Foundations in Microbiology

Sixth Edition

Chapter 4An Introduction to Cells

and Procaryotic Cell Structure and Function

Lecture PowerPoint to accompany

Talaro

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Characteristics of Cells and LifeAll 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: eucaryotic and procaryotic

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Characteristics of CellsEucaryotic cells: animals, plants, fungi, and protists

– contain double-membrane bound nucleus with DNA chromosomes

– contain membrane-bound organelles that compartmentalize the cytoplasm and perform specific functions

Procaryotic cells: bacteria and archaea – no nucleus or other membrane-bound organelles

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Characteristics of Life• Growth and development• Reproduction and heredity – genome composed of

DNA packed in chromosomes; produce offspring sexually or asexually

• 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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External Structures

• Appendages– two major groups of appendages:

• Motility – flagella and axial filaments (periplasmic flagella)

• Attachment or channels – fimbriae and pili

• Glycocalyx – surface coating

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Flagella• 3 parts:

– filament – long, thin, helical structure composed of protein flagellin

– hook- curved sheath

– basal body – stack of rings firmly anchored in cell wall

• Rotates 360o

• Number and arrangement of flagella varies:– monotrichous, lophotrichous, amphitrichous, peritrichous

• Functions in motility of cell through environment

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Flagellar Arrangements

1. Monotrichous – single flagellum at one end

2. Lophotrichous – small bunches arising from one end of cell

3. Amphitrichous – flagella at both ends of cell

4. Peritrichous – flagella dispersed over surface of cell; slowest

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Flagellar FunctionGuide bacteria in a direction in response to external

stimulus: chemical stimuli – chemotaxis; positive and negativelight stimuli – phototaxis

Signal sets flagella into rotary motion clockwise or counterclockwise:counterclockwise – results in smooth linear direction –

runclockwise - tumbles

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Insert figure 4.5Chemotaxis in bacteria

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Axial Filaments

• Periplasmic, internal flagella, enclosed between cell wall and cell membrane of spirochetes

• Produce cellular motility by contracting and imparting twisting or flexing motion

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Fimbriae• Fine, proteinaceous, hairlike bristles from

the cell surface

• Function in adhesion to other cells and surfaces

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Pili• Rigid tubular structure made of pilin protein

• Found only in Gram negative cells

• Function to join bacterial cells for partial DNA transfer called conjugation

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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

• Functions:– protect cells from dehydration and nutrient loss

– inhibit killing by white blood cells by phagocytosis contributing to pathogenicity

– attachment - formation of biofilms

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The Cell Envelope• External covering outside the cytoplasm• Composed of two basic layers:

– cell wall and cell membrane

• Maintains cell integrity• Two generally 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

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Insert figure 4.12Comparative cell envelopes

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Structure of Cell Walls

• Determines cell shape, prevents lysis (bursting) or collapsing due to changing osmotic pressures

• Peptidoglycan is primary component:– unique macromolecule composed of a repeating

framework of long glycan chains cross-linked by short peptide fragments

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Gram-positive Cell Wall

• Thick, homogeneous sheath of peptidoglycan– 20-80 nm thick– 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

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Gram-negative Cell Wall• Composed of an outer membrane and a thin

peptidoglycan layer• Outer membrane is similar to cell membrane bilayer

structure – outermost layer contains lipopolysaccharides and

lipoproteins (LPS)• endotoxin that may become toxic when released during

infections• may function as receptors and blocking immune response• contains porin proteins in upper layer – regulate molecules

entering and leaving cell

– Bottom layer composed of phospholipids and lipoproteins

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Gram-negative Cell Wall• Single, thin sheet of peptidoglycan

• Protective structure while providing some flexibility and sensitivity to lysis

• Periplasmic space surrounds peptidoglycan

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The Gram Stain

• Differential stain that distinguishes cells with a Gram-positive cell wall from those with a Gram-negative cell wall– Gram-positive - retain crystal violet and stain purple– Gram-negative - lose crystal violet and stain red from

safranin counterstain

• Important basis of bacterial classification and identification

• Practical aid in diagnosing infection and guiding drug treatment

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Atypical 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 i.e. Mycoplasma – cell wall is stabilized by sterols– pleomorphic

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Cell Membrane Structure

• Phospholipid bilayer with embedded proteins – fluid mosaic model

• Functions in:– providing site for energy reactions, nutrient processing, and

synthesis

– transport into and out of the cell

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Bacterial Internal Structures

• Cell cytoplasm:– dense gelatinous solution of sugars, amino acids,

and salts– 70-80% water

• serves as solvent for materials used in all cell functions

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Bacterial Internal Structures

• Chromosome– single, circular, double-stranded DNA molecule

that contains all the genetic information required by a cell

– DNA is tightly coiled around a protein, aggregated in a dense area called the nucleoid.

