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Aguilar Peña, Claudio Andrés
(2003).
DOI: https://doi.org/10.21954/ou.ro.0000f710
Abstract
Bacterial strains belonging to Burkholderia cepacia can be human opportunistic pathogens, plant growth promoting and have remarkable catabolic activity. Recently, B. cepacia has been thus far subdivided into several Genomovars comprising what is now known as the B. cepacia complex. In this thesis, the quorum sensing system of a rot onion Genomovar I type strain, ATCC 25416 is described. Quorum sensing is a cell-density dependent regulatory response, which involves the production of A-acyl homoserine lactones signal molecules (HSLs). The cep locus of B. cepacia ATCC 25416 coding for LuxI family CepI and LuxR family CepR proteins has been identified and characterised. The two genes have been inactivated in the chromosome and shown that CepI is responsible for the biosynthesis of a C6-HSL and a C8-HSL and that the cep locus regulates protease production as well as onion pathogenicity. A cep-lacZ based sensor plasmid has been constructed and used to demonstrate that CepR was specific for C8-HSL and not for C6-HSL, that a cepR knock out mutant synthesised 70 % less HSLs and that CepR had a higher specificity towards long chain HSLs. With the aid of this sensor plasmid, a novel technique aimed to identify quorum sensing-controlled (QSC) genes in B. cepacia is described.
In addition, the cloning and characterization of the stationary phase sigma factor gene rpoS of B. cepacia ATCC 25416 is also described. This RpoS was found to be 74 % identical to the RpoS of Ralstonia solanacearum but rather distant from other RpoS of gram-negative y-Proteobacteria. It was established that quorum sensing in B. cepacia has a negative effect on rpoS expression as determined using an rpoS-lacZ transcriptional fusion; on the other hand, rpoS-null mutants displayed no difference in the accumulation of HSL signal molecules.
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