Chapter 4. A Survey of Prokaryotic Cells and Microorganisms

Transcription

1 Chapter 4 A Survey of Prokaryotic Cells and Microorganisms

2 Characteristics of cells and life All living things (single and multicellular) are made of cells that share some common characteristics: Basic shape spherical, cubical, cylindrical Internal content cytoplasm, surrounded by a membrane DNA chromosome(s), ribosomes, metabolic capabilities Two basic cell types: eukaryotic and prokaryotic

3 Characteristics of cells Eukaryotic cells: animals, plants, fungi, and protists Contain membrane-bound organelles that compartmentalize the cytoplasm and perform specific functions Contain double-membrane bound nucleus with DNA chromosomes Prokaryotic cells: bacteria and archaea No nucleus or other membrane-bound organelles

4 Characteristics of life Reproduction and heredity genome composed of DNA packed in chromosomes; produce offspring sexually or asexually Growth and development Metabolism chemical and physical life processes Movement and/or irritability respond to internal/external stimuli; self-propulsion of many organisms Cell support, protection, and storage mechanisms cell walls, vacuoles, granules and inclusions Transport of nutrients and waste

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6 Whittaker s 5 kingdom classification

7 Bacterial shapes, arrangements, and sizes Vary in shape, size, and arrangement but typically described by one of three basic shapes: Coccus spherical Bacillus rod Coccobacillus very short and plump Vibrio gently curved Spirillum helical, comma, twisted rod, Spirochete spring-like

8 Common bacterial shapes

9 Variation in cell shape and size within a single species Pleomorphism Some species are noted for their pleomorphism: Corynebacterium diphteriae what are metachromatic granules? Volutin granules Inorganic polyphosphate granules Appear red when stained with methylene blue

10 Bacterial arrangements Arrangement of cells is dependent on pattern of division and how cells remain attached after division: Cocci: Singles Diplococci in pairs Tetrads groups of four Irregular clusters Chains Cubical packets (sarcina) Bacilli: Diplobacilli Chains Palisades

11 Classification systems for Prokaryotes 1. Microscopic morphology 2. Macroscopic morphology colony appearance 3. Bacterial physiology 4. Serological analysis 5. Genetic and molecular analysis

12 Bacterial taxonomy based on Bergey s Manual Bergey s Manual of Determinative Bacteriology five volume resource covering all known prokaryotes Classification based on genetic information phylogenetic Two domains: Archaea and Bacteria Five major subgroups with 25 different phyla Table 4.3

13 Structural plan of a prokaryote

14 Structure of a bacterial cell

15 Model of a bacterial cell Cytoplasm = consists of a gel-like network Cell membrane = encloses the cytoplasm Cell wall = covers the cell membrane Nucleoid = area of the cytoplasm that contains the chromosome in the form of looped coils Flagellum = external helical filament whose rotary motor propels the cell

16 External structures Appendages Motility Flagella Axial filaments (periplasmic flagella) Attachment Fimbriae Channels Pili Surface coating- Glycocalyx

17 Flagella Prokaryotes that are motile generally swim by means of flagella Rotates 360 o Functions in motility of cell through environment

18 Flagella Filament long, thin, helical structure composed of protein flagellin The filament is rotated by a motor driven by the proton motive force Hook curved sheath Basal body stack of rings firmly anchored in cell wall

19 Flagellar arrangements Monotrichous single flagellum at one end Vibiro cholerae Amphitrichous flagella at both ends of cell Aquaspirillum sp. Lophotrichous small bunches emerging from the same site Peritrichous flagella dispersed all over surface of cell Proteus vulgaris Spirillum serpens

20 Flagellar responses Guide bacteria in a direction in response to external stimulus: Chemical stimuli chemotaxis; positive and negative Light stimuli phototaxis Signal sets flagella into motion clockwise or counterclockwise: Counterclockwise results in smooth linear direction run Clockwise tumbles

21 Periplasmic flagella Internal flagella, enclosed in the space between the outer sheath and the cell wall peptidoglycan Produce cellular motility by contracting and imparting twisting or flexing motion Examples: Spirochetes like Borrelia burgdorferi

22 Fimbriae Fine, proteinaceous, hairlike bristles emerging from the cell surface Function in adhesion to other cells and surfaces

23 Pili Rigid tubular structure made of pilin protein Mostly found in Gram-negative bacteria Function to join bacterial cells for partial DNA transfer called conjugation Pili involved in conjugation are called sex pilus or F-pilus

24 Glycocalyx Coating of molecules external to the cell wall, made of sugars and/or proteins Two types: 1. Slime layer - loosely organized and attached 2. Capsule - highly organized, tightly attached

25 Functions of the Glycocalyx Protect cells from dehydration and nutrient loss Inhibit killing by white blood cells by phagocytosis, contributing to pathogenicity Attachment - formation of biofilms

