Chapter Overview • A synopsis of the bacterial cell • How cell parts are studied • The plasma membrane and transport • The cell wall and other outer layers • The nucleoid: structure and expression • How bacterial cells divide • Specialized structures, including pili and stalks • Bacterial flagella and chemotaxis
Introduction • Most bacteria share fundamental traits. • Thick, complex outer envelope • Compact genome • Tightly coordinated cell functions • Archaea, like bacteria, are prokaryotes • Have unique membrane and envelope structures • Eukaryotic cells have a nucleus and extensive membranous organelles
3.1 The Bacterial Cell: An Overview • In the early twentieth century, the cell was envisioned as a bag of “soup” full of floating ribosomes and enzymes. • Modern research shows that the cell’s parts fit together in a structure that is ordered, though flexible.
Model of a Bacterial Cell • Cytoplasm = Consists of a gel-like network • Cell membrane = Encloses the cytoplasm • Cell wall = Covers the cell membrane • Nucleoid = Non-membrane-bound area of the cytoplasm that contains the chromosome in the form of looped coils • Flagellum = External helical filament whose rotary motor propels the cell
Observing Cell Parts • Cell study requires isolation and analysis of cell parts. • Cell fractionation • Cells must be broken up by techniques that allow subcellular parts to remain intact. • Examples of such techniques include: • Mild detergent analysis • Sonication • Enzymes • Mechanical disruption
A key tool of subcellular fractionation is the ultracentrifuge. • The high rotation rate produces centrifugal forces strong enough to separate particles by size. • Parts are then subjected for structural and biochemical analysis. Figure 3.2
An approach that is complementary to cell fractionation is genetic analysis • Different types of strains can be used: • Mutant strains which are selected for loss of a given function • Strains that are intentionally mutated as to lose or alter a gene • Strains that are constructed with “reporter genes” fused to a gene encoding a protein of interest • The phenotype of the mutant cell may yield clues about the function of the altered part
Biochemical Composition of Bacteria • All cells share common chemical components. • Water • Essential ions • Small organic molecules • Macromolecules • Cell composition varies with species, growth phase, and environmental conditions.
3.2 The Cell Membrane and Transport • The structure that defines the existence of a cell is the cell membrane. Figure 3.4
Membrane Lipids • Membranes have approximately equal parts of phospholipids and proteins. • A phospholipid consists of glycerol with ester links to two fatty acids and a phosphoryl head group • May have side chain • The two layers of phospholipids in the bilayer are called leaflets. Figure 3.5
Membrane Proteins • Membrane proteins serve numerous functions, including: • Structural support • Detection of environmental signals • Secretion of virulence factors and communication signals • Ion transport and energy storage • Have hydrophilic and hydrophobic regions that lock the protein in the membrane
Transport across the Cell Membrane • The cell membrane acts as a semipermeable barrier. • Selective transport is essential for survival. • Small uncharged molecules, such as O2 and CO2, easily permeate the membrane by diffusion. • Water tends to diffuse across the membrane in a process called osmosis.
Weak acids and weak bases exist partly in an uncharged form that can diffuse across the membrane and change the pH of the cell. Figure 3.6
Polar molecules and charged molecules require transport through specific protein transporters. • Passive transport = Molecules move alongtheir concentration gradient • Active transport = Molecules move against their concentration gradient • Requires energy Figure 3.7
Membrane Lipid Diversity • Phospholipids vary with respect to their phosphoryl head groups and their fatty acid side chains. • Cardiolipin or diphosphatidylglycerol • A double phospholipid linked by a glycerol • Concentration increases in bacteria grown to starvation • Localizes to the cell poles • Fatty acid chains may be unsaturated • And may also contain cyclic structures
Figure 3.8 Figure 3.9
Membranes also include planar molecules that fill gaps between hydrocarbon chains. • In eukaryotic membranes, the reinforcing agents are sterols, such as cholesterol. • In bacteria, the same function is filled by hopanoids, or hopanes.
Archaea have the most extreme variations in phospholipid side-chain structures. • Ether links between glycerol and fatty acids • Hydrocarbon chains are branched terpenoids.
3.3 The Cell Wall and Outer Layers • How do prokaryotes protect their cell membrane? • For most species, the cell envelope includes at least one structural supporting layer • The most common structural support is the cell wall • Nevertheless, a few prokaryotes, such as the mycoplasmas, have a cell membrane with no outer layers
The Cell Wall Is a Single Molecule • The cell wall confers shape and rigidity to the cell, and helps it withstand turgor pressure. • The bacterial cell wall, or the sacculus, consists of a single interlinked molecule. Figure 3.13
Peptidoglycan Structure • Most bacterial cell walls are made up of peptidoglycan (or murein). • The molecule consists of: - Long polymers of two disaccharides called N-acetylglucosamine and N-acetylmuramic acid, bound to a peptide of 4-6 amino acids • The peptides can form cross-bridges connecting the parallel glycan strands.
