Prokaryotic Cell Structure Bacteria shape and size Bacteria

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 • (Mg 2+, Ca 2+) 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 β 1 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 appendages Smaller than flagella Adhere bacteria to surfaces Used in conjugation for Exchange of genetic information • Aid Flotation by increasing buoyancy

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 50 S and 30 S

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 cell Arrangement 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 successive stages • Preparatory stage • Forespore stage • Stage of cell wall formation • Maturation stage occurs in four

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 Hyphae 3. Agar Surface 4. 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 r. RNA 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