26 How biofilms form

27 Biofilm on a catheter

28 Inside the bacterial cell Cell cytoplasm: Dense gelatinous solution of sugars, amino acids, and salts 70-80% water Serves as solvent for materials used in all cell functions

29 Chromosome Single, circular, double-stranded DNA molecule that contains all the genetic information required by a cell Plasmids Free small circular, double-stranded DNA Not essential to bacterial growth and metabolism Used in genetic engineering - readily manipulated and transferred from cell to cell Nucleoid

30 Ribosomes Bacterial ribosome Made of 60% ribosomal RNA and 40% protein Consist of two subunits: large and small Prokaryotic differ from eukaryotic ribosomes in size and number of proteins Site of protein synthesis Found in all cells

31 A portion of an E. coli chromosome being transcribed (left to right) and being simultaneously translated. The arrow points to the putative site where RNA polymerase is first bound to the DNA. The chains of dark bodies are polysomes, that is, several ribosomes on the same mrna molecule. Miller OL Jr, Hamkalo BA, Thomas CA Jr. Visualization of bacterial genes in action. Science Jul 24;169 (3943) :

32 Bacterial internal structures Inclusions and granules Intracellular storage bodies Vary in size, number, and content Bacterial cell can use them when environmental sources are depleted Magnetosomes Aquaspirillum sp.

33 poly β hydroxyl butyrate (PHB) granules Ralstonia eutropha Wahl et al. BMC Microbiology 2012, 12:262

34 Gas vesicles = to increase buoyancy Bacterial internal structures Filaments (chains of cells) of the Gram-negative phototroph Halochromatium roseum. Gas vesicles (seen as white spots) provide buoyancy, enabling the phototroph to remain at the surface of the water, exposed to light. Source: P. Anil Kumar et al Int. J. Syst. Evol. Microbiol. 57:

35 Cytoskeleton Bacterial internal structures Many bacteria possess an internal network of protein polymers that is closely associated with the cell wall

36 Bacterial internal structures Endospores Inert, resting, cells produced by some G+ genera: Clostridium, Bacillus, and Sporosarcina Have a 2-phase life cycle: Vegetative cell metabolically active and growing Endospore when exposed to adverse environmental conditions; capable of high resistance and very long-term survival Sporulation - formation of endospores Hardiest of all life forms Withstands extremes in heat, drying, freezing, radiation, and chemicals Not a means of reproduction Germination - return to vegetative growth

37 Sporulation cycle

38 Endospores Dehydrated, metabolically inactive Thick coat Longevity verges on immortality, 250 million years Resistant to ordinary cleaning methods and boiling Pressurized steam at 120 o C for minutes will destroy them

39 The cell envelope External covering outside the cytoplasm Composed of two basic layers: Cell wall and cell membrane Maintains cell integrity Two different groups of bacteria demonstrated by Gram stain: Gram-positive bacteria: thick cell wall composed primarily of peptidoglycan and cell membrane Gram-negative bacteria: outer cell membrane, thin peptidoglycan layer, and cell membrane

40 Determines cell shape, prevents lysis due to changing osmotic pressures Structure of cell walls Peptidoglycan is the primary component: Unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments

41 Structure of cell walls Determines cell shape, prevents lysis due to changing osmotic pressures The bacterial cell wall (sacculus) consists of a single interlinked molecule. Peptidoglycan (murein) is the primary component: Unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments

42 Peptidoglycan (Murein) Peptidoglycan is unique to bacteria Thus, the enzymes responsible for its biosynthesis make excellent targets for antibiotics Penicillin inhibits the transpeptidase that cross-links the peptides Vancomycin prevents cross-bridge formation by binding to the terminal D-Ala-D-Ala dipeptide Unfortunately, the widespread use of such antibiotics selects for evolution of resistant strains

43 Gram-positive cell wall nm thick peptidoglycan Includes teichoic acid and lipoteichoic acid: function in cell wall maintenance and enlargement during cell division; move cations across the cell envelope; stimulate a specific immune response Some cells have a periplasmic space, between the cell membrane and cell wall.

44 Gram-negative cell wall Inner and outer membranes and periplasmic space between them contains a thin peptidoglycan layer Outer membrane contains lipopolysaccharides (LPS) Lipid portion (endotoxin) may become toxic when released during infections May function as receptors and blocking immune response Contain porin proteins in upper layer regulate molecules entering and leaving cell

45 Periplasmic space High magnification image of the envelope showing the plasma membrane (PM) enclosed by a low-density inner wall zone (IWZ) which is bound by a high-density outer wall zone (OWZ). Matias, V. R. F. and Beveridge, T. J. (2005), Cryo-electron microscopy reveals native polymeric cell wall structure in Bacillus subtilis 168 and the existence of a periplasmic space. Molecular Microbiology, 56:

