Biological Role of Type IV secretion systems in Burkholderia cenocepacia

Abstract

ABSTRACT : Burkholderia cenocepacia is an opportunistic pathogen in patients with cystic fibrosis, causing infections with difficult treatment due to its high virulence and its multiple antibiotic resistance. Over the years, various mechanisms have been described which help to understand how B. cenocepacia establishes an appropriate infection environment. Bacterial secretion systems, machineries allowing effector delivery to eukaryotic host cells and DNA exchange, are highly related to pathogenesis. In B. cenocepacia, various secretion systems have been described, grouped into different types according to their structure and function: one Type II, one Type III, two Type IV, and one Type VI Secretion System. There are contradictory reports about their biological role and contribution to virulence. B. cenocepacia presents two Type IV Secretion Systems, one encoded on chromosome II (VirB/D4 T4SS) and the other encoded in a plasmid (Ptw pT4SS). Previous work has suggested a DNA transfer function for VirB/D4 T4SS. pT4SS was named Ptw according to its involvement in the Plant tissue watersoaking phenotype. Moreover, it was proposed to translocate effectors to the host and to contribute to intracellular survival. However, other experiments have shown that there is no difference in intracellular bacterial survival in strains with pT4SS mutations. In addition, previous assays carried out in our laboratory, have demonstrated a DNA transfer function for ptw genes. Therefore, in the present work we establish an analytical approach in order to study the biological role of T4SSs of B. cenocepacia. Strains with deletions in various secretion systems were assessed in onion watersoaking assays and bacterial killing assays in order to compare the corresponding phenotypes. In contrast to what was previously reported, no significant differences were found in pT4SS mutant and wild type strain, both of them causing watersoaking phenotype. This clear-cut result confirms that the pT4SS does not contribute to the watersoaking phenotype. Interestingly, in bacterial killing assays, an increase in survival of the prey (E. coli) was observed in ΔT4SS mutants compared with wild type strains. This phenotype was also observed in T6SS mutants, suggesting that both secretion systems are coordinated for bacterial killing. Experiments addressing the expression of both sets of genes in the mutant strains, as well as experiments to test intracellular infections and DNA transfer to eukaryotic cells, could not be performed due to the lockdown; the proposed experiments are detailed, and expected outcomes are discussed.