MEB Seminar | Dr. Nicole Ratib

Dr. Nicole Ratib
Post-Doc, USC, MEB (PI: Dr. Cameron Thrash)
Research Profile

Genomic and physiological characterization of Escherichia coli evolving in long-term batch culture

Tuesday, March 10, 12 PM, AHF 153 (Torrey Webb Room)

Abstract: In most natural environments, bacteria spend much of their time under conditions of starvation and stress. Long-term batch cultures are an excellent laboratory system to study adaptation during nutrient stress because cells are incubated for relatively long periods of time, 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. To characterize the population dynamics and identify beneficial alleles, we analyzed the genomes of 1117 clones isolated from a single long-term batch culture inoculated with a single clone and incubated for 1200 days. A total of 679 mutations were identified including single nucleotide polymorphisms, indels, movement of mobile genetic elements, large deletions up to 64kbp and amplifications up to ~500kbp. During the 3.3-year incubation, two main lineages diverge and continuously evolve. There is evidence of at least two instances of a fixed mutation reverting back to the wild type allele, suggesting beneficial mutations may later become maladaptive. 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 mutated genes were detected in three 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.