Chapter 4

Characteristics of cells

• All living things are made up of cells that share some common characteristics:

  • Basic shape - spherical, cubical, cylindrical

  • Internal content - cytoplasm, surrounded by a membrane

  • DNA chromosome(s), ribosomes, metabolic capabilities

• Two types:

  • Eukaryotic

  • Prokaryotic

Eukaryotic cells

• Animals, plants, fungi, and protists

• Contain membrane-bound organelles that compartmentalize the cytoplasm and perform specific functions

• Contain double-membrane bound nucleus with DNA chromosomes

Prokaryotic cells

• Bacteria and archaea

• No nucleus or other membrane-bound organelles

Characteristics of life

• Reproduction and heredity

  • Genome composed of DNA packed in chromosomes

  • Produce offspring sexually or asexually

• Growth and development

• Metabolism

  • Chemical and physical life processes

• Movement and/or irritability

  • Respond to internal/external stimuli

  • Self-propulsion of many organisms

• Cell support, protection, and storage mechanisms

  • Cells walls, vacuoles, granules, and inclusions

• Transport of nutrients and waste

A typical prokaryotic cell contains:

• Cell wall

• Cell membrane

• A single circular chromosome that is concentrated in a nucleoid

• 70s ribosomes

• Some may also possess: plasmids, flagella, pili, fimbriae, inclusion bodies, and capsules

External structures of prokaryotes

• Appendages, composed of two major groups:

  • Motility - flagella and axial filaments (periplasmic flagella)

  • Attachment or channels - fimbriae and pili

• Glycocalyx - Surface coating

Structure of flagella

• Filament - long, thin, helical structure composed of protein flagellin

• Hook - curved sheath

• Basal body - stack of rings firmly anchored in cell wall

Flagellar arrangements of prokaryotes

• Monotrichous - single flagellum at one end

• Amphitrichous - flagella at both ends of cell

• Lophotrichous - small bunches emerging form the same site

• Peritrichous - flagella dispersed over surface of cell

Directional movement of bacteria

• Achieved by changing the rotation of their flagella

• In a cell wall with peritrichous flagella:

  • Flagella bundle when they rotate in a counterclockwise direction, resulting in a run

  • Flagella are no longer bundled when they rotate in a clockwise direction, resulting in tumbles

Flagellar responses

• Guide bacteria in a direction in response to external stimulus

  • Chemical stimuli - chemotaxis - positive and negative

  • Light stimuli - phototaxis

• Signal sets flagella into motion clockwise or counterclockwise:

  • Counterclockwise - results in smooth linear direction - run

  • Clockwise - tumbles

Periplasmic flagella

• Internal flagella - enclosed in the space between the outer sheath and the cell wall peptidoglycan

• Produce cellular motility by contracting and imparting twisting or flexing motion

Fimbriae

• Fine, proteinaceous, hair-like bristles emerging from the cell surface

• Function in adhesion to other cells and surfaces

Pili

• Rigid tubular structure made of pilin protein

• Found only in gram-negative cells

• Function to join bacterial cells for partial DNA transfer called conjugation (sex pilus) and attachment

Glycocalyx

• Coating of molecules external to the cell wall, made of sugars and/or proteins

• Two types:

  • Slime layer - loosely organized and attached

  • Capsule - highly organized, tightly attached

• Protects cells from dehydration and nutrient loss

• Inhibit killing by white blood cells by phagocytosis, contributing to pathogenicity

• Attachment - formation of biofilms

Cell envelope

• External covering outside the cytoplasm

• Composed of two basic layers:

  • Cell wall

  • Cell membrane

• Maintains cell integrity

• Two different groups of bacteria demonstrated by the Gram stain:

  • Gram-positive bacteria - thick cell wall composed of primarily peptidoglycan and cell membrane

  • Gram-negative bacteria - outer cell membrane, thin peptidoglycan layer, and cell membrane

Structure of cell walls

• Determines cell shape

• Prevents lysis due to changing osmotic pressures

• Peptidoglycan is the primary component

Peptidoglycan

Unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments

Gram-positive cell wall

• 20-80nm thick peptidoglycan

• One major layer

• Includes teichoic acid and lipoteichoic acid

• Some cells have a periplasmic space, which is a space between the cell membrane and the cell wall

