Chapter 4

A Survey of

Prokaryotic Cells and

Microorganisms

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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4.1 Basic Characteristics of Cells and Life Forms

All living things (single and multicellular) are made of

cells that share some common characteristics:

– Basic shape: spherical, cubical, cylindrical

– Internal content: cytoplasm, an internal matrix

surrounded by a membrane

– DNA chromosome(s), ribosomes, metabolic capabilities

Two basic cell types: eukaryotic and prokaryotic

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Characteristics of Cells 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

Cell membrane

Nucleus Mitochondria

Ribosomes

Cell

membrane

Cell wall

Flagellum Flagellum

Chromosome

Prokaryotic Eukaryotic

Ribosomes

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What is 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: cell walls, vacuoles, granules and inclusions

• Transport of nutrients and waste

4.2 Prokaryotic Profiles: The Bacteria and Archaea

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Ribosomes Cell wall Cell membrane

Chromosome

(DNA)

Actin

filaments

Pilus

Cytoplasmic matrix

Capsule

Inclusion

body

Flagellum

Fimbriae

Slime

layer

Mesosome

Figure 4.1 – structure of a bacterial cell

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Cell Extensions and Surface Structures (move outside of the cell to inside the cell)

• Appendages

– Two major groups of appendages:

• Motility: flagella and axial filaments (periplasmic flagella)

• Attachment or channels: fimbriae and pili

• Glycocalyx – surface coating to protect the cell,

and in some cases help it adhere to its

environment

Hook

Filament

Periplasmic

space

L ring

Cell wall

Cell

membrane

Rings Rod

Outer

membrane

Basal

body

22 nm (a) (b)

Rings

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Flagella – Bacterial Propellers

• 3 parts:

– Filament: long, thin, helical structure composed of

protein flagellin

– Hook: curved sheath

– Basal body: stack of rings firmly anchored in cell wall

(a)

(b)

(c)

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Flagella – Bacterial Propellers

• Rotates 360o

• Functions in motility of cell through environment

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a. / b. flagella rotates

CCW, cell runs (swims)

c. Flagella rotates CW, cell

tumbles

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Figure 4.3 Flagellar Arrangements

Monotrichous (a) – single flagellum at one end

Lophotrichous (b) – small bunches emerging from the same site

Amphitrichous (c) – flagella at both ends of cell

Peritrichous (d) – flagella dispersed over surface of cell

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Flagellar Responses

Guide bacteria in a direction in response to external stimulus,

taxis:

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

Run (R)

Key

Tumble (T)

T

T

Tumble (T)

(b) Gradient of attractant concentration

T

T

R

R

(a) No attractant

or repellent

Figure 4.5 Chemotaxis in

bacteria

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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, seen in spirochetes (b)

(a)

Cell membrane

PF PC OS

Periplasmic

flagella (PF)

Outer sheath (OS)

Protoplasmic

cylinder (PC)

Peptidoglycan

E. coli cells

Intestinal

microvilli

G

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Other Appendages: Fimbriae (in some texts known as attachment

pili)

• Fine, proteinaceous,

hairlike bristles

emerging from the

cell surface

• Function in adhesion

to other cells and

surfaces

– Ex. Neisseria

gonorrhoeae

(a)

(b)

© Eye of Science/Photo Researchers, Inc.

Dr. S. Knutton from D.R. Lloyd and S. Knutton, Infection and Immunity, January 1987, p 86-92. © ASM

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Other Appendages: 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

Fimbriae

Pili

© L. Caro/SPL/Photo Researchers, Inc.

Figure 4.8

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The Bacterial Surface Coating, or Glycocalyx

• Coating of molecules external to the cell wall,

made of sugars and/or proteins

• Two types:

1. Slime layer - loosely organized and attached

2. Capsule - highly organized, tightly attached

Slime layer

(a) Capsule (b)

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Functions of the Glycocalyx

• Protect cells from dehydration and nutrient loss

• Inhibit killing by white blood cells by phagocytosis,

contributing to pathogenicity

• Attachment - formation of biofilms

Colony without a capsule

Colonies with a capsule

© Kathy Park Talaro

Capsule

Cell body

© John D. Cunningham/Visuals Unlimited

(a) (b)

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Catheter

surface

Staphylococci

Fungal

cells

Figure 4.11 Biofilm on a catheter – the biofilm

allows bacteria to colonize and stick to surfaces

16 Janice Carr/CDC

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4.3 The Cell Envelope: The Boundary Layer of Bacteria

• External covering outside the cytoplasm

• Composed of two basic layers:

– Cell wall and cell membrane

• Maintains cell integrity

• Two different groups of bacteria demonstrated by

Gram stain:

– Gram-positive bacteria: thick cell wall composed

primarily of peptidoglycan and cell membrane

– Gram-negative bacteria: outer cell membrane, thin

peptidoglycan layer, and cell membrane

(b) This shows the

molecular pattern of

peptidoglycan. It has

alternating glycans (NAG

and NAM) bound

together in long strands.

