Bacterial Cell Structure

Cell envelop

Encloses the cytoplasm and internal structures of the cell

Bacterial cell envelop includes:

Plasma membrane, cell wall, periplasmic space, glycocalyx

Glycocalyx

polysaccharide outer layer outside the bacterial cell wall that aids in protection, adhesion, and virulence

Components of animal plasma membrane

1. phospholipids bilayer

2. cholesterol

3. proteins

4. carbohydrate

Bacterial plasma membrane

• retains the cytoplasm and separates the cell from its enviroment

• selectively permeable barrier

• contains transport systems used for nutrient uptake, waste excretion, and protein secretion

• location of a variety of crucial metabolic processes

• contains special receptor molecules

Bacterial membrane structure

lipid bilayer in which proteins floats

Asymmetry of most membrane lipids

Cell wall

maintains shape, helps protect cell from osmotic lysis and toxic materials, may contribute to pathogenicity

Gram positive cell walls

Composed primarily of a thick layer of peptidoglycan

Gram negative cell walls

Composed primarily of a thick layer of peptidoglycan, outer membrane composed of lipids, lipoproteins, and LPS

Peptidoglycan

mesh-like polymer that forms the bacterial cell wall, composed of alternating sugar derivatives (NN-acetylglucosamine [NAG] and NN-acetylmuramic acid [NAM]) cross-linked by short peptides

LPS (lipopolysaccharide)

• covers almost the entire outer surface of the gram-negative outer membrane

• acts as protective barrier, potent endotoxin, immune recognition molecule

• LPS causes severe inflammation and sepsis by stimulating Toll-like receptor 4 (TLR4)

LPS consists of ?

1. lipid A

2. core polysaccharide

3. O side chain (O antigen)

Lipid A

• endotoxic component of lipopolysaccharide (LPS) that anchors this molecule within the outer membrane of Gram-negative bacteria

• it serves as the primary activator of the host immune system (via TLR4) during infection, with its structure (acyl chains/phosphate groups) determining its toxicity

• recognize by TLR4/MD2 receptors

Core polysaccharide

• acts as a structural bridge,

• covalently linking the hydrophobic Lipid A (embedded in the membrane) to the outermost O-antigen

• the core is divided into an inner part containing KDO (3-deoxy-D-manno-octulosonic acid) and heptose, and an outer part with hexoses, often phosphorylated to stabilize the membrane

O antigen (O-polysaccharide)

• outermost layer

• consisting of repeating sugar units

• it acts as a primary barrier against host immune defenses (complement-mediated killing, phagocytosis) and serves as a major target for antibody recognition

• determines serotype

LPS triggers inflamation

Slime layer

loose, unorganized, and easily removed layer of extracellular material (mainly polysaccharides, glycoproteins, and glycolipids) that surrounds some bacterial cells. It functions in bacterial adherence to surfaces, protection against dehydration, entrapment of nutrients, and evasion of immune cell

Biofilm

protective, slimy communities of bacteria (and other microbes) that stick to surfaces, encased in a self-produced glue called Extracellular Polymeric Substances (EPS), allowing them to thrive, resist antibiotics, and cause infections like dental plaque, cystic fibrosis lung infections, or catheter infections

Three types of external structures

fiambre, pilli and flagella

Fiambre

• short, thin, hairlike

• composed of fimbrilin protein

• primary involved in attachment to surfaces

• biofilm formation

• important virulence factor in pathogenis bacteria

Pilli

• hair-like protein appendages

• composed mainly of pilin protein

• used in transfers of DNA to other cells and in cell adhesion

Conjugation

specific type of pilus, called F pilus or sex pilus, is important in the transfer of DNA between bacterial cells

Flagella

• responsible for bacterial mobility

• attached to the cell by a basal body that holds a long rotali

Structure of flagella

Filament: external helical structure

Hook: flexible connector

Basal body: motor embedded in cell envelope

Flagellar arrangement

Chemotaxis

movement towards a chemical attractant or away from a chemical repellent