Tesis doctoral de Alvaro Hernandez Fernandez
Stenotrophomonas maltophilia is an opportunistic pathogen of growing relevance due to its increasing prevalence in the nosocomial environment. The capability of this organism to colonize different medical devices, such as catheters or breathing tubes, is responsible for the distressing probabilities of acquiring infections caused by this bacterium at hospitals. A typical feature of s. Maltophilia is its broad antibiotic resistant phenotype (what makes difficult the treatment of the infected patients). This resistance is inherent to the species and has not been acquired by horizontal gene transfer or by mutations. Our data from the first part of this work, focused in the s. maltophilia d457 genome sequencing, suggest that the different strains of this microorganism share a large core genome but the elements that allow the bacteria to occupy several habitats are codified in a small accessory genome. we describe that the s. Maltophilia chromosome encodes a qnr protein, smqnr, that confers resistance to quinolones when is expressed in the heterologous host escherichia coli. A bioinformatic search for qnr genes in bacterial genomes and environmental metagenomes has pointed to the sea as the reservoir for the plasmid encoded qnrb genes that are causing problems in the clinical environment. we also report the mechanism that regulates the expression of the most important multidrug efflux pump in this bacterial species, smedef, which is responsible for the expulsion of tetracycline, chloramphenicol, erythromycin and quinolones. strikingly our results show that none of these antibiotics is capable to induce the expression of the pump. On the contrary, the molecules that act as pump inducers are biocides and secondary metabolites from plants. Based on these results, we propose a model that explains why the susceptibility of s. Maltophilia to quinolones decreases in presence of triclosan. We also demonstrate that pump expression is triggered by the binding of such molecules to the transcriptional repressor of this system, the protein smet. We have published the structure of smet and defined the dna operator that this protein recognizes in the s. Maltophilia chromosome. The structure of smet reveals a dimeric protein that folds forming 9 helices, the first 3 helices at the n-terminal are responsible for the dna binding, the 2 helices at the cterminal are the most important for protein dimerization and the central helices conform an efector binding pocket. The protein binds specifically to a 28 bp pseudopalindromic sequence, located in the dna region where the promoters of the smedef and smet overlap, and in this way represses simultaneously the transcription of smedef and smet. We have suggested a stoichiometry ratio for the complex smet-dna (4:1) and also determined that the sequence tgtatgt, in the strand that codifies the pump, is critical for the binding of the protein. the last part of the work has raised the probability that commonly used biocides might select for antibiotic resistance in s. Maltophilia. Our data show that this risk exists in the case of triclosan and benzalkonium chloride, but not for the mutants selected by hexachlorophene.
Datos académicos de la tesis doctoral «Elementos de resistencia intrínseca a antibiórticos en stenotrophomonas maltophilia«
- Título de la tesis: Elementos de resistencia intrínseca a antibiórticos en stenotrophomonas maltophilia
- Autor: Alvaro Hernandez Fernandez
- Universidad: Autónoma de Madrid
- Fecha de lectura de la tesis: 14/06/2010
Dirección y tribunal
- Director de la tesis
- José Luís Martínez Menéndez
- Tribunal
- Presidente del tribunal: fernando Baquero mochales
- José Antonio Ainsa claver (vocal)
- gracia Morales kucharski (vocal)
- renata Moreno albiger (vocal)