Introduction to Prokaryotes
Two domains
Bacteria
Archaea
Definition
A group of organisms that lack a cell nucleus or any other membrane-bound organelles
Most are unicellular except a few prokaryotes (e.g. Mycobacterium have multicellular stages in their life cycles
Size, shape and arrangement
Size: 0.2 to 2.0 µm in diameter and 2 to 8 µm in length
Three basic shapes
Coccus
Bacillus
Spiral
Arrangement
Cocci (s. coccus) - spheres
Bacilli (s. bacillus) - rod shape
Spirillum: rigid spiral shape
Diplococci - pairs
Streptococci - chains
Staphylococci - grape-like clusters
Tetrads - 4 cocci in a square
Sarcinae - cubic configuration of 8 cocci
Diplobacilli - Two bacilli
Streptobacilli - chains of bacilli
Palisades - rods side by side or in X, V or Y figures
Spirochete: flexible & undulating spiral shape
Additional shapes
Unusual
Star-shapped
Square
Triangular
Pleomorphic
Within a population, various shapes (Corynebacteria)
No fixed shape
Cellwall-less: Mycoplasma; Ureaplasma
Cell structure
External structures
Glycocalyx
Flagella
Axial filaments (endoflagella)
Fimbriae and Pili
Surrounds the cell
Made inside the cell and excreted to the surface
Called capsule when it is organized & firmly attached to the cell wall
Called slime layer when it is unorganized & only loosely attached to the cell wall
Made of sugars called an extracellular polysaccharide (EPS)
Functions
Protection from phagocytosis
Attachment to various surfaces
Source of nutrients
Protect a cell against dehydration
Threadlike, locomotor appendages extending outward from plasma membrane & cell wall
Functions
Motility & swarming behaviour
Attachment to surfaces
May be virulence factors
Have different numbers & arrangements
Monotrichous - one flagellum
Polar flagellum - flagellum at end of cell
Amphitrichous - one flagellum at each end of cell
Lophotrichous - cluster of flagella at one or both ends
Peritrichous - spread over entire surface of cell
Types of motility
'Run' / 'Swim' - moves in one direction for a length of time
'Tumbles' - runs interrupted by periodic, abrupt, random changes in direction
'Swarm' - rapid wavelike movement
Bundles of fibrils that arise at the ends of the cell beneath the outer sheath
Spiral around the cell
Present in a spirochetes group of bacteria
Hairlike appendages that are shorter, straighter and thinner than flagella
Consist of a protein called pilin arranged helically around a central core
Used for attachment rather than for motility
Two types
Fimbriae
Can occur at the poles of the bacterial cell
Can be evenly distributed over the entire surface of the cell
Can number anywhere from a few to hundred per cell
Pili
Longer than fimbriae
Only one or two per cell
Pili joint bacterial cells in preparation for the transfer of DNA from one cell to another
Sometimes are also called sex pili
Cell wall
Surrounds the cytoplasmic membrane
Not a regulatory structure like cytoplasmic membrane
Not selectively permeable - anything that can fit, can pass through the cell wall
Functions
Prevent bacterial cell from rupturing when the water pressure inside the cell is greater than that outside the cell
Contributes to pathogenicity
Classification
Maintains characteristics shape
Provides a rigid platform (a point of anchorage)
Counters the effects of osmotic pressure
The cell wall of all bacteria are not identical
Cell wall composition is an important factor in analysis & differentiation of bacteria
Clinically the cell wall
Contributes to ability to cause disease
Is a site of action for antibiotics
2 major types of walls
Gram-positive
Gram-negative
Thick peptidoglycan layer
90% of the cell wall is peptidoglycan
Consist of many layers of peptidoglycan
Periplasmic space of Gram-positive bacteria
Lies between plasma membrane and cell wall & is smaller than that of Gram-negative bacteria
Periplasm has relatively few proteins
Enzymes secreted by Gram-positive bacteria are called exoenzymes - aid in degradation of large nutrients