Teichoic and lipoteichoic acids

• Function in cell wall maintenance and enlargement of cell during cell division

• Move cations across the cell envelope

• Stimulate a specific immune response

Gram-negative cell wall

• 8-11nm thick

• Two major layers

• Inner and outer membranes and periplasmic space between them contains a thin peptidoglycan layer

• Outer membrane contains lipopolysaccharides (LPS)

Lipopolysaccharides (LPS)

• Lipid portion (endotoxin) may become toxic when release during infections

• May function as receptors and blocking immune response

• Contain porin proteins in upper layer - regulate molecules entering and leaving the cell

• Composed of:

  • Lipid A embedded in the outer membrane

  • A core polysaccharide

  • The O side chain

• Elicits and strong immune response

Gram stain

• Differential stain

  • Distinguishes cells with a gram-positive cell wall from those with a gram-negative cells wall

  • Gram-positive - retain crystal violet and stain purple

  • Gram-negative - lose crystal violet and stain red from safranin counterstain

• Important basis of bacterial classification and identification

• Practical aid in diagnosing infection and guiding drug treatment

Nontypical cell walls (acid-fast bacteria)

• Some bacterial groups lack typical cell wall structure

• i.e. Mycobacterium and Nocardia

  • Gram-positive cell wall structure with lipid mycolic acid (cord factor)

    • Pathogenicity and high degree of resistance to certain chemicals and dyes

    • Basis for acid-fast stain used for diagnosis of infections caused by these microorganisms

• Others have no cell wall

• i.e. Mycoplasma pneumoniae

  • Cell wall is stabilized by sterols

  • Pleomorphic

Cell membrane structure

• Phospholipid bilayer with embedded proteins - fluid mosaic model

• Functions in:

  • Providing site for energy reactions, nutrient processing, and synthesis

  • Passage of nutrients into the cell and discharge of wastes

  • Cell membrane is selectively permeable

• Contains glycoproteins and glycolipids

Passive transport

• Does not require energy

• Substances exist in a gradient and move from areas of higher concentration toward areas of lower concentration

• Diffusion

• Osmosis - diffusion of water

• Facilitated diffusion - requires a carrier

Active transport

• Requires energy and carrier proteins

• Gradient independent

• Active transport

• Group translocation - transported molecule chemically altered

• Bulk transport - endocytosis, exocytosis, pinocytosis

Cell wall-lacking cells and changes in osmotic pressure

• In hypertonic environments, cells without a cell wall can lose a lot of water, resulting in crenation

• In hypotonic environments, cells without a cell wall can gain excessive amounts of water, resulting in cell lysis

Facilitated diffusion

• Diffusion down a concentration gradient through a membrane protein

• Does not require energy

Active transport

• Pumps against the concentration gradient via a membrane pump

• Requires energy

Carrier Mediated Active transport

Molecules are pumped against the concentration gradient via membrane proteins using energy

Group Translocation

• Molecules are actively captured, but along the route of transportation, they are chemically altered

• Pairs transport with alteration, conserving cell energy

Endocytosis

• Bringing substances into the cell through a vesicle or phagosome

• Phagocytes - ingests substances or cells

• Pinocytes - ingests liquids

Cell cytoplasm

• Dense gelatinous solution of sugars, amino acids, metabolic enzymes, and salts

• 70% water

  • Serves as solvent for materials used in all cell functions

Nucleoid region

A condensed area where the single circular chromosome is found within prokaryotic cells

Chromosome

• Found in the nucleoid region

• Essential

• Single, circular, double stranded DNA molecule

• Contains all the genetic information required by a cell

Plasmids

• Free small circular, double-stranded DNA

• Not essential to bacterial growth and metabolism

• Used in genetic engineering - readily manipulated and transferred from cell to cell

Bacterial ribosomes

• 70S ribosomes - consists of a small 30S subunit and a large 50S subunit

• Made of 60% ribosomal RNA and 40% protein

• Prokaryotic differ from eukaryotic ribosomes in size and number of proteins

• Site of protein synthesis

• Found in all cells

Cytoskeleton

Many bacteria possess an internal network of protein polymers that is closely associated with the cell wall

