Evolution in long-term stationary phase batch culture: Emergence of divergent Escherichia coli lineages over 1200 days

In natural environments, bacteria survive conditions of starvation and stress. Long-term batch cultures are an excellent laboratory system to study adaptation during nutrient stress because cells can incubate for months to years without the addition of nutrients. During long-term batch culture cells adapt to acquire energy from cellular detritus, creating a complex and dynamic environment for mutants of increased relative fitness to exploit. Here, we analyzed the genomes of 1117 clones isolated from a single long-term batch culture incubated for 1200 days. A total of 679 mutations include single nucleotide polymorphisms, indels, mobile genetic element movement, large deletions up to 64kbp and amplifications up to ~500kbp. During the 3.3-year incubation, two main lineages diverge evolving continuously. At least two twice, a previously fixed mutation reverted back to the wild type allele, suggesting beneficial mutations may later become maladaptive due to the dynamic environment and changing selective pressures. Most of the mutated genes encode proteins involved in metabolism, transport, or transcriptional regulation. Clones from the two lineages are physiologically distinct based on outgrowth in fresh medium and competition against the parental strain. Similar population dynamics and mutations in hfq, rpoS, paaX, lrp, sdhB, and dtpA were detected in three additional parallel populations sequenced through Day 60, providing evidence for positive selection. These data provide new insight into the population structure and mutations that may be beneficial during periods of starvation in evolving bacterial communities.