chapter 22 prokaryotes: bacteria and archaea

prokaryotes

-first organisms on earth ~3.5-3.8 BYA

-millions of species (named or not), found everywhere

-live on and in every other living organism

-most benign and many essential to all life

-the size of prokaryotic cells led to their being undiscovered for most of human history

two technology strands that allows study for microbes

-microscopy for visualization

→electron microscopes allowed the study of cell substructure

-infectious disease investigations

history of microbiology

-german physician Robery Koch is credited with discovering the techniques for pure culture. including staining and using growth media

growing prokaryotes in the lab

-culture medium

-pure culture

culture medium

-contains all the nutrients needed by the target microorganism, can be liquid (broth) or solid

→after an incubation time at the right temperature, there should be evidence of microbial growth in the culture medium

pure culture

a laboratory culture containing a single species of microorganism

Robert Koch

-proposed four postulates to prove a casual relationship between a microorganism and an individual

Koch’s postulates:

1. the microorganism must be found in abundance in all organisms suffering from disease, but should not be found in healthy organisms.

2. the microorganism must be isolated from a diseased organism and grown in pure culture

3. the cultured microorganism should cause disease introduced into a healthy organism

4. the mircoorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent

prokaryote structure: cell size

-prokaryotic cells (0.1-5.0 um in diameter) are significantly smaller than eukaryotic cells (10-100 um diameter)

-the predominantly single-celled organisms of the domains Bacteria and Archaea are classified as prokaryotes

prokaryote structure-Shape

-many different shapes, but most fall into 3 main:

1. cocci

2. bacilli

3. spirilli

prokaryotes structure 2

often occur in characteristic aggregates (pairs, chains, tetrads, clusters, etc)

cell structure

-all cells have four common structures:

1. the plasma membrane

2. the cytoplasm

3. the double stranded DNA genome

4. ribosomes

the plasma membrane

functions as a barrier for the cell and separates the cell from its enviornment

the cytoplasm

a complex solution or organism molecules and salts inside the cell

double-stranded DNA genome

the informational archive of the cell

ribosomes

sites of protein synthesis

for prokaryotes:

-almost all have a cell wall

-many have capsule or slime layer

-some have hairlike pili: fimbriae or sex pilus

-many have flagellum

-may have plasmids

-NO membrane-bound organelles

→no nucleus: DNA in nucleoid

→ribosomes “free”

-no microtubules

cell wall

-almsot all pro have cell wall

-lies outside plasma membrane

-protects and prevents cell lysis (rupture)

-bacteria: w/ peptidoglycan (PG)

-archea: w/ other structural polysaccharides (do not have peptidoglycan)

gram positive bacteria

-gram-stain reflects cell wall type

-gram positive +: bacteria w/ thick PG layer

-stains purple in Gram stain

gram negative bacteria

-gram negative: bacteria with thinner PG layer plus outer lipid bilayer membrane

-pink in gram stain; outer lipopolysaccharide layer often toxic, resists drugs and immune system

capsule or slime layer

-many pro have

-sticky carbs and proteins secreted outside cell wall

-adheres (glues) cells together or to surface

-resists attack from immune system

-holds in moisture

fimbriae

-some have hair-like protein called fimbriae (short pili)

-helps cells stick to surfaces and each other

sex pilus

-many form sex pilus (pl. pili) pulls two bacteria cells together for DNA transfer (conjugation)

-forms mating bridge

flagellum

-not homologous to eukaryote flagellum

-allows movement!

Movement

-many are capable of movement (motility)

-taxis=movement directed toward or away from (+ or -) stimulus

-chemotaxis: chemical

-phototaxis: light

-geotaxis/magnetotaxis: gravity, towards something

plasmids

-extra tiny DNA ringss w/ few genes

-replicated independently

-not essential for life, but add diversity

→e.g. drug resistance genes