Escherichia coli cell envelope growth by spatiotemporal coordination of peptidoglycan synthesis and degradation

The cell envelope serves as a protective boundary that encases the cell, providing not only structural integrity to maintain cell shape but also functioning as a selective barrier. Gram-negative bacteria, such as Escherichia coli, exhibit a more intricate cell envelope compared to Gram-positive bacteria. This complexity forms a formidable barrier, impeding the entry of drugs into cells and imparting innate resistance to certain antibiotics. The consensus is that the mechanical properties of the gram-negative bacterial envelope are primarily attributed to the peptidoglycan layer. However, recent studies are reshaping our understanding of the bacterial envelope. The outer membrane significantly influences the overall stiffness and mechanical properties of the bacterial envelope. In addition, the intricate regulation between peptidoglycan and the outer membrane has been preliminarily unveiled. Therefore, It is important not only to comprehend the regulation of peptidoglycan metabolism but also to understand the intricate regulation between the outer membrane and peptidoglycan.
In chapter 2, we talked about the regulation of NlpI-Prc proteolytic complex to peptidoglycan synthesis and degradation. The NlpI-Prc proteolytic complex was proved to be involved in both peptidoglycan synthesis and degradation, appearing to be playing a mediating role between these two processes. In addition, new substrates (PBP7 and MltD) of NlpI-Prc complex were proved.
In chapter 3, PBP7 was proven can affect the timing of divisome assembly. The absence of PBP7 slowed down the assembly of FtsN in the divisome. By a few different methods, we speculate that the slower assembly speed of FtsN in the divisome is caused by weaker transpeptidase activity in the pbpG mutant.
In chapter 4, the sensitivity of wild-type and mutants to D-met was analyzed. We discovered that PBP1B exhibits the capability to incorporate D-met into the peptidoglycan layer. By summarizing the common changes between the mutant strains and the wild-type strain, along with the variations observed in the wild-type strain under different growth conditions, we found that the outer membrane may be the cause of the cellular changes to D-met sensitivity. Based on the speculations, we unveil the potential regulatory mechanisms between peptidoglycan and the outer membrane in Escherichia coli.