Lab 17 on Thursday

Prokaryotes and Viruses

Chapter 25

Learning Objectives

• Describe prions and the diseases they cause• Describe general characteristics of Prokaryotes• Compare and contrast bacteria, archeae and

eukarya• Compare and contrast gram positive and gram

negative bacteria• Describe gram + and gram- bacteria • Describe the three main bacterial shapes

Why It Matters

• Prokaryotes: The smallest organisms

Prions

• Infectious proteins with no associated nucleic acids

• Misfolded versions of normal cellular proteins that can induce other normal proteins to misfold

Prion Diseases

Degenerate nervous system in mammals– Scrapie: Brain disease in sheep– Mad cow disease (Bovine spongiform

encephalopathy): Spongy holes and protein deposits in brain tissue

– Creutzfeldt-Jakob disease: Rapid mental deterioration, loss of vision and speech, paralysis

– Kuru-cannibalistic tribe in New Guinea,

Brain Tissue Damaged by BSE

25.1 Prokaryotic Structure and Function

• Prokaryotes are simple in structure compared with eukaryotic cells

• Prokaryotes have the greatest metabolic diversity of all living organisms

• Prokaryotes differ in whether oxygen can be used in their metabolism

25.1 (cont.)

• Prokaryotes fix and metabolize nitrogen

• Prokaryotes reproduce asexually or, rarely, by a form of sexual reproduction

• In nature, bacteria may live in communities attached to a surface

Prokaryotes

• Colonize a great diversity of habitats

• Are small but complex cells

• Have great metabolic diversity

• Adapt rapidly to their environments

Three Common Shapes in Prokaryotes

• Spherical: cocci• Rodlike: bacilli• Spiral: vibrios (curved) and spirilla (helix)

Prokaryotic Genomes

• Prokaryotic chromosome– Single, circular DNA molecule– Packaged into nucleoid

• No nucleolus• No nuclear membranes

• Plasmids– Small circles of DNA– Genes supplement nucleoid genes– Replicate independently (along with main DNA)

Fig. 25-3, p. 528

Cell wall

Capsule

Outermembrane

Cytoplasmcontainingribosomes

Folded DNAmolecule(in the nucleoid)

Peptido-glycan layer

Plasmamembrane

Plasmid Pili

Flagellum

Prokaryotic Ribosomes

• Bacterial ribosomes– Smaller than eukaryotic ribosomes– Protein synthesis similar to eukaryotes

• Archaeal ribosomes– Size similar to bacteria– Different structure– Protein synthesis is combination of bacterial

and eukaryotic processes

Prokaryotic Cell Wall

• Protects plasma membrane– Helps withstand osmotic pressure– Prevents action of detergent-like chemicals

• Made of peptidoglycans– Polysaccharide polymers connected by short

polypeptides

Gram Stain

• Gram stain technique– Stain with crystal violet, then with iodine

• Fixes dye to cell wall

– Wash with alcohol– Stain again with fuchsin or safranin

• Gram-positive bacteria– Appear purple because crystal violet retained

• Gram-negative bacteria– Appear pink because crystal violet lost

Gram-Positive Bacteria

• Single, relatively thick peptidoglycan layer

Fig. 25-4, p. 529

Peptidoglycan layer

a. Gram-positive bacterial cell wallCapsulemay bepresent

Cellwall

Cytoplasm

Plasma membrane

Gram-Negative Bacteria

• Two-layered walls; relatively thin peptidoglycan sheath surrounded by outer membrane

Fig. 25-4, p. 529

Peptidoglycan layer

b. Gram-negative bacterial cell wall

Cell wall

Cytoplasm

Plasma membrane

Capsule

Outer membrane

Slime Coat

• Capsule– Slime attached to cells

• Slime layer– Loosely associated with cells

• Protects bacteria from desiccation, antibiotics, viruses, antibodies, and enzymes

• Helps bacteria adhere to surfaces

Pili

• Rigid protein shafts extend from cell walls

• Mostly in Gram-negative bacteria

• Help bacteria attach to each other or to surfaces

Obtaining Carbon and Energy (1)

• Autotrophs (auto = self; troph = nourishment) – Use carbon dioxide as their carbon source

• Heterotrophs – Obtain carbon from organic molecules

Obtaining Carbon and Energy (2)

• Chemoautotrophs – Obtain energy by oxidizing inorganic or

organic substances

• Phototrophs – Use light as energy source

Fig. 25-8, p. 531

CO

2

PHOTOHETEROTROPH

Found in somephotosyntheticbacteria

Found in some photo-synthetic bacteria, insome protists, andin plants

PHOTOAUTOTROPH

Light

Include some bacteriaand archaeans, andalso in protists, fungi,animals, and plants

Found in some bacteriaand archaeans; notfound in eukaryotes

CHEMOHETEROTROPH

CHEMOAUTOTROPH

Oxidation of molecules

Energy source

Org

anic

mo

lecu

les

Car

bo

n s

ou

rce

* Inorganic molecules for chemoautotrophsand organic molecules for chemoheterotrophs.