Peptidoglycan Structure • Peptidoglycan is unique to bacteria • Thus, the enzymes responsible for its biosynthesis make excellent targets for antibiotics • Penicillin inhibits the transpeptidase that cross-links the peptides • Vancomycin prevents cross-bridge formation by binding to the terminal D-Ala-D-Ala dipeptide • Unfortunately, the widespread use of such antibiotics selects for evolution of resistant strains
Gram-Positive & Gram-Negative Bacteria • Most bacteria have additional envelope layers that provide structural support and protection. • Envelope composition defines: • Gram-positive bacteria (thick cell wall) • Example: The phylum Firmicutes • Gram-negative bacteria (thin cell wall) • Example: The phylum Proteobacteria
Gram-Positive Cell Envelope • Has multiple layers of peptidoglycan • Threaded by teichoic acids • The capsule • Made of polysaccharide and glycoprotein • Protects cells from phagocytosis • Found also in Gram-negative cells Figure 3.16
Gram-Positive Cell Envelope • S-layer • An additional protective layer commonly found in free-living bacteria and archaea • Crystalline layer of thick subunits consisting of protein or glycoprotein • May contribute to cell shape and help protect the cellfrom osmotic stress Figure 3.17
Mycobacterial Cell Envelopes • Mycobacterium tuberculosis and M. leprae have very complex cell envelopes • Include unusual membrane lipids (mycolic acids) and unusual sugars (arabinogalactans) Figure 3.18
The Gram-Negative Outer Membrane • The thin peptidoglycan layer consists of one or two sheets • Covered by an outer membrane, which confers defensive abilities and toxigenic properties on many pathogens • Inward facing leaflet includes lipoprotein • Outward facing leaflet contains: • Lipopolysaccharides • Porins
Eukaryotic Microbes • Eukaryotic microbes possess their own structures to avoid osmotic shock. • Algae form cell walls of cellulose. • Fungi form cell walls of chitin. • Diatoms form exoskeletons of silicate. • Paramecia possess a contractile vacuole to pump water out of the cell
Bacterial Cytoskeleton • Shape-determining proteins • FtsZ = Forms a “Z ring” in spherical cells • MreB = Forms a coil inside rod-shaped cells • CreS “Crescentin” = Forms a polymer along the inner side of crescent-shaped bacteria Figure 3.23
3.4 The Nucleoid, RNA and Protein Synthesis • Eukaryotic cells have a well defined nucleus delimited by a nuclear membrane • In contrast, prokaryotic cells have a nucleoid region that extends throughout the cytoplasm Figure 3.24
DNA Is Organized in the Nucleoid • The E. coli nucleoid appears as clear regions that exclude the ribosome and contain the DNA strands. • The nucleoid forms about 50 loops or domains. Within each domain, the DNA is supercoiled by DNA-binding proteins. Figure 3.25 Figure 3.26
Transcription and Translation • RNA polymerase transcribes DNA into a single strand of RNA. • For most genes, it is messenger RNA. • mRNA immediately binds to a ribosome for translation into a polypeptide. • This is aided by transfer RNA (tRNA), which brings the amino acids to the ribosome. • In prokaryotes, translation is tightly coupled to transcription.
Protein Synthesis and Secretion • In prokaryotes, membrane proteins and secreted proteins are synthesized in association with the cell membrane. • This is aided by the signal recognition particle (SRP), which binds to the growing peptide.
3.5 Cell Division • Cell division, or cell fission, requires highly coordinated growth and expansion of all the cell’s parts. • Unlike eukaryotes, prokaryotes synthesize RNA and proteins continually while the cell’s DNA undergoes replication. • Bacterial DNA replication is coordinated with the cell wall expansion and ultimately the separation of the two daughter cells.
DNA Is Replicated Bidirectionally • In prokaryotes, a circular chromosome begins to replicate at its origin, or ori site. • Two replications forks are generated, which proceed outward in both directions. • At each fork, DNA is synthesized by DNA polymerase with the help of accessory proteins (the replisome). • As the termination site is replicated, the two forks separate from the DNA.
Septation Completes Cell Division • Replication of the termination site triggers growth of the dividing partition, or septum. • The septum grows inward, at last constricting and sealing off the two daughter cells. Figure 3.31
The spatial orientation of septation has a key role in determining the shape and arrangement of cocci. • Parallel planes • Streptococci • Random planes • Staphylococci • Perpendicular planes • Tetrads • Sarcinae Figure 3.32
3.6 Cell Polarity and Aging • Bacterial cell poles differ in their origin and “age” • This phenomenon is called polar aging • In bacteria that appear superficially symmetrical, polar differences may appear at cell division • Bacillus species can undergo an asymmetrical cell division to form an endospore at one end. • Other bacteria expand their cells by extending one pole only
Some bacteria generate two kinds of daughter cells: one stationary and the other mobile. • Example: The flagellum-to-stalk transition of the bacterium Caulobacter crescentus Figure 3.34