Prokaryotic Cell Structure

Bacteria – shape and size

• Bacteria are believed to be the first cell to evolve – have no clear membrane bound nucleus or organelles

• Bacteria vary in size and shape• Coccus• In pairs diplococcus. eg. Neisseria.sp. • Long chains – Streptococcus sp. • Irregular grape like clumps - Staphylococcus sp. • Tetrads eg. Micrococcus sp.

Epulopiscium fishelsoni grows as large as 600 µm by 800 µm, a little smaller than a printed hyphen. Exceptionally large bacteris

• Bacillus: rod shape eg. Bacillus spp. – Coccobacilli

– The shape of the rod’s end often varies

– Some bacteria form long multinucleate filaments – eg. Actinomycetes

• Sprillum: long rods twisted into spirals or helix– spirilla (rigid) spirochetes (if flexible)

• Size: – Mycoplasma are only 100 - 200 nm in diameter

– E. coli is 1.1 to 1.5 µm wide by 2.0 to 6.0 µm. long,

– Spirochetes - size reaches 500 µm in length

Structure and function of prokaryotes

• Membrane systems • prokaryotic and eukaryotic membranes are

similar in structure• Membranes of eukaryotic microorganisms serve

to compartmentalize cell contents into organelles

• Prokaryotic organisms contain only a single membranous structure, cytoplasmic mebrane or plasma membrane

• measures 4 – 5 nm thick

• permeability barrier of the cell• involve in complex biochemical processes

respiration• membranes are formed of a lipid bilayer, made

of phospholipids• fatty acid portion hydrophobic, glycerol

phosphate part hydrophilic• hydrophilic parts are exposed to the aqueous

external environment

• the inner and outer sides of the cytoplasmic membrane have different properties

• property of ‘sidedness’ is of great importance• overall structure of a membrane is maintained by

hydrogen bonds and hydrophobic interactions• (Mg2+, Ca2+) help to stabilize the structure• Eucaryotic membranes differentiated from those

of prokaryotes with sterols• Mycoplasmas, contain sterols

Plasma Membrane• Layer of phospho-lipids

and proteins that separates cytoplasm from external environment.

• Regulates flow of material in and out of cell.

Cell walls

• Bacterial cell wall is unique two broad categories, Gram positive and gram negative

• Gram positive bacteria have a thick, single layered wall. Gram negative - complex multilayered wall thin

• Peptidoglycan layer is present in the cell walls• In Gram positive bacteria, bulk of the wall is

peptidoglycan Gram-negative it accounts for only the innermost layer

• Peptidoglycan consists N-acetylmuramic acid (NAM) and N-acetylgucosamine (NAG) linked by bonds described as β1-4 linkages

• Gram positive cell walls contain another polymer called teichoic acid

• Mycobacterium, Corynebacterium contain waxy esters of mycolic acids

Cell Wall

• Rigid peptidoglycan - polysaccharide coat that gives the cell shape and surround the cytoplasmic mem-brane. Offers protection from environment.

Bacterial cell surface (Fimbriae and Pili)

• Some bacteria possess additional hair like structures called fimbriae

• shorter than flagella but numerous• to stick to a surface• Pili – specialised pili - conjugation process

• Glycocalyx (Slime / capsule) • Glycoclyx consists of polysaccharides, with

glycoprotein• Hinders the engulfing (phagocytosis)• Also prevents desiccation.

PILI

• Short protein appendagesShort protein appendages• Smaller than flagellaSmaller than flagella• Adhere bacteria to surfacesAdhere bacteria to surfaces• Used in conjugation for Exchange of Used in conjugation for Exchange of

genetic informationgenetic information• Aid Flotation by Aid Flotation by increasingincreasing buoyancybuoyancy

Nucleoid

• Region of the cytoplasm where chromosomal DNA is located. Usually a singular, circular chromosome.

• Smaller circles of DNA called plasmids (extra chromosomal DNA) are also located in cytoplasm.

Nucleoid

• Prokaryotic DNA is in circular form• lack a nuclear envelope• Bacterial DNA not associated with proteins• The DNA is highly coiled• Plasmids• extrachromosdmal circular DNA

Ribosomes

• Translate the genetic code into proteins.

• Free-standing and distributed throughout the cytoplasm.