Endospores

• Dehydrated, metabolically inactive

• Thick coat

• Longevity verges on immortality, 250 million years

• Resistant to ordinary cleaning methods and boiling

• Pressurized steam at 120^oC for 20-30 minutes will destroy

• Inert, resting, cells produced by some G+ genera: Clostridium, Bacillus, and Sporosarcina

  • Have a 2-phase life cycle:

    • Vegetative cell - metabolically active and growing

    • Endospore - when exposed to adverse environmental conditions - Capable of high resistance and very long-term survival

• Sporulation - Formation of endospores

  • Hardiest of all life forms

  • Withstands extremes in heat, drying, freezing, radiation, and chemicals

  • Not a means of reproduction

• Germination - return to vegetative growth

Sporulation cycle

1. Vegetative cell

2. Chromosome is duplicated and separated

3. Cell is septated into a sporangium and forespore

4. Sporangium engulfs forespore for further development

5. Sporangium begins to actively synthesize spore layers around forespore

6. Cortex and outer coat layers are deposited

7. Mature endospore

8. Free spore is released with the loss of the sporangium

9. Germination spore swells and releases vegetative cell

Classification systems for prokaryotes

1. Microscopic morphology

2. Macroscopic morphology - colony appearance

3. Bacterial physiology

4. Serological analysis

5. Genetic and molecular analysis

Bacterial shapes

• Vary in shape, but typically described by one of these three shapes:

  • Coccus - spherical

  • Bacillus - rod

    • Coccobacillus - very short and plump

    • Vibrio - gently curved

  • Spirillum - helical, comma, twisted rod

    • Spirochete - spring-like

Bacterial arrangements

• Arrangement of cells is dependent on pattern of division and how cells remain attached after division:

  • Cocci:

    • Singles

    • Diplococci - in pairs

    • Tetrads - groups of four

    • Irregular clusters

    • Chains

    • Cubical packets (sarcina)

  • Bacilli

    • Diplobacilli

    • Chains

    • Palisades

Streptococcus

Chain of cocci

Staphylococcus

Cluster of cocci

Streptobacillus

Chain of rods

Pleomorphism

• Variation in cell shape and size within a single species

• Some species are noted for their pleomorphism

Symbiotic ecological associations

• Two organisms live together in a close partnership

• Mutualism

  • Obligatory, dependent

  • both members benefit

• Commensalism

  • commensal member benefits

  • Other member neither harmed nor benefited

• Amensalism

  • One population harms another but remains unaffected itself

• Parasitism

  • Parasite is dependent and benefits

  • Host is harmed

Non-symbiotic ecological associations

• Organisms are free-living

• Relationships are not required for survival

• Synergism

  • Members cooperate to produce a result that none of them could do alone

• Antagonism

  • Actions of one organism affect the success or survival of others in the same community (competition)

    • Antibiosis

Interrelationships between microbes and humans

• Human body is a rich habitat for symbiotic bacteria, fungi, and a few protozoa - normal microbial flora, microbiome, or microbiota

• Commensal, parasitic, and synergistic relationships

Microbial biofilms

• Result when organisms attach to a substrate by some form of extracellular matrix that binds them together in complex organized layers

• Dominate the structure of most natural environments on earth

• Communicate and cooperate in the formation and function of biofilms - quorum sensing

Biofilm formation and quorum sensing

1. Free-swimming cells settle on a surface and remain there

2. Cells synthesize a sticky matrix that holds them tightly to the substrate

3. When biofilm grows to a certain density (quorum) the cells release inducer molecules that can coordinate a response

4. Enlargement of one cell to show genetic induction - inducer molecule stimulates expression of a particular gene and synthesis of a protein product, such as an enzyme