46 Structures of Gram-positive and Gram-negative bacterial cell walls

47 Structures of Gram-positive and Gram-negative bacterial cell walls

48 Comparison of Gram-positive and Gram-negative cell walls

49 Nontypical cell walls Some bacterial groups lack typical cell wall structure, i.e., Mycobacterium and Nocardia Gram-positive cell wall structure with lipid mycolic acid (cord factor) Pathogenicity and high degree of resistance to certain chemicals and dyes Basis for acid-fast stain used for diagnosis of infections caused by these microorganisms Some have no cell wall, e.g. Mycoplasma sp. Cell wall is stabilized by sterols Pleomorphic

50 Cell membrane structure Phospholipid bilayer with embedded proteins fluid mosaic model Functions in: Providing site for energy reactions, nutrient processing, and synthesis Passage of nutrients into the cell and discharge of wastes Cell membrane is selectively permeable

51 Major prokaryote taxonomic groups Domain Archaea primitive, adapted to extreme habitats and modes of nutrition Domain Bacteria: Phylum Proteobacteria Gram-negative cell walls Phylum Firmicutes mainly Gram-positive with low G + C content Phylum Actinobacteria Gram-positive with high G + C content

52 Diagnostic scheme for medical use Uses phenotypic qualities in identification Restricted to bacterial disease agents Divides bacteria based on (i) cell wall structure, (ii) shape, (iii) arrangement, and (iv) physiological traits

53 Medically important bacteria

54 Species and Subspecies Species a collection of bacterial cells which share an overall similar pattern of traits in contrast to other bacteria whose pattern differs significantly Strain or variety a culture derived from a single parent that differs in structure or metabolism from other cultures of that species (biovars, morphovars) Type a subspecies that can show differences in antigenic makeup (serotype or serovar), susceptibility to bacterial viruses (phage type) and in pathogenicity (pathotype)

55 Prokaryotes with unusual characteristics Free-living nonpathogenic bacteria Photosynthetic bacteria use photosynthesis, can synthesize required nutrients from inorganic compounds Cyanobacteria (blue-green algae) Green and purple sulfur bacteria Gliding, fruiting bacteria

56 Gram-negative cell walls Cyanobacteria (blue-green algae) Extensive thylakoids with photosynthetic chlorophyll pigments and gas inclusions

57 Green and Purple Sulfur bacteria Photosynthetic Contain photosynthetic pigment bacteriochlorophyll Do not give off oxygen as a product of photosynthesis Unlike plants, algae, and cyanobacteria, do not use water as their reducing agent, so no oxygen is produced Use hydrogen sulfide, which is oxidized to produce granules of elemental sulfur. This sulfur may be oxidized to form sulfuric acid.

58 Gram-negative E.g.: Myxobacteria sp. Glide over moist surfaces Gliding and Fruiting bacteria

59 L- forms L- forms arise from mutations of cell wall forming genes Can be induced in lab by treating bacteria with lysozyme or penicillin Treated Gram positive cells lose cell walls completely forming protoplast fragile cell bound by only membrane Treated Gram negative cells lose peptidoglycan but retain outer membrane forming spheroplast less fragile than protoplast

60 Unusual forms of medically significant bacteria Mycoplasmas Lack cell wall Cell membrane has sterols- prevents lysis of cells Pleomorphic Mycoplasma pneumoniae: primary atypical pneumonia Colony shows fried egg morphology G.A. Meloni et al. J. Gen Microbiol. 1980

61 Unusual forms of medically significant bacteria Rickettsias Obligate intracellular parasites Very tiny, Gram-negative bacteria Most are pathogens Cannot survive or multiply outside of a host cell Rickettsia rickettisii Rocky Mountain spotted fever Transmitted by ticks and fleas

62 Unusual forms of medically significant bacteria Chlamydias Tiny, Obligate intracellular parasites, Gram-negative bacteria Not transmitted by arthropods Chlamydia trachomatis severe eye infection and one of the most common sexually transmitted diseases Chlamydia pneumoniae lung infections Chlamydia psittaci - psittacosis (parrot fever) is a zoonotic infectious disease contracted from parrots

63 HeLa cells infected with Clamydia trachomatis Suchland R J et al. Infect. Immun. 2000;68:

64 Comparing mycoplasma, rickettsias, chalmydia, viruses Mycoplasma Rickettsia Chlamydia Viruses Size µm µm 0.3 µm nm Cell wall No Yes Yes No Gram stain N/A Gram negative Gram negative N/A Obligate intracellular parasites Growth in cell free media No Yes Yes Yes Yes No No No 1 µm = 1000 nm

65 Archaea: The other prokaryotes More closely related to Eukarya than to Bacteria Have unique membrane lipids and cell walls

66 Archaea Live in the most extreme habitats in nature, extremophiles Adapted to heat, salt, acid ph, pressure, and atmosphere Includes: methane producers, hyperthermophiles, extreme halophiles, sulfur reducers