Techoic acid
Primarily of an alcohol & phosphate
Negatively charged (from the Phosphate group)
Provide much of the wall'antigenic specificity
For classifying species of Gram-positive bacteria
Makes the Gram-positive cell wall acidic
Cell walls do not degrade as easily
Do not contain outer membrane
Two classes of techoic acids
Lipoteichoic acid (spans the peptidoglycan layer & is linked to the plasma membrane)
Wall techoic acid (link to the peptidoglycan layer)
Consist of one or very few layers of peptidoglycan (more susceptible to mechanical breakage)
The peptidoglycan is bonded to lipoprotein
Do not contain teichoic acids
Peptidoglycan layer
10% of cell wall
bonded to lipoproteins
located between outer membrane & the cytoplasmic membrane
area called the periplasmic space
Because of the minimal amount of peptidoglycan, Gram-negative cells are more vulnerable to mechanical breakage
Periplasmic space
contains a high concentration of degrading enzymes
large number of transport proteins
20-40% of cell volume
Consist of outer membrane
consists of lipoproteins, lipopolysaccharides (LPS), phospholipids & porins
strong negative charge (evade phagocytosis)
provide a barrier to certain antibiotic (penicillin), lysozyme, detergent, heavy metals, bile salts & certain dyes
Porins form channels that permit the passage of molecules (e.g. nucleotides, disaccharides, peptides, amino acids)
LPS (lipopolysaccharide)
The polysaccharide portion is composed of sugars called O polysaccharides
function as antigens
useful for distinguishing species of Gram-negative bacteria
this role is comparable to that of teichoic acids in Gram-positive bacteria
The lipid portion of the LPS called Lipid A
referred to as endotoxin
is toxin when in the host's bloodstream or gastrointestinal tract which causes fever and shock
Unusual features of peptidoglycan
D-alanine & D-glutamic acid used
N-acetylmuramic acid & diaminopimelic acid only found in Bacteria, not Archaea or Eukarya
DAP mostly found in Gram-negative bacteria
Peptidoglycan layer is porous, unlike a membrane
Atypical cell walls
Mycoplasma
Have no walls or have very little wall material
Membrane contains sterols, which impart rigidity to the membrane
Smallest known bacteria
Can pass through most bacterial filters
Plasma membranes are unique in having lipids called sterols which protect them from osmotic lysis
Chlamydiaceae (Chlamydia, Chlamydophila)
Two membranes, like Gram
Some genes for peptidoglycan synthesis found in genome
Obligate intracellular parasites
Non-replicative elementary body (EB) form has extensive crosslinking in outer membrane proteins; analogous to spore
Archaea
Lack peptidoglycan in their cell walls
Archaea
Some species have cell walls consisting of polysaccharide, glycoprotein, or protein but not peptidoglycan
However contain a substance similar to peptidoglycan called pseudopeptidoglycan (pseudomurein)
Most common wall type is paracrystalline surface layer (S-layer) made of protein or glycoprotein with hexagonal symmetry
Archaea are naturally resistant to lysozyme & penicillin
S-layer
Surface or outermost layer forming 'lipidless membrane'
Found on Archaea, a few Gram-positive & Gram-negative
Composed of repeating subunitss of protein or glycoprotein
Protects against osmotic stress, pH, enzymes
May aid in attachment and inhibit phagocytosis
Cell wall of Archaea
Gram-negative
A layer of thick protein or glycoprotein outside plasma membrane
Gram-positive
Pseudomurein (in methanogens)
L-amino acid
N-acetylalosaminuronic acid (not N-acetylmuramic acid as in peptidoglycan)
Unique feature
Surface with protein or glycoprotein subunits
Outer membrane
Complex peptidoglycan network
Unique feature
Single thick homogenous layer
Unique feature
Resistant to lysozyme & B-lactam antibiotics
Bacteria vs Archaea membrane
Archaea contain ether-linked lipids
No fatty acids but isoprene (5-carbon hydrocarbon) in Archaea
Some Archaea have lipid monolayers