• Bacterial ribosomes have two sub units 50S and 30S

Flagella

• hair-like structures called flagella• (14 – 20 nm diameter) rotate like a ship’s propeller• protein called Flagellin, flagellar subunits• basal body rotates the flagellum to cause movement

of the cellArrangement of flagella

• Monotrichous: Eg. Vibrio cholerae• Amphitrichous : Eg. Spirillum volutans• Lophotrichous: Eg. Alcalegenes faecalis• Peritrichous: Eg. E.coli

Monotrichous Lophotrichous

Amphitrichous Peritrichous

Chemotaxis and Motility

• Chemotaxis is the movement of an organism towards or away from a chemical

• Positive chemotoxis movement towards a chemical (attractant); negative chemotaxis movement away from a chemical (repellent)

• Bacterial movement is characterized by runs and tumbles

• when an attractant present it is marked by larger runs and less frequent tumbles

Mesosome

• Infolding of cell membrane.

• Possible role in cell division.

• Increases surface area.• Photosynthetic

pigments or respira-tory chains here.

• Http://www.med.sc.edu:85/fox/protobact.jpg

Other structures

• Inclusion bodies for storage of materials• Poly-β – hydroxybutyric acid (PHB), Granules of

polyphosphate, volutin granules matachromatic granules generation of ATP and other cell costitutions

Bacterial endospores

• Bacterial endospore is not a reproductive structure

• Resistant to harsh environmental conditions

• Bacillus and Clostridium produce endospores

• Endospore is more complex than the vegetative cell

• Dipicolinic acid (DPA)• Sporulation occurs due to

environmental stress

Spore formation

• The sporulation process occurs in four successive stages

• Preparatory stage• Forespore stage • Stage of cell wall

formation• Maturation stage

Other Prokaryotes • Actinomycetes (The Filamentous Bacteria) • aerobic, high G-C percentage gram -positive bacteria

form branching filaments or hyphae and asexual spores

• closely resemble fungi in overall morphology• aerial hyphae, substrate hyphae• Septa • aerial hyphae reproduce asexually• Most actinomycetes are non-motile• they break down hard organic materials like

newspaper

Growth of Actinomycetes on agar plate

1. Chain of Conidiospores (Conidia) 2. Aerial Hyphae3. Agar Surface4. Substrate Hyphae

Spirochaetes

• Gram-negative bacteria, long, helically coiled (spiral-shaped) cells.

• Chemoheterotrophic lengths between 5 and 250 µm diameters around 0.1-0.6 µm

• Flagella called axial filaments, cell membrane and outer membrane

• cause a twisting motion which spirochaete will undergo asexual transverse binary fission

• Most spirochaetes are free-living and anaerobic

• Classification three families (Brachyspiraceae, Leptospiraceae, Spirochaetaceae),

• Disease-causing members of this phylum Leptospira species, Borrelia burgdorferi, Borrelia recurrentis, Treponema pallidum

Spirochaetes

Treponema pallidum spirochetes

Cyanobacteria

• Cyanobacteria , blue-green algae, blue-green bacteria obtain their energy through photosynthesis

• significant component of the marine nitrogen cycle and an important primary producer

• The cyanobacteria were classified into five sections, I-V.

• Chlorococcales, Pleurocapsales, Oscillatoriales, Nostocales and Stigonematales

Mycoplasma

• Mycoplasmas lack a cell wall• unaffected by many common antibiotics such as

penicillin beta-lactam antibiotics• parasitic or saprotrophic• pathogenic in humans, M. pneumoniae, • Mycoplasma is by definition restricted to

vertebrate hosts• Cholesterol is required for the growth

Cell wall structure

• M. pneumoniae cells are of small size and pleomorphic

• Mycoplasmas are unusual among bacteria – possess sterols for the stability of their cytoplasmic membrane

• low GC-content

Rickettsiae

• Rickettsia is a genus of motile, Gram-negative, pleomorphic bacteria present as cocci, rods thread-like

• Obligate intracellular parasites, survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells

• cannot live in artificial nutrient environments are grown either in tissue or embryo cultures

• Rickettsia carried as parasites by

cause diseases typhus, rickettsialpox, Boutonneuse fever, African Tick Bite Fever, Rocky Mountain spotted fever, Australian Tick Typhus

Archaebacteria (Archaea)

• Archaea although look like bacteria are not closely related to them

• divided into two evolutionary lineages based on rRNA sequences, crenarchaeotae, Euryarcheotae

• Crenarchaeotae grow at high temperatures and metabolize elemental sulfur

• Euryarchaetoes are methanogens some grow aerobically very high concentrations of salt

• Archaea possess membrane lipids of branched-chain hydrocarbons bound to one or two glycerol molecules by ether bonds

ARCHAEA

Halobacteria sp