A 28-year evolution experiment on Burkholderia pseudomallei survival in nutrient-depleted sterile water
Environmental persistence allows opportunistic pathogens to survive a range of harsh conditions, increasing the likelihood of eventual infection. Burkholderia pseudomallei, the causative agent of melioidosis, can endure long-term nutrient-depletion in the environment, but its adaptive mechanisms remain poorly understood. Here, we investigated the evolutionary trajectory of a clinical B. pseudomallei strain maintained in sterile water since 1994. The strain was inoculated into nine individual tubes at nine initial concentrations (102-1010 CFU/mL) and has remained viable to date. Liquid chromatography-mass spectrometry analysis of the water identified potential carbon sources, including phthalic acid -- a plastic degradation product likely leached from inoculation tubes -- which the strain can metabolise via an intact catabolic operon. Genomic variations accumulated between 1994 and 2022 were characterised using both single-colony and plate-sweep sequencing which provided complementary insights. Across all tubes, we identified 249 single-nucleotide polymorphisms (SNPs), 73 indels, and a large-scale deletion. Cultures from each tube displayed a consistently low mutation rate (3.18 x 10- SNPs per site per year), suggesting that cells entered a dormant or slow-growth state. Of 393 genes with mutations, 193 were independently mutated in more than one tube, particularly those involved in signal transduction, cell wall and membrane biogenesis, and secondary metabolite synthesis. These patterns indicate parallel adaptation to long-term nutrient deprivation through modulation of cell-density-related functions and loss of metabolically costly pathways. Remarkably, B. pseudomallei from this experiment remain viable after nearly three decades, providing a rare natural model for understanding how environmental bacteria endure and adapt in extremely nutrient-depleted conditions.