Bacteria

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

BacteriaStaphylococcus bacteria in nose


Structure

• Unicellular

• Some species form colonies

• 0.5–10 µm, much smaller than the 10–100 µm of many eukaryotic cells

• Most common shapes:

• spheres (cocci)

• rods (bacilli)

• and spirals


Bacterial Cell Wall

• maintains cell shape

• provides physical protection,

• prevents the cell from bursting in a hypotonic environment

• Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides (proteins)

sugar

sugar

polypeptide


• Using the Gram stain, scientists classify bacterial into Gram + or Gram - groups based on cell wall composition

• Gram - bacteria:

• less peptidoglycan

• outer membrane that can be toxic

more likely to be antibiotic resistant

• Many antibiotics target peptidoglycan and damage bacterial cell walls

Gram Stain Classification

Gram +Gram -

Cellwall

Peptidoglycanlayer

Plasma membrane

Protein

(a) Gram-positive: peptidoglycan traps crystal violet.

Cellwall Peptidoglycan

layerPlasma membrane

Protein

(b) Gram-negative: crystal violet is easily rinsed away, revealing red dye.

Outermembrane

Carbohydrate portionof lipopolysaccharide

Gram-positivebacteria

Gram-negativebacteria

20 µm


• A polysaccharide or protein layer called a capsule covers many bacteria

Structure


Bacterial Motility

• Most motile bacteria use flagella to propel themselves

• Many exhibit taxis, the ability to move toward or away from certain stimuli

Video: Prokaryotic Flagella (Video: Prokaryotic Flagella (Salmonella typhimuriumSalmonella typhimurium))

Flagellum

Filament

Hook

Basal apparatus

Cell wall

Plasmamembrane

50 nm


Fimbriae (also called attachment pili)

• allows them to stick to their substrate or other individuals in a colony

Fimbriae


Internal Organization

• lack complex compartmentalization (ex. no nucleus, ER, mitochondrion, etc.)

• Often perform metabolic functions using highly folded extensions of plasma membrane

(a) Aerobic prokaryote (b) Photosynthetic prokaryote

Thylakoidmembranes

Respiratorymembrane

0.2 µm 1 µm


• Most of the genome consists of a circular chromosome located in a nucleoid region

• Some have smaller rings of DNA called plasmids

• Plasmids w/ short codes of DNA that may be beneficial to bacteria (Ex. some code for antibiotic resistance)

Genome


• Sex pili are longer than fimbriae and allow prokaryotes to exchange DNA

Genome


• Cells containing the F plasmid function as DNA donors during a process called conjugation

• Cells without the F factor function as DNA recipients during conjugation

• The F factor is transferable during conjugation

Plasmid F factor

F plasmid

F+ cell

F– cell

Matingbridge

Bacterial chromosome

Bacterialchromosome

(a) Conjugation and transfer of an F plasmid

Plasmid F factor

F plasmid

F+ cell

F– cell

Matingbridge


Bacterialchromosome


Plasmid F factor

F plasmid

F+ cell

F– cell

Matingbridge


Bacterialchromosome


F+ cell

F+ cell

Plasmid F factor


• R plasmids carry genes for antibiotic resistance

• Antibiotics select for bacteria with genes that are resistant to the antibiotics

R Plasmids and Antibiotic Resistance


Reproduction and Adaptation

• Quick reproduction by binary fission

• every 1–3 hours

• Beneficial mutations can accumulate rapidly in a population, allowing for rapid evolution

• ex. Antibiotic resistant strains are becoming more common

• Many form inactive endospores

• to remain viable in harsh conditions

Endospore

0.3 µm


Genetic Recombination

• DNA from different individuals can be brought together by conjugation, transformation, transduction


Transformation and Transduction

• A prokaryotic cell can take up and incorporate foreign DNA from the surrounding environment in a process called transformation

• Transduction is the movement of genes between bacteria by vectors like bacteriophages (viruses that infect bacteria)

Donorcell

A+ B+

A+ B+

Phage DNA

A+

Donorcell

A+ B+

A+ B+

Phage DNA

Recipientcell

B–

A+

A–

Recombination

A+

Donorcell

A+ B+

A+ B+

Phage DNA

Recombinant cell

Recipientcell

A+ B–

B–

A+

A–

Recombination

A+

Donorcell

A+ B+

A+ B+

Phage DNA


Pathogenic Prokaryotes

• Prokaryotes cause about half of all human diseases

• Lyme disease is an example

5 µm

Fig. 27-21a

Deer tick

Fig. 27-21c

Lyme disease rash


• Bacteria typically cause disease by releasing exotoxins or endotoxins

• Exotoxins cause disease even if the prokaryotes that produce them are not present

• Endotoxins are released only when bacteria die and their cell walls break down

• Many pathogenic bacteria are potential weapons of bioterrorism

Pathogenic Prokaryotes

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Bacteria