Microbial Cell Structure, Division, Biofilm Formation, and Antibiotic Targets

Bacterial Cell Division and the Divisome

Divisome Complex and Its Role

The divisome is a protein complex essential for bacterial cell division, ensuring the formation of two identical daughter cells. It forms late in the cell cycle, after DNA replication, and orchestrates the constriction of the cell membrane and synthesis of new peptidoglycan cell wall at the division site.

• FtsZ: Forms the Z-ring at the cell center, uses GTP for energy, and initiates septum formation.

• Min Proteins (MinC, MinD, MinE): Prevent division at incorrect locations, guiding FtsZ to the cell midpoint.

• FtsK: Assists in chromosome separation during division.

The divisome ensures equal partitioning of cellular contents and chromosomes, resulting in two identical daughter cells.

Cytoskeleton and Cell Shape

Bacterial Cytoskeleton Proteins

Bacteria possess cytoskeletal proteins analogous to those in eukaryotes, which are crucial for maintaining cell shape and structure.

• MreB: Similar to actin, forms filaments under the membrane, recruits cell wall synthesis proteins, and determines rod shape. Inactivation leads to cocci shape.

• CrvA: In Vibrio species, adds peptidoglycan to one side, causing cell curvature.

• Crescentin: Similar to keratin, forms filaments on the concave side, creating crescent-shaped cells.

These proteins provide evolutionary evidence for relatedness between domains of life.

Peptidoglycan Biosynthesis and Cell Wall Growth

Mechanisms of Peptidoglycan Synthesis

Peptidoglycan synthesis is vital during cell division and elongation, involving two main complexes:

• Divisome: Produces peptidoglycan during division.

• Elongasome: Produces peptidoglycan during cell growth.

Rod-shaped bacteria grow their wall along the cell length, while cocci expand outward from the FtsZ ring.

Insertion of New Peptidoglycan

1. Break old cell wall: Autolysins degrade existing peptidoglycan, creating space for new material.

2. Transport new building blocks: Bactoprenol carries peptidoglycan precursors (Lipid I, Lipid II) across the membrane. Flippase flips bactoprenol. Bacitracin blocks bactoprenol.

3. Insert new wall: Transglycosylase adds sugar chains, forming glycosidic bonds. Transpeptidation cross-links peptidoglycan, strengthening the wall. Penicillin inhibits transpeptidation.

Endospore Formation and Germination

Endospore Formation

Some Gram-positive bacteria, such as Bacillus and Clostridium, form endospores under stress for survival.

1. Environmental stress activates Spo0A.

2. Spo0A triggers over 500 genes.

3. Asymmetric division produces a small endospore and a larger mother cell.

• Endospore develops a thick coat, dipicolinic acid, and extreme resistance.

Endospore Germination

Endospores are dormant and metabolically inactive, surviving for years. Upon favorable conditions, they undergo:

1. Activation

2. Germination

3. Outgrowth

Result: Return to vegetative cell.

Biofilm Formation and Regulation

Stages of Biofilm Formation

A biofilm is a community of bacteria attached to a surface, such as dental plaque or medical devices. Formation occurs in four stages:

1. Attachment

2. Colonization

3. Development

4. Dispersal

Attachment Mechanisms

• Initial attachment via random collisions, flagella, pili, and cell surface proteins.

• Cells lose flagella and become non-motile after attachment.

Regulation by c-di-GMP

Biofilm formation is regulated by cyclic di-GMP (c-di-GMP), a signaling molecule that controls the switch between motile and biofilm states.

Low c-di-GMP	High c-di-GMP
Motile cells	Biofilm formation

• Reduces flagella activity

• Promotes extracellular matrix production

Biofilm Examples

• Pseudomonas aeruginosa: Forms strong biofilms at high cell density, high antibiotic resistance, releases DNA into matrix.

• Vibrio cholerae: Biofilm formation activated at low cell density, repressed at high density via quorum sensing and HapR repressor.

Antibiotic Targets and Resistance

Major Antibiotic Targets

• Cell membrane

• Cell wall

• DNA replication

• RNA synthesis

• Ribosomes

• DNA gyrase

Examples of Antibiotic Action

• DNA Replication: Quinolones inhibit DNA gyrase.

• Protein Synthesis: Bacterial ribosome (70S) targeted by puromycin and aminoglycosides; eukaryotic ribosome (80S) is not affected.

• Cell Membrane: Daptomycin forms pores; polymyxins target LPS layer.

• Cell Wall: β-lactam antibiotics (penicillin, cephalosporins) block transpeptidation; vancomycin blocks precursor; bacitracin blocks bactoprenol.

Antibiotic Resistance Mechanisms

• Drug target modification

• Drug inactivation (e.g., β-lactamase)

• Efflux pumps (e.g., AcrAB-TolC in E. coli)

• Metabolic bypass (e.g., MRSA with mecA gene)

Persistence, Dormancy, and Toxin–Antitoxin Systems

Persistence and Dormancy

Some bacteria survive antibiotics by entering a dormant state (persister cells), which are genetically identical but temporarily inactive. After treatment, they can resume growth and cause recurring infections.

• Examples: Mycobacterium tuberculosis, Pseudomonas aeruginosa

Toxin–Antitoxin Modules

Bacteria possess genes regulating dormancy through toxin-antitoxin systems:

• Toxin: Stops cell growth

• Antitoxin: Neutralizes toxin

• Example: HipA–HipB system

Stringent Response

Response to Amino Acid Starvation

During amino acid starvation, bacteria activate the stringent response:

1. Ribosome stalls

2. RelA produces alarmone (p)ppGpp

3. Stringent response activates

• Decreases rRNA and tRNA synthesis

• Reduces protein production

• Slows DNA replication

• Stops cell division

Result: Cell enters dormancy and resumes growth once conditions improve.

Summary Table: Bacterial Cytoskeleton Proteins and Functions

Bacteria Protein	Similar Eukaryotic Protein	Function
FtsZ	Tubulin	Cell division
MreB	Actin	Cell shape
Crescentin	Keratin	Intermediate filaments

This suggests evolutionary relatedness between domains of life.

Summary Table: c-di-GMP Regulation of Biofilm Formation

Low c-di-GMP	High c-di-GMP
Motile cells	Biofilm formation

• Reduces flagella activity

• Promotes extracellular matrix production

Key Equations

Transpeptidation Reaction (Peptidoglycan Cross-linking)

The transpeptidation reaction forms peptide cross-links between peptidoglycan chains:

$\text{Peptidoglycan}_n + \text{Peptidoglycan}_m \xrightarrow{\text{Transpeptidase}} \text{Cross-linked Peptidoglycan}$

Stringent Response Alarmone Synthesis

RelA synthesizes (p)ppGpp from GTP and ATP:

$\text{GTP} + \text{ATP} \xrightarrow{\text{RelA}} (p)ppGpp + \text{AMP}$

Additional info:

• Tables and images were recreated and expanded for clarity and completeness.

• Academic context was added to ensure self-contained study notes.