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Prokaryotes The bacterium Clostridium butyricum, one of the Clostridium species that produces the toxin botulin (colorized TEM). Learning Objectives • Describe the two domains of prokaryotes. • Describe the unique characteristics of prokaryotes and their metabolic diversity. • Discuss how prokaryotes reproduce. • Give specific examples of pathogens that cause human illness and how they do so. • Understand the role between natural selection and antibiotic resistance in bacteria. • Explain why horizontal gene transfer could affect the phylogenetic tree of prokaryotes. 2 Eubacteria Familiar, a few members cause human disease Live almost anywhere Archaebacteria Similar to Eubacteria in size, shape, and cellular content but more closely related to eukaryotic cells than traditional bacteria Many occupy extremely harsh environments A.#70×#magnifica.on# B.#350×#magnifica/on# C.#14,000×#magnifica/on# Plasmid$ Pili$ Folded$DNA$ molecule$ (in$the$nucleoid)$ Cytoplasm$ containing$ ribosomes$ Flagellum$ Capsule$ Pep>do?$ Plasma$ membrane$ glycan$layer$ Outer$ membrane$ Cell$wall$ Prokaryotes have no membrane-bound nucleus or organelles – their DNA and some proteins are localized in the cytoplasm They vary in how their cell membrane is protected – some species have specialized surface structures that protect them from their environment or that enable them to move Prokaryotes also show great diversity in the ways they obtain energy and in their metabolic activities Prokaryotic cells are bounded by a plasma membrane, surrounded by a cell wall Prokaryotes lack membrane-bound organelles – genetic material is localized to a non-membrane enclosed area Prokaryotic cytoskeletons maintain cell shape, function in cell division, and sometimes determine polarity of the cell Most can use a variety of substances as energy and carbon sources Prokaryote Shape • Three shapes are common among prokaryotes: • Spherical prokaryotes are cocci • Cylindrical or rod-‐shaped prokaryotes are bacilli • Spiral prokaryotes are spirilla (corkscrew-‐shaped) and vibrios (comma-‐shaped) • Some live singly, others link into chains or aggregates of cells A.#Cocci# B.#Bacilli# C.#Spirilla# Prokaryotic Cell Wall Plasma membrane surrounded by layer of peptidoglycan cross-linking chains of polypeptides and sugar polymers Gram Stain is used to classify prokaryotes Gram Positive – Large amounts of peptidoglycan = stain violet Gram Negative – Less peptidoglycan, 2°membrane of polysaccharide = stain pink Outer membrane contains Lipopolysaccharides that add to virulence, protection from stomach acid Antibiotics like Penicillin target peptidoglycan cross links Prokaryotic Cell Wall Plasma membrane surrounded by layer of peptidoglycan cross-linking chains of polypeptides and sugar polymers Gram Stain is used to classify prokaryotes Gram Positive – Large amounts of peptidoglycan = stain violet Gram Negative – Less peptidoglycan, 2°membrane of polysaccharide = stain pink Outer membrane contains Lipopolysaccharides that add to virulence, protection from stomach acid Antibiotics like Penicillin target peptidoglycan cross links A.#Gram(posi-ve#bacterial#cell#wall# Pep-doglycan#layer# # #Plasma#membrane# Cell# wall# Cytoplasm# B.-Gram3nega$ve-bacterial-cell-wallCapsuleOuter-membranePep$doglycan-layerPlasma-membraneCytoplasm- Cellwall- Prokaryotic Cell Walls • Most pathogenic bacteria are Gram-‐negaBve species – their outer membrane protects them against the body’s defense systems and blocks the entry of drugs such as anBbioBcs • Bacterial cells walls may be coated with an external layer of polysaccharides (glycocalyx) that may be loosely associated (slime layer) or firmly aIached (capsule) Flagella • Movement through liquids and across wet surfaces occurs by the acBon of flagella • A bacterial flagellum consists of a helical fiber of protein that rotates in a socket in the cell wall and plasma membrane • Archaeal flagella have a different structure and mechanism Bearings( Rod( Motor( Motor( protein( Flagellum( Cytoplasm(( Plasma( membrane( Outer( membrane( Pep3doglycan( layer( Figure 26.6 A flagellum of a Gram-negative bacterium. The ring of motor proteins that surround the motor provide torque to rotate the flagellum in a counterclockwise direction. Bearings Rod Motor Energy for rotation comes from a proton (H1) gradient. ! Pili • Some bacteria and archaeans have rigid, hairlike shaNs of protein (pili) extending from their cell walls • Among bacteria, pili are characterisBc primarily of Gram-‐negaBve bacteria • Some types of pili allow bacterial cells to adhere to other cells, or transfer geneBc material (sex pili) Prokaryotic Metabolic