The NAG stands for

N-acetyl glucosamine,

and the NAM stands for

N-acetyl muramic acid.

Adjacent muramic acid

molecules on parallel

chains are bound by a

cross-linkage of peptides

(green spheres)

O

NAM O

C

C

H H3C H3C NH

C

CH3 CH3

O

CH2OH CH2OH

NAM

O

O

C

C

H NH

NAG NAG

C O

O O NAG NAG O O

L–alanine

D–glutamate

L–lysine

D–alanine

–glycine

–glycine

–glycine –glycine

–glycine

L–alanine

D–glutamate

L–lysine

D–alanine

Interbridge

(c) An enlarged view of

the links between the

NAM molecules.

Tetrapeptide chains

branching off the

muramic acids

connect by amino acid

Interbridges. The

amino acids in the

interbridge can vary or

may be lacking

entirely. It is this

linkage that provides

rigid yet flexible

support to the cell.

(a) The peptidoglycan of a

cell wall is a huge,

3-dimensional lattice

work that is actually one

giant molecule to

surround and support

the cell.

Tetr

apeptide

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Structure of Cell Walls

• Determines cell shape,

prevents lysis due to

changing osmotic

pressures

• Peptidoglycan is the

primary component:

– Unique

macromolecule

composed of a

repeating framework

of long glycan chains

cross-linked by short

peptide fragments

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Gram-Positive Cell Wall

– 20-80 nm thick peptidoglycan

– Includes teichoic acid and

lipoteichoic acid: function in

cell wall maintenance and

enlargement during cell division;

move cations across the cell

envelope; stimulate a specific

immune response

– Some cells have a periplasmic

space, between the cell

membrane and cell wall

Peptidoglycan

Cell

membrane

Gram (+)

Cell membrane

(a) Cell wall (peptidoglycan)

© S.C Holt/Biological Photo Service

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Gram-Negative Cell Wall

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

– Outer membrane contains lipopolysaccharides (LPS)

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

• May function as receptors and blocking immune response

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

Cell membrane

Peptidoglycan

Outer membrane

Cell membrane

Cell wall Periplasmic space

Peptidoglycan

Gram (–)

(b) Outer membrane

© T. J. Beveridge/Biological Photo Service

Figure 4.14 Structures of Gram-Positive and

Gram-Negative Bacterial Cell Walls

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Peptidoglycan

Teichoic acid

Phospholipid

Membrane

proteins

Lipopolysaccharide

Porin

Lipoprotein

Membrane

proteins Periplasmic

space

Lipoproteins

Cell membrane

Periplasmic space

Peptidoglycan

Outer

membrane layer

Lipoteichoic acid

Wall

Teichoic acid

Envelo

pe

Lipopolysaccharides Porin proteins Phospholipids

Membrane

protein

Gram positive Gram negative

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The Gram Stain

• Differential stain that distinguishes cells with a gram-positive cell wall from those with a gram-negative cell 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

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The Gram Stain

Microscopic Appearance of Cell Chemical Reaction in Cell

(very magnified view)

Step

1 Crystal

Violet

(primary

dye)

2 Gram’s

iodine

(mordant)

3 Alcohol

(decolorizer)

4 Safranin

(red dye

counterstain)

Both cell walls stain with the dye.

Dye crystals

trapped in cell

Crystals remain

in cell.

Red dye

has no effect.

Gram (+) Gram (–) Gram (+) Gram (–)

Outer wall is

weakened; cell

loses dye.

Red dye stains

the colorless cell.

No effect

of iodine

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Nontypical Cell Walls

• 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 - acid-fast bacteria stain red

• Some have no cell wall, i.e., Mycoplasma

– Cell wall is stabilized by sterols

– Pleomorphic

Phospholipid Peripheral

protein

Glycolipid

Integral

protein

Carbohydrate

receptor

Integral

protein

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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

Figure 4.16 Cell membrane

structure

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4.4 Bacterial Internal Structure

• Cell cytoplasm: – Dense gelatinous solution of sugars, amino acids, and

salts

– 70-80% water

• Serves as solvent for materials used in all cell functions

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• Chromosome – Single, circular, double-

stranded DNA molecule that 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 Chromosomes and Plasmids: The Sources of

Genetic Information

Courtesy of Michael J. Daly

Figure 4.17

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Ribosomes: Sites of Protein Synthesis

• Ribosomes

– Made of 60% ribosomal

RNA and 40% protein

– Consist of two subunits:

large and small

– Prokaryotic differ from

eukaryotic ribosomes in

size and number of

proteins

– Site of protein synthesis

– Found in all cells

Ribosome (70S)

Large

subunit

(50S)

Small

subunit

(30S)

Figure 4.18

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Bacterial Internal Structures

• Inclusions and granules – Intracellular storage bodies

– Vary in size, number, and content

– Bacterial cell can use them when environmental sources are depleted, source of carbon and/or ATP

MP

(a) (b)

© D. Balkwill and D. Maratea

Figure 4.19 inclusion bodies

Bacterial Internal Structures

• Cytoskeleton

– Many bacteria possess an internal network

of protein polymers that is closely

associated with the cell wall

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Actin

filaments

© Rut CARBALLIDO-LOPEZ/I.N.R.A. Jouy-en-Josas, Laboratoire de Génétique Microbienne

Figure 4.20 bacterial

cytoskeleton of Bacillus

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Bacterial Endospores: An Extremely Resistant

Life Form

• Endospores – 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

Figure 4.22 Sporulation cycle

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Cell wall

Chromosome

Vegetative cell

Chromosome is

duplicated and

separated.