Prokaryotes and Oxygen: Aerobes

• Aerobes– Require oxygen for cellular respiration– Oxygen is the final electron acceptor

• Obligate aerobes – Cannot grow without oxygen

Prokaryotes and Oxygen: Anaerobes

• Anaerobes– Do not require oxygen to live

• Obligate anaerobes (poisoned by oxygen)– Use fermentation or type of respiration in which

inorganic molecules (NO3– or SO4

2–) are final electron acceptors

• Facultative anaerobes – Use O2 when present– Use fermentation under anaerobic conditions

Prokaryotes and Nitrogen (1)

• Nitrogen fixation

– Conversion of atmospheric nitrogen (N3) to ammonia (NH3)

– Ammonia ionized to ammonium (NH4+) for biosynthesis

• Nitrogen-fixing bacteria include

– Some cyanobacteria

– Free-living Azotobacter

– Bacteria such as Rhizobium that are symbiotic with plants

Prokaryotes and Nitrogen (2)

• Nitrification– Conversion of ammonium (NH4

+) to nitrate (NO3

–)– Two-step conversion by nitrifying bacteria

• Some types of bacteria convert ammonia to nitrite (NO2

–)• Other types convert nitrite to nitrate

Prokaryote Reproduction

• Binary fission– Asexual reproduction– Produces exact copies of parent

• Conjugation– Two cells connected by pilus– Part of DNA of one cell is transferred to

another cell (usually plasmids)

Endospore

• Develops inside some bacteria when environmental conditions are unfavorable

• Metabolically inactive

• Highly resistant to heat, desiccation, attack by enzymes or chemicals

Endospore: Clostridium tetani

25.2 Domain Bacteria

• Molecular studies reveal more than a dozen evolutionary branches in the Bacteria

• Bacteria cause diseases by several mechanisms

• Pathogenic bacteria commonly develop resistance to antibiotics

Classification of Prokaryotes

Bacteria

• 12 separate evolutionary branches

• Six most important groups:– Proteobacteria– Green bacteria– Cyanobacteria– Gram-positive– Spirochetes– Chlamydias

The Proteobacteria (1)

• Gram-negative bacteria– Purple sulfur (photoautotrophic)– Purple nonsulfur (photoheterotrophic)– Purple photosynthetic pigment

• Free-living proteobacteria (chemoheterotrophs)– Some cause human diseases

• Bubonic plague, Legionnaire’s disease, gonorrhea, gastroenteritis, dysentery

– Some plant pathogens• Rot, scabs, wilts

The Cyanobacteria

• Gram-negative photoautotrophs

• Blue-green color

• Photosynthesis similar to plants

• Release oxygen as byproduct of photosynthesis

Fig. 25-13, p. 535

Heterocystc.

b.

Resting spore

The Gram-Positive Bacteria (1)

• Primarily chemoheterotrophs• Many pathogenic species

– Anthrax– Staphylococcus

• Food poisoning, skin infections, toxic shock syndrome, pneumonia, meningitis

– Streptococcus• Strep throat,

pneumonia, scarlet fever, kidney infections

The Gram-Positive Bacteria (2)

• Some beneficial species– Lactobacillus

• Lactic acid fermentation used to produce pickles, sauerkraut, yogurt

• Mycoplasmas– Naked cells that have lost their cell walls– Smallest known cells (0.1 to 0.2 µm in diameter)

The Spirochetes

• Gram-negative spiral-shaped bacteria– Propelled by rotation of flagella– Enables movement in thick mud and sewage

• Beneficial or harmless species– Spirochetes in termite intestine digest plant fiber– Treponema in mouth

• Pathogenic species– Syphilis, relapsing fever, Lyme disease

The Chlamydias

• Gram-negative bacteria– Cell walls with membrane outside– Lack peptidoglycans

• Intercellular parasites that cause diseases in animals

Bacterial Disease Mechanisms

• Exotoxins

– Toxic proteins leaked or secreted

• Clostridium botulinum (botulism exotoxin)

• Endotoxins

– Toxins only released when bacteria die or lyse

• E. coli, Salmonella, Shigella

• Exoenzymes

– Enzymes secreted that digest plasma membrane

• Streptococcus, Staphylococcus, Clostridium

Resistance to Antibiotics

• Pathogenic bacteria may develop resistance to antibiotics– Mutation of their own genes– Acquiring resistance genes from other bacteria

• Resistant strains difficult to treat with conventional antibiotics

• Resistance is a form of evolutionary adaptation

25.3 Domain Archaea

• Archaea have some unique characteristics

• Molecular studies reveal three evolutionary branches in the Archaea

The Archaea

• Archaea are more closely related to domain Eukarya than domain Bacteria

• Characteristics– Some features like bacteria– Some features like eukaryotes– Some unique features

Characteristics of Bacteria, Archaea,

and Eukarya