Acute versus persistent infection by Burkholderia cepacia complex bacteria in cystic fibrosis patients: underlying mechanisms, in particular the role of exopolysaccharide biosynthesis

Principal Investigator: Isabel Sá-Correia

Contract: PTDC/SAU-MII/69591/2006

Start date: 01/10/2007

Duration: 36 months

 

The Burkholderia cepacia complex (Bcc) is a heterogenous group of nine closely related bacterial species recognized as relevant opportunistic pathogens, particularly in patients with cystic fibrosis (CF), the most common life-threatening autosomal recessive disease. Among other clinical manifestations, CF predisposes to recurrent and chronic respiratory infections, leading to progressive airway obstruction, long-term tissue damage and early death. Although Pseudomonas aeruginosa remains the most prevalent CF pathogen, an increase in morbidity and mortality rates is registered in Bcc infected patients. The clinical outcome of CF patients following colonization with Bcc bacteria is highly variable and, so far, unpredictable. Some patients experience asymptomatic carriage, others develop a chronic infection that can last for years, leading to a progressive loss of lung function. However, in approximately 30% of the infected patients, the bacteria can cause the “cepacia syndrome”, a fatal necrotizing pneumonia frequently accompanied by septicemia. The systemic infections caused by some Bcc isolates suggest that the bacterium is not only capable to adhere to host epithelial cells but is able to penetrate and gain access to deeper tissues. However, the determinants that mediate the entry process, at the host or pathogen levels, are not known. In this project we expect to contribute to the understanding of the mechanisms utilized by the Bcc to cause acute versus chronic infections under experimental conditions mimicking those present in the CF lung. We intend to investigate the molecular mechanisms underlying the initiation of chronic versus acute infections and to elucidate adaptive strategies used by these bacteria when, in chronic latestage CF lung disease, they are exposed to an anaerobic, acidic and nutrient depleted environment leading to slower growth and anaerobic metabolism. We want to establish a relationship between the production of the exopolysaccharide (EPS) cepacian and anaerobiosis and the involvement of cepacian biosynthesis in anaerobic growth in biofilms or as planktonic cells and in bacterial protection from all sorts of aggressions in the CF lung, in particular from nitrosative stress. A large collection of isolates known to have been involved in acute or chronic infections, and previously characterized by genotyping at the species and strain levels, is available for these studies. Post-genomic approaches based on the screening of transposon insertion mutants, expression proteomics and in silico analysis of the distribution of close DNA repeat sequences across the genome, will be exploited. Studies will be complemented by the detailed analysis of gene sequence, gene transcription and enzyme activities in different clinical isolates and in cells grown in anaerobic biofilms or in suspension and exposed to nitrosative stress. Studies will also involve the comparison of virulence of selected mutants using the Caenorhabditis elegans model of infection. Bacteria of the Bcc remain highly problematic CF pathogens as the adaptive strategies employed by these bacteria to adapt to the CF environment by adopting a chronic, biofilm growth strategy, or, alternatively, by growing planktonically, spreading rapidly in the host, are still not understood. This project will contribute to the understanding of these crucial questions in Bcc pathogenesis in CF.