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Bacterial Internal Structures

• Plasmids– small circular, double-stranded DNA– free or integrated into the chromosome– duplicated and passed on to offspring– not essential to bacterial growth and metabolism– may encode antibiotic resistance, tolerance to toxic

metals, enzymes and toxins– used in genetic engineering- readily manipulated

and transferred from cell to cell

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Bacterial Internal Structures

• Ribosomes– made of 60% ribosomal RNA and 40% protein– consist of two subunits: large and small– procaryotic differ from eucaryotic ribosomes in

size and number of proteins– site of protein synthesis– present in all cells

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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.– examples: glycogen, poly--hydroxybutyrate, gas

vesicles for floating, sulfur and phosphate granules (metachromatic granules)

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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

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Endospores• Resistance linked to high levels of calcium and

dipicolinic acid• Dehydrated, metabolically inactive• thick coat• Longevity verges on immortality - 25,250 million

years.• Resistant to ordinary cleaning methods and boiling• Pressurized steam at 120oC for 20-30 minutes will

destroy

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Bacterial Shapes, Arrangements, and Sizes

• Variety 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

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Bacterial Shapes, Arrangements, and Sizes

• 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

– bacilli:• chains

• palisades

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Classification Systems in the Procaryotae

1. Microscopic morphology

2. Macroscopic morphology – colony appearance

3. Physiological / biochemical characteristics

4. Chemical analysis

5. Serological analysis

6. Genetic and molecular analysis• G + C base composition

• DNA analysis using genetic probes

• Nucleic acid sequencing and rRNA analysis

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Bacterial Taxonomy Based on Bergey’s Manual

• Bergey’s Manual of Determinative Bacteriology – five volume resource covering all known procaryotes– classification based on genetic information –

phylogenetic– two domains: Archaea and Bacteria– five major subgroups with 25 different phyla

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Major Taxonomic Groups of Bacteria

• 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

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Diagnostic Scheme for Medical Use

• Uses phenotypic qualities in identification– restricted to bacterial disease agents– divides based on cell wall structure, shape,

arrangement, and physiological traits

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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)

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Procaryotes with Unusual Characteristics• Free-living nonpathogenic bacteria• Photosynthetic bacteria - use photosynthesis, can

synthesize required nutrients from inorganic compounds– Cyanobacteria (blue-green algae)

• Gram-negative cell walls• extensive thylakoids with photosynthetic chlorophyll pigments

and gas inclusions

– Green and purple sulfur bacteria• contain photosynthetic pigment bacteriochlorophyll• do not give off oxygen as a product of photosynthesis

– Gliding, fruiting bacteria• Gram-negative• Glide over moist surfaces

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Procaryotes with Unusual Characteristics

• Unusual forms of medically significant obligate intracellular parasites– Rickettsias

• Very tiny, Gram-negative bacteria• Most are pathogens that alternate between mammals and

fleas, lice or ticks.• Obligate intracellular pathogens • Cannot survive or multiply outside of a host cell• Cannot carry out metabolism on their own • Rickettsia rickettisii – Rocky Mountain spotted fever• Rickettsia prowazekii – epidemic typhus• Coxiella burnetti – Q fever

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Unusual Forms of Medically Significant

– Chlamydias• Tiny

• Obligate intracellular parasites

• Not transmitted by arthropods

• Chlamydia trachomatis – severe eye infection and one of the most common sexually transmitted diseases

• Chlamydia psittaci – ornithosis, parrot fever

• Chlamydia pneumoniae – lung infections

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Archaea: The Other Procaryotes

• Constitute third Domain Archaea• Seem more closely related to Domain Eukarya than to

bacteria• Contain unique genetic sequences in their rRNA• Have unique membrane lipids and cell wall construction• Live in the most extreme habitats in nature,

extremophiles• Adapted to heat, salt, acid pH, pressure and atmosphere• Includes: methane producers, hyperthermophiles,

extreme halophiles, and sulfur reducers