Bacterial Communication

Bacteria are no longer thought to exist as individual single celled organisms; in fact, they can co-ordinate and communicate in a multicellular fashion when it is advantageous for them to do so. The majority of microbes are found thriving in complex polymicrobial biofilm communities. In the natural environment bacteria predominantly live adhered to a surface as part of a biofilm. A biofilm is a multicellular community of cells encased in a self-produced extracellular matrix. Life in a biofilm offers advantages to the population, including supply of nutrients and protection from environmental stress.

Bacteria have evolved elaborate means to communicate with each other, both within and between species, by releasing a wide range of diffusible signalling compounds into their external environment. Once outside the bacterial cell, these molecules freely diffuse within the milieu where they can be perceived by other microorganisms. Bacteria can utilize such systems to monitor their population density (the process of quorum sensing) and/or their confinement in particular environmental niches, and to coordinate specific population-wide alterations in gene expression and bacterial behaviour. Cell–cell signalling allows a colony or group of organisms to behave in a coordinated fashion to regulate processes contributing to virulence, antibiotic production, biofilm formation and other developmental programmes. In some cases, bacteria are capable of responding to signals produced by other species of bacteria; eavesdropping on their competitors or synchronizing with collaborators.

The autoinducer signal molecules produced by bacteria are a structurally diverse group of compounds. Many Gram-negative bacteria use N-acylhomoserine lactones as signals, although other fatty acid derivatives such as cis-unsaturated fatty acids are also found¹.

In order to sense these chemical signals, bacteria often employ two-component signal transduction systems allowing them to respond and adapt to a wide range of signals and stimuli including nutrient availability, cellular redox state, changes in osmolarity, quorum signals and antibiotics. The canonical two-component signal transduction systems are composed of two proteins; a sensor histidine kinase which responds to a signal and a cognate response regulator which mediates the response via altered gene expression (a schematic of this is shown in Fig 1). The perceived signal is directly translated into a change in transcriptional regulation, the decision-making level of a prokaryotic cell at which cell-cell communication is integrated with other sensory inputs.

Microbial Infections Associated with CF

In a healthy respiratory system, the upper respiratory tract is colonized by a wide variety of microorganisms, while the lower respiratory tract is maintained in a sterile state by the various immune defences of the host. Failure of any of these defences results in susceptibility to pulmonary infection. CF patients have an abnormal mucous composition in the airways and are particularly vulnerable to microbial infection. Such infections are responsible for the early mortality associated with CF.

Ultimately 80 to 95% of patients with CF succumb to respiratory failure brought on by chronic bacterial infection and associated airway inflammation³. Staphylococcus aureus, a Gram-positive bacterium, is the most common pathogen that colonizes the CF lung in the first few years of life (Fig 2), however Pseudomonas aeruginosa is considered the most important pathogen in progressive, severe CF lung disease⁴. P. aeruginosa is a Gram-negative opportunistic pathogen and chronic colonization occurs in 60% of CF patients by 18 years of age (see Fig 2). These patients can also be co-infected by other pathogens such as Burkholderia cenocepacia and Stenotrophomonas maltophilia.

One of the most problematic of CF pathogens is B. cenocepacia, which accounts for between 50% and 90% of all Burkholderia cepacia complex (Bcc) infection diagnoses in CF patients⁵. B. cenocepacia is a Gram-negative opportunistic pathogen which presents by low responsiveness to antibiotic therapy and the ability to cause significant reductions in lung function, additionally it includes strains exhibiting a high virulence potential as well as strains with a high patient to patient transmission⁵.

S. maltophilia is a Gram-negative bacterium that occurs ubiquitously with P. aeruginosa in the environment and both can co-infect the CF lung. The treatment of S. maltophilia infections is problematic as isolates are also resistant to many clinically useful antibiotics⁶.

Communication in Pseudomonas, Burkholderia and Stenotrophomonas

The signalling network of P. aeruginosa consists of multiple interconnected signalling layers that co-ordinately regulate virulence and persistence of infections. The master virulence signalling systems in P. aeruginosa are the N-acyl homoserine lactone (AHL) responsive systems Las and Rhl, which together control the expression of multiple virulence factors in response to cell density. Another group of signalling molecules related to virulence factor production are the quinolones, including Pseudomonas Quinolone Signal 2-heptyl-3-hydroxy-4(1H)-quinolone (PQS), a quorum-sensing (QS) signal that regulates numerous virulence genes and genes involved in iron scavenging⁷.

In addition to producing AHL signal, B. cenocepacia produces a diverse set of 2-alkyl-4(1H)-quinolones (AHQs), including the PQS precursor 2-heptyl-4(1H)-quinolone (HHQ), but does not synthesize 2-heptyl-3-hydroxy-4(1H)-quinolone⁸. B. cenocepacia also produces cis-2-dodecenoic acid, a molecule structurally related to the diffusible signal factor (DSF; 11-methyl-cis-2-dodecenoic acid) found in Xanthomonads⁵. S. maltophilia also possesses a DSF-mediated cell–cell signalling system that was first identified in the related plant pathogen Xanthomonas campestris pv. campestris⁶.

Bacterial Chatter Matters

There is an increasing appreciation of the polymicrobial nature of many bacterial infections, such as those associated with CF, and of the potentially important role that interspecies interactions have in influencing both bacterial virulence and response to therapy. Patients with CF are often co-infected with P. aeruginosa and other pathogens such as B. cenocepacia and S. maltophilia.

P. aeruginosa responds to DSF produced by S. maltophilia and BDSF produced by B. cenocepacia leading to changes in biofilm architecture and increased resistance to cationic antimicrobial peptides⁹. Such interspecies interactions occur in the CF lung where they may influence the efficacy of antibiotic treatment for chronic P. aeruginosa infections⁹.