Diversity • All organisms take in carbon: • Autotrophs obtain carbon from CO2 (inorganic) • Heterotrophs obtain carbon from organic molecules • All organisms take in energy: • Chemotrophs obtain energy by oxidizing inorganic or organic substances • Phototrophs obtain energy from light Prokaryotic Metabolic Diversity • All organisms take in carbon: • Autotrophs obtain carbon from CO2 (inorganic) • Heterotrophs obtain carbon from organic molecules • All organisms take in energy: • Chemotrophs obtain energy by oxidizing inorganic or organic substances • Phototrophs obtain energy from light 16 Prokaryotic Metabolism • Aerobes require oxygen for cellular respiraBon • Obligate aerobes cannot live without oxygen • Anaerobes do not require oxygen to live • Obligate anaerobes are poisoned by oxygen – they survive either by fermentaBon or anaerobic respiraBon • Faculta<ve anaerobes use O2 when it is present, but can live by fermentaBon under anaerobic condiBons Nitrogen Fixers • Some bacteria and archaeans are able to reduce atmospheric nitrogen (N2) to ammonia (NH3) by nitrogen fixa<on • Ammonia is ionized to ammonium (NH4+), which the cell uses to produce nitrogen-‐containing molecules such as amino acids and nucleic acids • Some bacteria carry out nitrifica<on, the conversion of ammonium (NH4+) to nitrate (NO3–) • Plants and fungi incorporate nitrate into organic molecules Nitrogen fixation by prokaryotes is the only means of replenishing the nitrogen sources used by most microorganisms and by all plants and animals Asexual Reproduction • Prokaryotes normally reproduce by asexual reproduction • A parent cell divides by binary fission into two daughter cells that are exact genetic copies of the parent Binary Fission – Asexual - produces identical offspring As short as 20 minutes for E.Coli Bacterial genome has high mutation rate 1 out of 10,000,000 (in 1 gene); many mutations can occur in 1 day Responsible for genetic diversity and adaptation Horizontal Gene Transfer • Horizontal gene transfer is the movement of geneBc material between organisms by means other than descent • In conjuga/on, two parent cells “mate”– usually only the plasmid is passed on during conjugaBon • In some bacteria, the plasmid integrates into the host chromosome – during conjugaBon, host genes are transferred from the donor to the recipient • This geneBc recombinaBon is a prokaryoBc form of sexual reproducBon Bacterial genome is highly recombinant Can incorporate and express foreign DNA Produces variation & responsible for success over billions of years Transformation and Transduction • In transforma<on, bacteria take up pieces of DNA that are released as other cells disintegrate • In transduc<on, DNA is transferred to recipient bacterial cells by an infecBng phage In conjugation, transformation, and transduction within the same species, genetic recombination between donor and recipient DNA occurs because the sequences match, allowing crossing-over to occur ! When different species are involved, crossing-over can lead to integration of a segment of transferred donor DNA if some sequences match sufficiently – in this way, a recipient species can acquire new genes Endospores • In a small number of bacteria, an endospore can develop within the cell when environmental condiBons are unfavorable • The endospore is a dormant cell containing the bacterial genome and some of the cytoplasm – it is resistant to desiccaBon or aIack by enzymes or other chemical agents • When environmental condiBons become favorable, the spore germinates and becomes metabolically acBve Endospore( Parent (cell( Protein(coat(of(endospore( Figure 26.9 A developing endospore of the bacterium Clostridium tetani, a dangerous pathogen that causes tetanus. Biofilms ✦ Communities of bacteria attached to a ✦ Benefits of biofilm formation include adherence of organisms to hospitable surfaces, transfer of genes between species, and living off products of other organisms in the biofilm ✦ Humans use biofilms in sewage treatment plants and in bioremediation of toxic organic contaminates in groundwater ✦ Biofilms also adhere to surgical equipment and synthetic implants, and are involved in certain serious infections 1" !!!!!!A!specific!! environmental! signal!changes! gene!expression! in!free!bacteria.! 2" !!!!!Bacteria!a8ach!to!a! surface!coated!with! polysaccharides!and! glycoproteins,!which! results!in!more!free! bacteria!a8aching.!A! monolayer!forms.! 3" !!!!!Bacteria!a8ach! more!firmly!to!the! surface!and!form! small!colonies.! 