Prokaryotic Profiles: The Bacteria and Archaea€¦ · The way their DNA is packaged No nucleus Not wrapped around histones The makeup of their cell wall Bacteria- peptidoglycan Archae-

The Main Themes of Microbiology Microbiology: A Systems Approach

Chapter 1, pages 1 to 26 1

Prokaryotic

Profiles: The

Bacteria and

Archaea

Chapter 4

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

Cellular Acellular

Prokaryotes Eukaryotes Viruses

Bacteria and archaea

Fungi,protozoa, helminths

Viruses and bacteriophage

Types of Microbes

(b) VirusTypes

Bacterial virus

AIDS virus

Envelope

Capsid

Nucleic

acid

(a) Cell Types

Prokaryotic

Chromosome

Ribosomes

Flagellum

Cellwall Cell

membrane

Eukaryotic

Flagellum

Mitochondria Ribosomes

Cell membrane

Nucleus

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



How are Prokaryotes Different from

Eukaryotes?

The way their DNA is packaged

No nucleus

Not wrapped around histones

The makeup of their cell wall

Bacteria- peptidoglycan

Archae- tough and made of other chemicals, distinct to them

Their internal structures

No complex, membrane-bound organelles

4.1 Prokaryotic Form and Function



Structures common to all bacterial cells

Cell membrane

Cytoplasm

Ribosomes

One (or a few) chromosomes

Structures found in most

bacterial cells

Cell wall

Surface coating or glycocalyx



Structures found in some bacterial cells

Flagella

Pili

Fimbriae

Capsules

Slime layers

Inclusions

Actin cytoskeleton

Endospores

Prokaryote Cell



4.2 External Structures

Appendages: Cell extensions

Common but not present on all species

Can provide motility (flagella and axial filaments)

Can be used for attachment and mating (pili and fimbriae)

Flagella

Three parts: Filament, hook (sheath), and basal body

Vary in both number and arrangement

Polar arrangement: flagella attached at one or both ends of the cell

Monotrichous- single flagellum

Lophotrichous- small bunches or tufts of flagella emerging from the same site

Peritrichous- dispersed randomly over the structure of the cell



Figure 4.2

Flagellar Arrangements



Flagellum Structure

Bacterial locomotion

Electric motor

Dozens of proteins

Rotates rapidly

Many are reversible

Propels the cell through its environment

Flagellar Function

Chemotaxis- positive and negative

Phototaxis

Move by runs and tumbles



Axial Filaments

Found in Spirochetes

A type of internal flagellum that is enclosed in the space between the cell wall and the cell membrane

Bundles of many flagella

Cause the bacterium to corkscrew

Can move through viscous media



Pili

Elongate, rigid tubular structures

Made of the protein pilin

Found on gram-negative bacteria

Used for attachment and genetic exchange

Used in conjugation

Figure 4.8



Fimbriae

Small, bristlelike fibers

Most contain protein

Used for attachment and sometimes motility

Tend to stick to each other and to surfaces

Figure 4.7



The Glycocalyx

Develops as a coating of repeating polysaccharide units, protein, or both

Protects the cell

Sometimes helps the cell adhere to the environment

Differ among bacteria in thickness, organization, and chemical composition

Slime layer- a loose shield that protects some bacteria from loss of water and nutrients

Capsule- when the glycocalyx is bound more tightly to the cell and is denser and thicker

Capsules and Slime Layers



Functions of the Glycocalyx

Formed by many pathogenic bacteria- protect the bacteria against phagocytes

Important in formation of biofilms

Capsule- important for pathogenesis, prevent phagocytosis

4.3 The Cell Envelope: The Boundary

layer of Bacteria

Majority of bacteria have a cell envelope

Lies outside of the cytoplasm

Composed of two or three basic layers

Cell wall (peptidoglycan)

Cell membrane

In some bacteria, the outer membrane (gram-negative material)



Figure 4.12

Structure of the Cell Wall

Helps determine the shape of a bacterium

Provides strong structural support

Most are rigid because of peptidoglycan content



Structure of the Cell Wall, cont.

Keeps cells from rupturing because of changes in pressure due to osmosis

Target of many antibiotics- disrupt the cell wall, and cells have little protection from lysis

Gram-positive cell wall

A thick (20 to 80 nm), homogeneous sheath of peptidoglycan

Gram-Negative Cell Wall

Single, thin (1 to 3 nm) sheet of peptidoglycan

Figure 4.14

Gram-positive and Gram-negative



Gram Staining

Nontypical Cell Walls

Some aren’t characterized as either gram-positive or gram-negative

Some don’t have a cell wall at all

For example, Mycobacterium and Nocardia- unique types of lipids

Archae- unusual and chemically distinct cell walls

Mycoplasmas- lack cell wall entirely



Mycoplasmas

Mycoplasma cell membrane is stabilized by sterols and is resistant to lysis

Very small bacteria (0.1 to 0.5 µm)

Range in shape from filamentous to coccus

Important medical species: Mycoplasma pneumonia (walking pneumonia)

Acid-Fast Bacteria

Mycobacteria

Contain mycolic acid (a wax)

Modified Gram-positive structure

Includes important pathogens

Tuberculosis

Leprosy

Opportunistic wound infections



Cell Membrane Structure

Also known as the cytoplasmic membrane

Very thin (5-10 nm)

