prokaryotes
very small is diameters 1-5 um
very numerous
-more prokaryotes in a handful of soil than total number of people who have every lived
they are able to tolerate a variety of extreme conditions
-acidic, basic, salty, hot, cold
bacteria cause half of all human diseases
-pathogens are disease causing agents
hundreds of species of beneficial bacteria in the human body
-skin, gut
bacteria and archaea
prokarotes used to be one group referred to as bacteria
studies of rRNA sequences of prokaryotes in the 1970s revelaed that many prokaryotes first classified as bacteria were actually more similar to eukaryotes
-these bacteria were then placed in a domain of their own referred to as archaea
-further comparisons revealed that some ancheael genes are more similar to eukaryotes, others are ore similar to prokaryotes, and others unique to archaea
curretn hypothesis: archaea and eukaryotes evolved from a common ancestor
differences in between bacteria and archaea
some key differences between bacteria and archaea include:
-bacteria have peptidogylan and archaea do not
this means that bacteria repsond to cell wall active antiboiotics archaea do not
-archaea have a number of different RNA polymerses bacteria have only one
archaea does not cause disease and can withstand extreme conditions
bacteria can withstand only moderate conditions
prokaryotic shapes
prokaryoktes come in a variety of shapes:
- cocci: sherical shape further characterized by arrangement
staphylococci: grape- like clusters of circles
streptococci: chains of circles - bacilli: rod- shaped - usually as a single rod
diplobacilli: two rods together
streptobacilli: rods arranged in a chain - spiral: helical shape
spirilla: short and rigid helical structure
spirochete: long, more flexible helical structure
bacterial structures
- bacterial cell wall
- capsule
- pili
- flagella
- bacterial reproduction
- endospores
- internal structures
bacterial cell wall
-composed of sugar called peptidoglcan
-maintians cell shape, provides physcia; protection, and prevents cell from bursting in hypotonic environments
-hypertonic environments cause bacteria to shrivel-inhibiting reproduction.. explains why salt can be used in food preservation
there are two main types of bacterial cell walls:
a) gram positive: thick layer of peptidogylan
b) gram negetive: thin layer of peptioglycan with outer memebrane
these two types of stain differently during gram stain- an identification tool used in microbio lab
-pos stain purple
-neg stain pink
capsule
- a sticky layer of polysaccaride or protein covering the cell wall
found in many but not all prokaryotes
allows the bacterium to adhere:
-to other organisms
-to medical devices such as catheters
-to other surfaces
the capsule also acts to shield the mirco organisms from host defenses
pili
-hair like appendeges
allows bacterium to adhere to one another or to thier substance
-allows adherence to rocks in streams or the lining of human intestine
-specialized pili called sex pili allow bacteria to stick to one another and transfer DNA
flagella
- this structure enables bacterial motility
- not present in all bacteria
- flaggellated bacteria may have one flagella, two flagella or many scattered all over the surface
- allows the bacteria to move towards the chemical and light stimuli called attractants away from repellent stimuli
bacterial reproduction
- replication occurs very quickly by a process called binary fission 1-2-4-8-16
- some species can reproduce in as little as 20 minutes others take 1-3 hours
- growth is limited:
- nutrients become exhausted, metabolic wastes build up
- competition from other organisms
- antibiotics are present in certain environments
endospores
- structure formed by some gram positive organisms only
- used to withstand harsh conditions
- outer cell will disintergrate and the inner cell will remain
- endospore has thick outer coat
- will dehydrate and become dormant until conditions become favourable
- can withstand heat, and cold
- when conditions are favorable it will absorb water and resume growth
internal membranes and internal structures
- prokaryotes have much simpler internal structure and genome than eukaryotes
- have specialized membranes to preform metabolic functions
- respiratory membrane: for cellualr respiration
- thylakoid membrane: for photosynthesis
- used in cyanobacteria
- prokaryotes have 1/1000 as much DNA as a eukaryote
- fewer genes highlight the simplicity of the cells required activities
- cells may have plasmids: smaller pieces of DNA acquired from other bacteria which contain specialized genes
ex) antibiotic resistance genes - prokaryotes have smaller ribosomes (70s) than eukarotes (80s)
- this difference in ribosomal size and structure make them an important antibiotic target
metabolic diversity of prokaryotes
two types of metabolic resources:
prokaryotes require two main metabolic resources :
- a source of energy
- a source of carbon
source of energy
prokaryotes can either use light or chemicals
- light: phototroph
- chemicals: chemotroph
source of carbon
some prokaryotes make their own carbon compounds while others require pre-formed carbon
- autotrophs: use an inorganic carbon dioxide is used to form organic carbon such as glucose
- this process is called photosynthesis - heterotrophs: obtain their carbon from organic compounds such as glucose
mode of nutrition
based on the source of energy a prokaryote uses and the form of carbon that it uses to ontian organic carbon, prokaryotes can be assigned a mode of nutrition, there are four classifications
- photoautotrophs
- chemoautotrophs
- photoheterotrophs
- chemoheterotrophs
biofilms
- bacteria lives as individual cells (stand alone)
- they may also live in surface coating colonies called biofilms
- they form when bacterial cells secrete signaling molecules which recruit nearby cells
- when the colony reaches a sufficient size the bacterial cells of the colony begin producing protiens that cause them to stick to a particular surfcae and to each other
- there are channels within the biofilm which all nutrients and wastes to leave
- in humans:
- biofilms allow bacteria to adhere to teeth causing cavity (dental carries)
- they also form on catheters and other medical devices causing serious problems
- -they are benefical when used in water sewage treatment facilities
archaea live in extereme environements
- archeaea are able to live in environments where few other organisms are able to survive
- they have unsusual protiens and other molecular adaptations that allow them to survive and reproduce where few other organisms can
- there are three main groups of archaea formed on the basis of the environemtn in which they are found:
1. extereme halophiles
2. extereme thermophiles
3. methanogens
extreme halophiles
these archaea that thrive in waters with high salt concentrations
-15-30% salt concentration
(sea water = 3% salt concentration)
extereme thermophiles
archaea that thrive in water of very high temps
some live near deep ocean vents where temps exceed 100 degrees
methanogens
archaea that live in anaerobic environments
produce methane as a waste product
can be found in anaerboic mud at the bottom of lakes and swamps
these also live in the intestinal tract of humans
responsible for human gas which results from methanogen metabolism
even though archaea are dubbed exteremeophiles they are also abundant in moderate environments as well (versitle)
bacteria are diverse
the domain bacteria is divided into 9 groups: (only 5) - proteobacteria -chlamydias -spirochetes -the gram positive -cynobacteria
proteobacteria
composed of gram negative organisms
5 subgroups:
alpha: including rhizobium species which live in plant roots and fix nitrogen from the atmosphere fro the plant
-gramma: including salmonella, ecoli, and photosynthetic sulfur bacteria that oxidize H2S
-delta: includes slime secreting myxobacteria which form elabroate colonies and bdellovirbiro which attack other bacteria
chlamydias
these bacteria live inside ekaryotic cells
chlamydia trachomatis causes blindess in developing areas and is responsible for most prevalent sexually transmitted infection
