Bacterial cells have protective cell walls, called peptidoglycans, a mesh made up of amino acids and sugar molecules. The peptidoglycan serves as an armour to the bacteria protecting them from unfavourable environmental conditions. It is also a target of several clinically used antibiotics. In the era of rising antibiotic resistance in bacteria, it is even more important for us to understand its structure and synthesis.

Manjula Reddy, Chief Scientist, CSIR-Centre for Cellular and Molecular Biology (CCMB) studies how bacterial cell walls grow and divide using a well-studied model bacterium, Escherichia coli (E. coli). This is a Gram-negative bacterium, in which the cell envelopes are made up of peptidoglycan layer sandwiched between two lipid-rich membranes. A new study from her lab published in the Proceedings of National Academy of Sciences (PNAS) details how these multiple layers of the cell envelope grow together to make the bacteria larger in size. 

When E. coli grows in size to divide further, their cell envelopes also need to grow to accommodate the additional cell material. Else, the cells burst and die. Earlier, the lab has discovered several enzymes that cut the peptidoglycan mesh to make space so that the new material can be stitched into it to make it larger in size. MepS is one such important peptidoglycan cutting enzyme. However, constant cutting of peptidoglycan by MepS is lethal. Hence, E. coli controls the level of MepS enzyme by a protein degradation system, NlpI-Prc. 

“We observed that when cells are rapidly growing in log phase, peptidoglycan synthesis and MepS levels are high. Similarly, when cells are in stationary phase, there is not as much need to produce peptidoglycans, and MepS is lower. This differential regulation of MepS is done by Nlpl-Prc. But it is not known how the status of the cell cycle is signalled to the Nlpl-Prc, and this is what we set out to discover,” said Nilanjan Som, the first author of the study.

Through genetic screening and biochemical assays, Som and Reddy tested several factors for their potential role in signalling NlpI-Prc to lower the MepS enzyme levels in stationary phase cells. They narrowed down to the genes involved in fatty acid biosynthesis, and finally showed that fatty acid availability controls MepS levels. 

Reddy, the lead investigator of the study, commented, ​“We know that fatty acid biosynthesis leads to phospholipid synthesis in cells. The phospholipids make the lipid-rich membranes that sandwich the peptidoglycan layer. Therefore, it makes sense to think that phospholipids control both, lipid membrane synthesis and MepS levels, and thus peptidoglycan synthesis.”

A variety of phospholipids are found in bacterial cell membranes, some more than others. ​“Although, now we know that phospholipid abundance controls the MepS levels, we still have to figure out the molecular identity of the signalling molecule, and how it signals NlpI-Prc system to control MepS,” said Reddy. 

Thomas Bernhardt, Professor at Harvard Medical School who studies bacterial physiology and is not associated with the study, commented on the findings. He said, ​“We still understand very little about how bacteria coordinate the biogenesis of their lipid membranes with assembly of the cell wall. This paper presents compelling evidence that the stability of enzymes that cut bonds in the cell wall to promote its expansion in E. coli is connected to phospholipid synthesis via an elegant mechanism involving the sensing of signals related to the growth of the outer membrane. These findings give us new insight into how Gram-negative bacteria like E. coli grow, and the findings suggest that interfering with the growth signals may be a new way to block bacterial growth for antibiotic development.”