4" !!!!!The!bacteria!produce! an!extracellular!matrix! that!enables!the!biofilm!to! mature,!producing!its! three?dimensional!shape.! A species-specific environmental signal induces changes in the pattern of gene expression The genetically reprogrammed bacteria attach to a surface The bacteria communicate (send and receive chemical signals) in a process called quorum sensing The bacteria produce an extracellular matrix of polysaccharide polymers that enables the biofilm to mature, leading to complex three-dimensional forms Healthy( Healthy(gums( Healthy(bone(level( Periodontal(disease( Plaque( Tartar( Pocket( Reduced(bone(level( Pathogenic bacteria in a biofilm are up to 1,000 times more resistant to antibiotics than the same bacteria in cultures Genomic and proteomic studies show that biofilm formation markedly changes a prokaryote’s gene expression pattern and physiology Eukaryotes) Korarchaeota) Thaumarchaeota) Crenarchaeota) Nanoarchaeota) Euryarchaeota) Cyanobacteria) Gram6posiEve)bacteria) Chlamydias) Spirochetes) Proteobacteria) ARCHAEA) EUKARYA) BACTERIA) COMMON)ANCESTOR) OF)ALL)PRESENT6DAY) ORGANISMS) Figure)26611,)p.)577) The Archaea have some eukaryotic features, some bacterial features, and some features that are unique to the group Proteobacteria ✦Highly diverse, Gram negative bacteria ✦Alphaproteo ✦Betaproteo ✦Gammaproteo (E.coli, Salmonella) ✦Deltaproteo ✦Epsilonproteo - inhabiters of animal guts Spirochetes & Chlamydias ✦Spirochetes - Gram negative, helically spiraled ✦Famous members: ✦Treponema pallidum (sypilis) ✦Borrelia burgdoferi (Lyme disease) ✦Chlamydia - Gram negative, no peptidogylcan ✦Parasites ✦Chlamydia trachomatis - chlamydia, most common STD Gram-Positive Bacteria • Gram-‐posiBve pathogens include: • Bacillus anthracis (anthrax) • Staphylococcus aureus (pneumonia, meningiBs, sepsis) • Streptococcus pyogenes (streptococcal pharyngiBs) • Lactobacillus species carry out lacBc acid fermentaBon used in the producBon of fermented foods (e.g. pickles and yogurt) Cyanobacteria Cyanobacteria are Gram-negative photoautotrophs that carry out photosynthesis using the same pathways and chlorophyll molecules as eukaryotic algae and plants Cyanobacteria ancestors were the first organisms to use water-splitting reactions of photosynthesis – critical to the appearance of abundant oxygen in the atmosphere ! B.)Chains)of)cyanobacterial)cells.)Some)cells)in)the) chains)form)spores.)The)heterocyst)is)a)specialized) cell)that)fixes)nitrogen.) Heterocyst) Res+ng)spore) Bacterial Diseases • Some bacteria produce exotoxins that interfere with biochemical processes of body cells in various ways • Clostridium botulinum • Botulism - Interferes with the transmission of nerve impulses, causing muscle paralysis • Some bacteria cause disease through endotoxins. • Endotoxins are natural components of all Gram-negative bacteria, including E. coli, Salmonella, and Shigella • Cause disease by overstimulating the host’s immune system (e.g. typhoid fever) exotoxins - toxic proteins that are secreted or released from the cell when it lyses endotoxins – lipopolysaccharides released from the outer membrane surrounding cell walls when the bacterium dies and lyses AnBbioBc-‐Resistant Bacteria • Antibiotics are used to kill or inhibit the growth of bacteria • Streptomycins block protein synthesis in target cells • Penicillins prevent formation bacterial cell walls • Many pathogenic bacteria develop resistance to antibiotics through mutations • Resistance is also acquired by horizontal gene transfer between bacteria, or by transformation and transduction • Resistance is a form of evolutionary adaptation – antibiotics alter the bacterium’s environment, conferring a reproductive advantage on strains best adapted to the altered conditions Factors that contribute to the development of resistance: Taking antibiotics routinely in mild doses Failing to complete a prescribed dosage of antibiotics Antibacterial agents in commercial products such as soaps, detergents, and deodorants Domain Archaea • Some archaeans (extremophiles) live in extreme environments such as hot springs, hydrothermal vents on the ocean floor, or salt lakes • Other archaeans (mesophiles) live in normal environments • Many archaeans are chemoautotrophs that obtain energy by oxidizing inorganic substances – others are chemoheterotrophs that oxidize organic molecules Archaea • Archaea were classified as a separate domain of life based on phylogeneBc analysis of small ribosomal subunit RNA • The domain Archaea is divided into five groups: Euryarchaeota, Nanoarchaeota, Crenarchaeota, Thaumarchaeota, and Korarchaeota • Extreme Halophiles -‐ live in highly saline environments Dead Sea, Great Salt Lake • Extreme Thermophiles -‐ temperatures from 70° to 95°C • Psychrophiles -‐ grow in cold temperatures in the AntarcBc and ArcBc oceans (10 to 20°C)