5. Cells secrete their enzymes in unison to digest food particles

Bergey’s Manual of Determinative Bacteriology

• Five volume resource covering all known prokaryotes

• Classification based on genetic information - phylogenetic

• Two domains: Archaea and Bacteria

• Five major subgroups with 25 different phyla

Domain Archaea

• Primitive, adapted to extreme habitats and modes of nutrition

• More closely adapted related to Eukarya than to bacteria

• Contain unique genetic sequences in their rRNA

• Have unique membrane lipids and cell walls

• Live in the most extreme habitats in nature, extremeophiles

• Adapted to heat, salt, acid pH, pressure, and atmosphere

• Includes:

  • Methane producers

  • Hyperthermophiles

  • Extreme halophiles

  • Sulfur reducers

Domain Bacteria

• Phylum Proteobacteria - Gram-negative cell walls

• Phylum Firmicutes - Mainly Gram-positive with low G + C content

• Phylum Actinobacteria - Gram-positive with higher G + C content

Species

A collection of bacterial cells which share an overall similar pattern of traits in contrast to other bacteria whose pattern differs significantly

Strain or variety

A culture derived from a single parent that differs in structure or metabolism from other cultures of that species (biovars, morphovars)

Type

A subspecies that can show differences in antigenic makeup (sereotype or serovar), susceptibility to bacterial viruses (phage type), and in pathogenicity (pathotype)

Diagnostic scheme for medical use

• Uses genetic and phenotypic qualities in identification

  • Determination of pathogens that cause disease

  • Divides bacteria based on cell wall structure, shape, and arrangement

  • Physiological traits such as flagella, capsules, and protein/enzyme panel

  • Test include metabolic capacity such as carbohydrate fermentation or oxidation

  • Presumptive serological test to detect the presence of specific antigens - ABS:Ag interaction

  • DNA/RNA sequencing for determination

Prokaryotes with unusual characteristics

• Free-living nonpathogenic bacteria

• Photosynthetic bacteria - use photosynthesis, can synthesize required nutrients from inorganic compounds

  • Cyanobacteria (blue-green algae)

  • Green and purple sulfur bacteria

  • Gliding, fruiting bacteria

Cyanobacteria (Blue-green Algae)

• Gram-negative cell walls

• Extensive thylakoids with photosynthetic chlorophyll pigments and gas inclusions

Green and purple sulfur bacteria

• Photosynthetic

• Contain photosynthetic pigment bacteriochlorophyll

• Do not give off oxygen as a product of photosynthesis

Gliding and fruiting bacteria

• Gram-negative

• Glide over moist surfaces

Deep Sea bacteria

• i.e. Shewenella

• Lives in the deep sea, where there is little oxygen diffused in the water

• Able to survive this harsh environment by attaching to the sea floor and using long appendages, called nanocables, to sense oxygen

• Can also digest toxic waste and generate electricity

Nitrogen-fixing bacteria

• i.e. Rhizobium

• Live in the root nodules of legumes such as clover

Fastidious and Capnophiles

• i.e. Neisseria meninditigis

• Thrive in 5% CO₂

Acidophiles

• i.e. Helicobacter pylori

• Can cause chronic gastritis, which can lead to ulcers and stomach cancer

• has a mechanism for resisting the low pH in the human stomach

Rickettsias

• Medically significant

• Obligate intracellular parasites

• Very tiny, gram-negative bacteria

• Most are pathogens

• Obligate intracellular pathogens

• Cannot survive or multiply outside of a host cell

• Rickettsia rickettisii - Rocky Mountain spotted fever

Chlamydias

• Medically significant

• Obligate intracellular parasites

• Tiny

• Not transmitted by arthropods

• Chlamydia trachomatis - severe eye infection and one of the most common sexually transmitted diseases

• Chlamydia pneumoniae - lung infections

Chlamydia life cycle

• Chlamydia begins infection of a host when the metabolically inactive elementary bodies enter an epithelial cell

• Once inside the host cell, the elementary bodies turn into active reticulate bodies

• The reticulate bodies multiply and release more elementary bodies when the cell dies after the Chlamydia uses all of the host cell’s ATP

Halophiles

Prefer extreme salt concentrations

Microaerophiles

Require a small amount of oxygen

Inorganic carbonophiles

Take advantage of inorganic carbon

Anaerobes

Harmed by oxygen

Facilitative anaerobes

Can use oxygen, but don’t need it