Contain primarily phospholipids and proteins

The exceptions: mycoplasmas and archae

Functions

Provides a site for functions such as energy reactions, nutrient processing, and synthesis

Regulates transport (selectively permeable membrane)

Secretion

Practical Considerations of Differences

in Cell Envelope Structure

Outer membrane- an extra barrier in gram-

negative bacteria

Makes them impervious to some antimicrobial chemicals

Generally more difficult to inhibit or kill than gram-positive bacteria

Cell envelope can interact with human tissues and cause disease

Corynebacterium diphtheriae

Streptococcus pyogenes



4.4 Bacterial Internal Structure

Contents of the Cell Cytoplasm

Gelatinous solution

Site for many biochemical and synthetic activities

70%-80% water

Also contains larger, discrete cell masses (chromatin body, ribosomes, granules, and actin strands)

Bacterial Chromosome

Single circular strand of DNA

Aggregated in a dense area of the cell- the nucleoid

Figure 4.17



Plasmids

Nonessential pieces of DNA

Double-stranded circles of DNA

Often confer protective traits such as drug resistance or the production of toxins and enzymes

Ribosomes

Made of RNA and protein

Special type of RNA- ribosomal RNA (rRNA)

Characterized by S units- the prokaryotic ribosome is 70S

Figure 4.18

//upload.wikimedia.org/wikipedia/commons/c/cf/Plasmid_%28english%29.svg



Inclusion Bodies

Non-membrane bound granules

Usually for storage of nutrients

Volutin P storage

Polyhydroxybutyrate Carbon storage

Magnetosomes Magnetic iron crystals

(a)

(b)

MP


a: © Paul W. Johnson/Biological Photo Service; b: © D. Balkwill and D. Maratea

Granules

A type of inclusion body

Contain crystals of inorganic compounds

Are not enclosed by membranes

Example- sulfur granules of photosynthetic bacteria

Polyphosphate granules of Corynebacterium and Mycobacterium are called metachromatic granules because they stain a contrasting color in methylene blue



The Cytoskeleton

Long polymers of actin

Arranged in helical ribbons around the cell just under the cell membrane

Contribute to cell shape

Bacterial Endospores: An Extremely

Resistant Stage

Dormant bodies produced by Bacillus, Clostridium, and Sporosarcina



Endospore-Forming Bacteria

These bacteria have a two-phase life cycle

Phase One- Vegetative cell

Metabolically active and growing

Can be induced by the environment to undergo spore formation (sporulation)

Phase Two- Endospore Formation

Phase Two: Endospore Formation

Stimulus for sporulation- the depletion of nutrients

Vegetative cell undergoes a conversion to a sporangium

Sporangium transforms in to an endospore

Hardiest of all life forms Withstand extremes in heat, drying, freezing, radiation, and

chemicals

Heat resistance- high content of calcium and dipicolinic acid

Some viable endospores have been found that were more than 250 million years old



Endospore Formation


Cortex

1

9

8

7

6

5

4

3

2

Spore coats

Chromosome

Vegetative cell

Chromosome

Cell membrane Cell wall Cortex Core of spore

Germination

spore swells

and releases

vegetative cell.

Free spore is

released with

the loss of the

sporangium.

Exosporium

Spore coat

Cortex

Core

Sporulation

Cycle

Forespore

Chromosome is

duplicated and

separated.

Cell is septated

into a sporangium

and forespore.

Sporangium

Sporangium engulfs

forespore for further

development.

Sporangium begins

to actively synthesize

spore layers around

forespore.

Early spore

Cortex and

outer coat layers

are deposited.

Mature

endospore

© George Chapman/Visuals Unlimited

Endospore Germination

Breaking of dormancy

In the presence of water and a specific germination agent

Quite rapid (1 ½ hours)

The agent stimulates the formation of hydrolytic enzymes, digest the cortex and expose the core to water



Endospore Medical Significance

Several bacterial pathogens

Bacillus anthracis

Clostridium tetani

Clostridium perfringens, Clostridium botulinum

Resist ordinary cleaning methods

4.5 Bacterial Shapes, Arrangements, and

Sizes

Three general shapes

Coccus- roughly spherical

Bacillus- rod-shaped

Coccobacillus- short and plump

Vibrio- gently curved

Spirillum- curviform or spiral-shaped

Pleomorphism- when cells of a single species vary to some extent in shape and size



Bacterial Shapes

4.6 Classification Systems in the

Prokaryotes

One of the original classification systems- shape,

variations in arrangement, growth characteristics, and habitat

Now compare sequence of nitrogen bases in rRNA

Definitive published source for bacterial classification

Bergey’s Manual

Since 1923

Early classification- the phenotypic traits of bacteria

Current version- combines phenotypic information with rRNA sequencing



4.7 Domain Archaea

Prokaryotic cells

Many are found in extreme environments

Important non- pathogens

Different from Bacteria in Cell structure Metabolism Genetics (more closely related to Eukarya)

How they differ from other cell types

Certain genetic sequences are found only in their rRNA

Unique membrane lipids and cell wall construction

The most primitive of all life forms

Most closely related to the first cells that originated on earth

Modern archaea live in habitats that share conditions as the ancient earth

Methane producers

Hyperthermophiles

Extreme halophiles

Sulfur reducers

Documents

Prokaryotic Profiles: The Bacteria and Archaea€¦ · The way their DNA is packaged No nucleus Not wrapped around histones The makeup of their cell wall Bacteria- peptidoglycan Archae-