Pseudomonas aeruginosa is a versatile bacterium that can inhabit many environments. In its natural environments, Pseudomonas predominantly grows in organized communities called biofilms. Growth as a biofilm is also an important requirement for the colonization of human tissues. For instance, P. aeruginosa grows as a biofilm in the lungs of cystic fibrosis (CF) patients. Previously, it has been shown that nfxC-phenotypic isolates of P. aeruginosa associated with decreased early attachment - the first phase of biofilm formation. This chloramphenicol resistant nfxC-phenotype is thought to be caused by the activation of a LysR-type transcriptional regulator, MexT, which regulates the expression of the MexEF-oprN efflux-system. Recently, we have shown that this biofilm phenotype is a result of the activation of MexT in a MexEF-oprN independent manner (Tian et al., 2009). To further establish the role of MexT in biofilm formation, preliminary flow-cell biofilm experiments have been performed. Analysis of the biofilm structure showed that biofilms of cells overexpressing MexT contain a higher microcolony-density with smaller average size as compared to control cells. These differences were independent of the MexEF-oprN efflux system. This project aims to dissect how MexT modulates biofilm formation. Several potential candidate genes have been identified using a transcriptome approach of cells overexpressing MexT, which will be subject to further evaluation (Tian et al., 2009). To further establish the role of MexT on virulence phenotypes, eukaryotic cell lines, as well as a ‘state of the art’ infection model that uses zebrafish embryos will be used. This study may identify potential targets for the modulation of biofilm formation. In the long term, these targets may provide new strategies to develop drugs that reduce the ability of Pseudomonas to form biofilms, thereby increasing the chance of successful antibiotic treatment therapies.