Microbial biofilms and c-di-GMP signalling
Work done in the last two decades has shown that bacteria in natural, industrial and clinical settings most often live in sessile structured microbial communities referred to as biofilms. Biofilms formed by opportunistic pathogenic bacteria are involved in devastating medical device-associated infections, and problematic infections in individuals who are immuno-compromised or otherwise impaired in the host defence such as in the case of cystic fibrosis and non-healing wounds. One of the most important characteristics of microbial biofilms is that the bacteria in these sessile communities display a remarkable increased tolerance to antimicrobial attack. Because the use of conventional antimicrobial compounds in many cases cannot eradicate biofilms, there is an urgent need to develop alternative measures to combat biofilm infections.
Evidence is accruing that the second messenger cyclic diguanosine monophosphate (c‑di‑GMP) regulates bacterial phenotypes associated with the biofilm lifestyle. The level of c‑di‑GMP in bacterial cells has been shown to be controlled by two classes of proteins with opposing enzymatic activities. The GGDEF domain proteins are diguanylate cyclases that synthesize c‑di‑GMP from GTP, whereas the EAL domain proteins are phosphodiesterases that degrade c‑di‑GMP. High intracellular c-di-GMP levels promote adhesin/extracellular matrix production and biofilm formation, whereas, low intracellular c-di-GMP levels suppress adhesin/matrix production and promote single cell motility.
Inhibition of c‑di‑GMP production or stimulation of c‑di‑GMP degradation are interesting targets for the development of novel anti-biofilm drugs. To identify such anti-biofilm drug candidates we have constructed a plasmid-based reporter that can function as an indicator of the intracellular c‑di‑GMP level. As a proof of concept, a P. auruginosa mutant that overproduces c‑di‑GMP showed a high level of fluorescence when it was harbouring the reporter plasmid in comparison to the corresponding wild-type harbouring the reporter plasmid. Moreover, treatment of the c‑di‑GMP overproducer with sodium nitroprusside (that has been shown to increase the turnover of c‑di‑GMP in P. aeruginosa through phosphodiesterase activation) resulted in a significant decrease in the fluorescent level. These results indicate that the monitor can be used to estimate the level of c‑di‑GMP in P. aeruginosa and should facilitate screening for novel anti-biofilm drug candidates.
Saarland University, Campus, Bldg. B2.1
Tim Tolker-Nielsen, University of Copenhagen
Rolf Hartmann (HIPS-DDOP)