Cell is septated

into a sporangium

and forespore.

Sporangium engulfs

forespore for further

development.

Sporangium begins

to actively synthesize

spore layers around

forespore.

Cortex and

outer coat layers

are deposited.

Mature

endospore

Free spore is

released with

the loss of the

sporangium.

Germination

spore swells

and releases

vegetative cell.

Forespore

Early spore Cortex

Exosporium

Spore coat

Cortex

Core

Sporangium

Cell membrane

Sporulation

Cycle

1

9

8

7

6

5

4

3

2

Exosporium

Core

Cortex

Spore

coats

SJ Jones, CJ Paredes, B Tracy,

N Cheng, R Sillers, RS Senger,

ET Papoutsakis, "The transcriptional

program underlying the physiology of

clostridial sporulation," Genome

Biol., 2008. 9:R114

33

Figure 4.21Endospores

• Dehydrated, metabolically inactive

• Thick coat

• Longevity verges on immortality, 250 million years

• Resistant to ordinary cleaning methods and boiling

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

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4.5 Bacterial Shapes, Arrangements, and Sizes

• Vary in shape, size, and arrangement but

typically described by one of three basic shapes:

– Coccus: spherical

– Bacillus: rod

• Coccobacillus – very short and plump

• Vibrio – gently curved

– Spirillum: helical, comma, twisted rod,

• Spirochete – spring-like

Figure 4.3 Common bacterial shapes

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(a) Coccus (b) Rod/Bacillus (c) Vibrio

(d) Spirillum (e) Spirochete (f) Branching filaments

Janice Carr/CDC Janice Carr/CDC

Photo by De Wood. Digital colorization by Chris Pooley © VEM/Photo Researchers, Inc. © Science VU/Frederick Mertz/Visuals Unlimited

Key to Micrographs Key to Micrographs

(a) Micrococcus luteus (22,000×) (b) Legionella pneumophila (6500×) (c) Vibrio cholerae (13,000×) (d) Aquaspirillum (7,500×)

(e) Spirochetes on a filter (14,000×) (f) Streptomyces species (6500×)

From Jacob S. Teppema, “In vivo adherence and colonization of Vibrio cholerae strains

that differ in hemagglutinating activity and motility, ” Journal of Infection and Immunity,

55(9): 2093-2102, Sept. 1987. Reprinted by permission of American Society for

Microbiology

Figure 4.24 Pleomorphism

• Variation in cell shape

and size within a

single species

• Some species are

noted for their

pleomorphism, such

as Corynebacterium

diphtheriae

Metachromatic

granules

Palisades arrangement

Palisades arrangement

Metachromatic

granules

© A.M. Siegelman/Visuals Unlimited

36

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Figure 4.25 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

Diplococci

(two cells)

(a) Division in

one plane

Streptococci (variable

number of cocci in chains)

Tetrad (cocci in

packets of four)

(b) Division in two

perpendicular planes

Sarcina (packet of 8 – 64

cells)

Staphylococci and

Micrococci

Irregular clusters (number of cells

varies)

(c) Division in

several planes

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4.6 Classification Systems of Prokaryotic Domains:

Archaea and Bacteria

1. Microscopic morphology

2. Macroscopic morphology – colony appearance

3. Bacterial physiology

4. Serological analysis

5. Genetic and molecular analysis

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Bacterial Taxonomy Based on Bergey’s Manual

• 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

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Diagnostic Scheme for Medical Use

• Uses phenotypic qualities in identification, a

bit more informal

– Restricted to bacterial disease agents

– Divides bacteria based: on 1)cell wall structure,

2)shape, 3) arrangement, and 4) physiological

traits

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Species and Subspecies

• 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 (serotype or serovar), susceptibility to bacterial viruses (phage type) and in pathogenicity (pathotype)

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4.7 Prokaryotes with Unusual Characteristics Unusual Forms of Medically Significant Bacteria

• Obligate intracellular parasites – Rickettsias

• 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

Vacuole

Nucleus

Rickettsial cells

Baca and Paretsky, Microbiological Reviews, 47(20);133, fig. 16, June 1983 © ASM

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Unusual Forms of Medically Significant Bacteria

– Chlamydias

• Tiny

• Obligate intracellular parasites

• Not transmitted by arthropods

• Chlamydia trachomatis – severe eye infection

and one of the most common sexually

transmitted diseases

• Chlamydia pneumoniae – lung infections