Burkholderia cenocepacia physiology and molecular adaptations to the acidic pH of the CF nutritional environment

Abstract

Burkholderia cenocepacia is an opportunistic pathogen associated with increased disease severity and mortality in cystic fibrosis (CF) patients. We have previously shown that elevated iron and acidic pH in the CF nutritional environment increases B. cenocepacia growth rate and decreases its susceptibility to some of the antimicrobials used clinically to treat CF infections. Here, we aimed to characterize B. cenocepacia physiology and its molecular response under acidic pH and increased zinc and iron concentrations using a modified synthetic CF sputum media (SCFM-FeZn). By investigating B. cenocepacia internal pH homeostasis, we found that it maintains a neutral internal pH when exposed to mildly acidic media at pH 5.5. We also assessed the effect of B. cenocepacia growth on the pH of SCFM-FeZn. When cultured at pH 6.8, B. cenocepacia maintained a media pH of ∼6.5. In contrast, when the culture pH value was initially 5.5, it increased to 6.5 during growth. Using comparative transcriptomics and metabolomics analysis, we identified 990 differentially expressed genes, and 23 differentially abundant metabolites in supernatants at acidic compared to neutral pH. Some of these genes and metabolites were involved in aromatic amino acid metabolism including the upregulated trpE gene, encoding a tryptophan biosynthetic enzyme. A tryptophan auxotrophic trpE deletion strain grew slower in SCFM-FeZn. Overall, this work identifies mechanisms involved in B. cenocepacia adaptation to acidic pH under conditions to model the CF nutritional environment. Some of these mechanisms are also associated with pathogenicity and virulence. Importance Pathogenic bacteria can be exposed to acidic pH inside and outside the host. Their ability to adapt to pH fluctuations contributes to success in host colonization. B. cenocepacia can grow at acidic pH (∼3.5) and has been recovered from intracellular acidic compartments of amoebas and macrophages. Adaptation to acidic pH depends on molecular mechanisms that maintain a near optimal pH inside the cell for the function of vital processes. A few mechanisms that contribute to its adaptation to acidic pH have been described, but not in conditions reflecting the CF nutritional environment. Here, we identified multiple differentially-regulated systems that are associated with bacterial susceptibility to antimicrobials and pathogenesis. This research provides a better understanding of the role of acidic pH on B. cenocepacia physiology in the CF nutritional context and highlights possible systems that should be further characterized.