Growth Condition Dependent Differences in Methylation Implies Transiently Differentiated DNA Methylation States in E. coli

DNA methylation in bacteria frequently serves as a simple immune system, allowing recognition of DNA from foreign sources, such as phages or selfish genetic elements. It is not well established whether methylation also frequently serves a more general epigenetic function, modifying bacterial phenotypes in a heritable manner. To address this question, here we use Oxford Nanopore sequencing to profile DNA modification marks in three natural isolates of E. coli. We first identify the DNA sequence motifs targeted by the methyltransferases in each strain. We then quantify the frequency of methylation at each of these motifs across the genome in different growth conditions. We find that motifs in specific regions of the genome consistently exhibit high or low levels of methylation. Furthermore, we show that there are replicable and consistent differences in methylated regions across different growth conditions. This suggests that during growth, E. coli transiently differentiates into distinct methylation states that depend on the growth state, raising the possibility that measuring DNA methylation alone can be used to infer bacterial growth states without additional information such as transcriptome or proteome data. These results provide new insights into the dynamics of methylation during bacterial growth, and provide evidence of differentiated cell states, a transient analogue to what is observed in the differentiation of cell types in multicellular organisms.