Bacterial Cell Wall Recycling

Abstract Cell wall recycling is a process whereby bacteria degrade their own wall during growth, recover released constituents by active transport and reutilise them either to rebuild the wall or to gain energy. Most knowledge about cell wall recycling comes from studies with the Gram‐negative bacterium Escherichia coli . Within one generation, this organism breaks down and efficiently recycles approximately 60% of the mature peptidoglycan of its side‐wall during cell elongation and approximately 30% of newly deposited septal peptidoglycan during cell division. The reason for the massive turnover of the peptidoglycan is still unclear, although many other bacteria, including Gram‐positives, have been reported to turnover their cell wall and release similar quantities of peptidoglycan fragments during growth and differentiation. Whether these fragments are also recycled is basically unknown. The presence of recycling genes on most bacterial genomes, however, suggests that cell wall recycling is a very common pathway of bacteria. Key Concepts: The peptidoglycan of the bacterial cell wall represents a single, giant, reticular macromolecule (i.e. the murein sacculus) that encases the entire bacterial cell. The peptidoglycan cell wall has to be cleaved continuously during growth to allow cell expansion by insertion of new wall material. Bacteria possess a huge and partially redundant set of cell wall lytic enzymes that potentially target every covalent bond connecting the amino acid and amino sugar building blocks within the peptidoglycan network (cell wall lytic complement). Many bacteria release a great amount of cell wall material during bacterial growth (cell elongation and division). The reason for the massive turnover of approximately 50% of the existing peptidoglycan in one generation is still unclear. Bacteria eventually recover cell wall turnover fragments. The pathways for the continuous recycling of peptidoglycan have been explored in great detail in Escherichia coli . Cell wall recycling in Gram‐positive bacteria has been appreciated just recently and apparently differs from the E. coli paradigm as well as between Gram‐positive species. Cell wall‐derived peptidoglycan fragments function as potent biological effectors. They are involved in sensing the cell wall and growth state, inducing expression of antibiotic resistance genes, and triggering cell differentiation and resuscitation of dormant cells.