Prokaryote Cell Structures and Bacterial Morphology: Key Concepts and Definitions

Chemotaxis

Movement of a cell in response to a chemical stimulus.

Positive Chemotaxis

Movement toward an attractant.

Negative Chemotaxis

Movement away from a repellent.

Periplasmic Flagella

Flagella located between the cell wall and outer membrane of spirochetes; produce corkscrew motion.

Fimbriae

Short hairlike appendages used for attachment to surfaces and host tissues.

Conjugation

Transfer of DNA between bacterial cells through direct contact.

Nanotubes (Nanowires)

Cell-to-cell connections that transfer nutrients, signals, and genetic material.

S-Layer

Protective crystalline protein layer outside the cell wall.

Glycocalyx

External coating made of polysaccharides and/or proteins.

Peptidoglycan

Polymer that provides bacterial cell walls with strength and rigidity.

Plasmids

Small circular DNA molecules separate from the chromosome.

Vegetative Cell

Actively growing bacterial cell.

Sporulation

Process of forming an endospore.

Germination

Process by which an endospore returns to a vegetative cell.

Bacterial Species

Population of cells with similar characteristics.

Strain

Descendants of a single bacterial isolate.

Serotype

Variant distinguished by unique surface antigens.

When did prokaryotes first appear?

Approximately 3.5-3.8 billion years ago.

Where are prokaryotes found?

Nearly every habitat on Earth including soil, water, human bodies, deep oceans, hot springs, ice, and acidic environments.

True or False: Most prokaryotes cause disease.

False.

True or False: Prokaryotes are the most diverse group of cellular microbes.

True.

What two domains contain prokaryotes?

Bacteria and Archaea.

How do prokaryotes differ from eukaryotes?

No nucleus, no membrane-bound organelles, smaller size, 70S ribosomes.

What do all cells possess?

Cell membrane, cytoplasm, DNA, and ribosomes.

Do all bacteria function as independent unicellular organisms?

No; some form colonies, biofilms, and multicellular associations.

Bergey's Manual of Systematic Bacteriology

Reference used to identify and classify prokaryotes.

What does Bergey's Manual rely on?

Morphology, physiology, biochemistry, genetics, and molecular sequencing.

Typical size of a prokaryotic cell

Usually 1-10 µm.

Morphology

Cell shape and appearance.

Monomorphic

Having a single consistent shape.

Pleomorphism

Having variable shapes.

Coccus

Spherical bacterium.

Bacillus

Rod-shaped bacterium.

Vibrio

Curved rod-shaped bacterium.

Spirillum

Rigid spiral-shaped bacterium.

Spirochete

Flexible spiral-shaped bacterium.

Diplococci

Pairs of cocci.

Streptococci

Chains of cocci.

Tetrads

Groups of four cocci.

Sarcinae

Cubic packets of eight cocci.

Staphylococci

Grape-like clusters of cocci.

Diplobacilli

Pairs of bacilli.

Streptobacilli

Chains of bacilli.

Palisades

Bacilli arranged side-by-side.

Cell Envelope

Structure surrounding the cytoplasm; includes membrane, wall, and sometimes outer membrane.

Cytoplasmic Membrane Location

Directly surrounds the cytoplasm.

Cytoplasmic Membrane Structure

Phospholipid bilayer with embedded proteins.

Do bacteria and archaea have the same membrane lipids?

No.

Three primary functions of the cytoplasmic membrane

Selective permeability, transport, and energy generation.

Selectively Permeable

Allows some substances to pass while restricting others.

Functions of the Cell Wall

Maintains shape, provides protection, and prevents osmotic lysis.

What happens when drugs target the cell wall?

The cell may burst from osmotic pressure.

Most common antibiotic target in bacteria

Peptidoglycan.

What gives the bacterial cell wall rigidity?

Peptidoglycan.

What provides an extra barrier in Gram-negative cells?

The outer membrane.

Why are Gram-negative bacteria harder to kill?

The outer membrane blocks many antibiotics and chemicals.

What can dissolve lipids in the outer membrane?

Detergents and alcohols.

Two unique components of the Gram-negative outer membrane

Lipopolysaccharide (LPS) and porins.

Which Gram-negative component contains Lipid A endotoxin?

LPS.

Gram-positive peptidoglycan layer

Thick peptidoglycan layer.

Unique components of Gram-positive bacteria

Teichoic acids and lipoteichoic acids.

Gram-negative peptidoglycan layer

Thin peptidoglycan layer.

How does the Gram-negative wall affect cell integrity?

Less rigid than Gram-positive walls.

Periplasmic Space

Region between the cytoplasmic membrane and outer membrane.

Common acid-fast bacteria

Mycobacterium tuberculosis and Mycobacterium leprae.

Unique lipids of acid-fast bacteria

Mycolic acids.

Stain required for acid-fast bacteria

Acid-fast stain (Ziehl-Neelsen stain).

Do mycoplasmas have a cell wall?

No.

What is present in mycoplasma membranes?

Sterols.

Common mycoplasma species

Mycoplasma pneumoniae.

Disease caused by Mycoplasma pneumoniae

Walking pneumonia.

Do L-forms have a cell wall?

No.

Medical importance of L-forms

May survive antibiotics that target cell walls.

What replaces peptidoglycan in many archaea?

Pseudomurein.

What cell wall materials may archaea possess?

Pseudomurein, proteins, or polysaccharides.

Which domains may contain S-layers?

Archaea and some bacteria.

Functions of S-layers

Protection, structural support, and adhesion.

Capsule

Organized glycocalyx firmly attached to the cell.

Slime Layer

Loose glycocalyx that is easily removed.

Functions of the glycocalyx

Protection from drying, attachment, and protection from phagocytosis.

Fimbriae function

Attachment to surfaces and host tissues.

Fimbriae are common in which bacteria?

Gram-negative bacteria.

Two types of motility involving pili

Twitching and gliding.

How do pili transfer DNA?

Through conjugation.

Pili involved in DNA transfer

Sex pili.

Type IV pili function

Twitching motility and adhesion.

Primary function of flagella

Motility.

Three parts of a flagellum

Filament, hook, and basal body.

Monotrichous flagella

One flagellum at one pole.

Lophotrichous flagella

Tuft of flagella at one pole.

Amphitrichous flagella

Flagella at both poles.

Peritrichous flagella

Flagella distributed over the entire cell surface.

Run

Straight movement of a bacterium.

Tumble

Random reorientation of a bacterium.

How do attractants affect movement?

Increase runs and decrease tumbles.

How do repellents affect movement?

Increase tumbles.

Which bacteria possess periplasmic flagella?

Spirochetes.

How do spirochetes move?

Corkscrew motion.

Archaella

Motility structures of archaea.

How are archaella different from bacterial flagella?

Function like flagella but are structurally more similar to pili.