These two autoinducers, cholerae autoinducer-1 (CAI-1) and autoinducer-2 (AI-2), function synergistically to control gene regulation, although CAI-1 is the stronger of the two signals. V. cholerae AI-2 is the furanosyl borate diester (2S,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran borate. Here we describe the purification of CAI-1 and identify the molecule as (S)-3-hydroxytridecan-4-one, a new type of bacterial autoinducer. We provide a synthetic route to both the R and S isomers of CAI-1 as well as simple homologues, and we evaluate their relative activities. Synthetic (S)-3-hydroxytridecan-4-one functions as effectively as natural CAI-1 in repressing production of the canonical virulence factor TCP (toxin co-regulated pilus). These findings suggest that CAI-1 could be used as a therapy to prevent cholera infection and, furthermore, that strategies to manipulate bacterial quorum sensing hold promise in the clinical arena.

Also: Cooperation and conflict in quorum-sensing bacterial populations. 2007 Nature 450: 411
Abstract: It has been suggested that bacterial cells communicate by releasing and sensing small diffusible signal molecules in a process commonly known as quorum sensing (QS). It is generally assumed that QS is used to coordinate cooperative behaviours at the population level. However, evolutionary theory predicts that individuals who communicate and cooperate can be exploited. Here we examine the social evolution of QS experimentally in the opportunistic pathogen Pseudomonas aeruginosa, and show that although QS can provide a benefit at the group level, exploitative individuals can avoid the cost of producing the QS signal or of performing the cooperative behaviour that is coordinated by QS, and can therefore spread. We also show that a solution to the problem of exploitation is kin selection, if interacting bacterial cells tend to be close relatives. These results show that the problem of exploitation, which has been the focus of considerable attention in animal communication, also arises in bacteria.

Related:

• Quorum Sensing in Bacteria: We Two Are One
• Pneumococcus: the quorum-sensing killer
• Quorum sensing in Serratia

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David Davies stood at the hospital bedside of his great-aunt, who had recently had all her toes amputated to try to prevent persistent wounds from spreading. It didn’t work; doctors would later amputate both of her feet, and she never returned to her independent life. As Davies thinks back to this episode from his high-school days, “I remember wondering how come, in this era of antibiotics, was it not possible to treat what was obviously, to me, an infection?”. Even today, such non-healing wounds are common in people with late-stage diabetes like Davies’ great-aunt, who have poor circulation, as well as in people with compromised immune systems. Davies, now an associate professor at Binghamton University, says that many doctors treat these debilitating wounds as a problem with the patient, rather than a sign of infection. “There’s no excuse for it,” he says.

Wounds are just one example of the huge impact of bacterial biofilms. The United States National Institutes of Health says that 80% of chronic infections are biofilm related. Unlike the more familiar planktonic lifestyle, in which bacteria float or swim freely, in biofilms they surround themselves with a complex polymeric matrix, better known as slime. As it grows thicker, the film often includes many bacterial species and the matrix develops a complex structure. Traditional antibiotics are often ineffective. We thought we had it all figured out, Davies says, but the past 20 years have shown that researchers are still in the dark ages when it comes to understanding and controlling bacteria.

As biofilms, bacteria routinely foul industrial equipment and medical devices like catheters and implants, where they form dense layers that cling tightly to the artificial surfaces. They also occur naturally within us, most familiarly as dental plaque. In addition, biofilms are increasingly blamed for recurrent and chronic infections. The case is clear for the Pseudomonas aeruginosa lung infections that haunt cystic fibrosis patients and for recurrent middle ear infections of Haemophilus influenzae. But biofilms are also prime suspects in a long list of other itises, including endocarditis, prostatitis, and conjunctivitis.

Researchers have learned much in recent years about the mechanisms that let bacteria establish a beachhead on a surface and work together to form a highly structured matrix that nourishes and protects them. The films are easily seen by electron microscopy on foreign surfaces like catheters. However, definitively establishing a role for biofilms in a particular diseaseor in chronic wounds like those of Davies’ great-auntis difficult, in part because traditional culture and assay techniques work best for planktonic forms. Even as the evidence comes in, however, many researchers are actively seeking unique vulnerabilities of the biofilm state, hoping to control these stubborn films wherever they occur.

Read more: Looking for Chinks in the Armor of Bacterial Biofilms. PLoS Biol 2007 5: e307

Candida parapsilosis is the second most common cause of invasive candidiasis worldwide. This fungus is particularly associated with disease in premature infants and immunocompromised adults and is a major cause of infections in intensive care units. Despite increasing clinical importance, little is known about the genetic basis of fungal virulence traits that enable C. parapsilosis to cause disease. Virulence factors of C. parapsilosis need to be identified in order to develop more effective therapy against this pathogen. The authors developed a gene deletion system for C. parapsilosis and used it to delete the lipase locus in the C. parapsilosis genome. They also reconstructed the missing genes, which restored lipase activity. Biofilm formation was inhibited with lipase-negative mutants and their growth was significantly reduced in lipid-rich media. The knockout mutants were more efficiently ingested and killed by macrophage-like cells. Additionally, the lipase-negative mutants were significantly less virulent in infection models that involve inoculation of reconstituted human oral epithelium or murine intraperitoneal challenge. These studies represent the first targeted disruption of a gene in C. parapsilosis and show that C. parapsilosis-secreted lipase is involved in disease pathogenesis. This system for targeted gene deletion holds great promise for rapidly enhancing our knowledge of the biology and virulence of this increasingly common invasive fungal pathogen.

Targeted gene deletion in Candida parapsilosis demonstrates the role of secreted lipase in virulence.
J Clin Invest. 2007 Sep 13

In one of the first potential applications of synthetic biology, which aims to design and build useful biomolecular systems, researchers have engineered viruses to attack and destroy the surface “biofilms” that harbor harmful bacteria in the body and on industrial and medical devices. Bacterial biofilms can form almost anywhere, such as on your teeth if you don’t brush. When they accumulate in hard to reach places such as the insides of food processing machines or medical catheters, however, they become persistent and dangerous sources of infection. The bacteria excrete a variety of proteins, polysaccharides, and nucleic acids that together with other materials form an extracellular matrix, a slimy layer that encases the bacteria. Traditional remedies such as antibiotics are not as effective on these bacterial biofilms as they are on free-floating bacteria. In some cases, antibiotics may even encourage bacterial biofilms to form.
For a bacteriophage to be effective against a biofilm, it must both attack the bacteria in the film and degrade the extracellular matrix. Recently, a group of researchers discovered several phages in sewage that meet both criteria because they carry enzymes capable of degrading the biofilm’s extracellular matrix. In the most recent edition of PNAS, researchers have defined a modular system that allows scientists to design phages to target specific biofilms. As a proof of concept, they used their strategy to engineer T7, an Escherichia coli-specific phage, to express dispersin B (DspB), an enzyme known to disperse a variety of biofilms. They found that their engineered T7 phage eliminated 99.997% of the bacterial cells in the biofilm, an improvement of more than a hundred-fold over the original phage.

Dispersing biofilms with engineered enzymatic bacteriophage. PNAS USA 2007 104: 11197-11202