Microbial Growth and Control: Cell Division and Peptidoglycan Biosynthesis

Microbial Growth and Control

Bacterial Cell Division

Bacterial cell division is a fundamental process that leads to an increase in the number of cells within a population. The primary mechanism for this process is binary fission, which ensures that each daughter cell receives the necessary cellular components to function independently.

• Growth: Defined as an increase in the number of cells, not cell size.

• Binary fission: A type of cell division where a cell enlarges to twice its minimum size and then divides into two daughter cells.

• Generation time: The time required for a microbial population to double in number.

• During cell division, each daughter cell receives a complete chromosome and sufficient copies of all other cell constituents to exist as an independent cell.

Peptidoglycan Biosynthesis

The synthesis of new cell wall material, particularly peptidoglycan, is a major feature of bacterial cell division. This process is essential for maintaining cell shape and integrity during growth and division.

• In cocci (spherical bacteria), cell walls grow outward from the FtsZ ring at the cell center.

• In rod-shaped cells, growth occurs at several points along the cell's length.

• Existing peptidoglycan must be severed to allow insertion of newly synthesized material.

• Autolysins create small openings in the cell wall, and new cell wall material is added across these openings.

Key Steps in Peptidoglycan Biosynthesis

• Bactoprenol: A hydrophobic carrier molecule (a C55 isoprenoid alcohol) that plays a major role in the insertion of peptidoglycan precursors into the growing cell wall. It binds to N-acetylglucosamine/N-acetylmuramic acid/pentapeptide peptidoglycan precursors and transports them across the cytoplasmic membrane.

• Transglycosylases: Enzymes that interact with bactoprenol to insert cell wall precursors into growing points of the cell wall and catalyze glycosidic bond formation.

• Transpeptidation: The final step in cell wall synthesis, forming peptide cross-links between muramic acid residues in adjacent glycan chains. This step is inhibited by the antibiotic penicillin.

Enzyme/Component	Function
Autolysin	Creates openings in the existing cell wall for new material insertion
Bactoprenol	Transports peptidoglycan precursors across the cytoplasmic membrane
Transglycosylase	Catalyzes glycosidic bond formation between sugar residues
Transpeptidase	Forms peptide cross-links between glycan chains (target of penicillin)

Example: Penicillin Action

• Penicillin inhibits the transpeptidase enzyme, preventing the formation of cross-links in peptidoglycan and leading to cell lysis.

Relevant Chemical Reaction (Transpeptidation)

The transpeptidation reaction can be summarized as:

$\text{Peptidoglycan-}D\text{-Ala} + \text{Peptidoglycan-Lys/meso-DAP} \rightarrow \text{Cross-linked peptidoglycan} + D\text{-Ala}$

Additional info: The FtsZ ring mentioned in the notes is a cytoskeletal protein structure that guides the site of cell division in bacteria. The process of peptidoglycan biosynthesis is a major target for antibiotics due to its essential role in bacterial viability and absence in eukaryotic cells.