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The Bacterial Cell

MORPHOLOGY OF BACTERIA

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Key WordsKey WordsProkaryote Outer membrane Eubacteria (Bacteria)

Periplasmic spaceArchaebacteria (Archaea) Oxidative phosphorylation Eukaryote Spheroplast/protoplast

Plasmid Flagella

Chromosome

Chemotaxis Ribosome Axial filament Peptidoglycan (murein, mucopeptide)

Gram stain

Gram negative

Storage Granules

Gram positive

Pili (fimbriae)

Cell envelope

Capsule (slime layer, glycocalyx)Cell membrane

Endospore (spore)

Cell wall

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PROKARYOTESPROKARYOTES

EUBACTERIAEUBACTERIA ARCHAEAARCHAEA

EUKARYOTESEUKARYOTES

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Prokaryotes (Bacteria)Prokaryotes (Bacteria)• EubacterEubacter "True" bacteria "True" bacteria

– human pathogenshuman pathogens

– clinical or environmentalclinical or environmental

– one kingdomone kingdom

• ArchaeaArchaea – Environmental organisms Environmental organisms

– second kingdomsecond kingdom

Introduction

• Bacteria despite their simplicity, contain a well developed cell structure which is responsible for many of their unique biological properties. Many structural features are unique to bacteria and are not found among eukaryotes.

Cell Morphology

•  The most elemental structural property of bacteria is cell morphology (shape).

• Bacteria are classified by shape into three basic groups:• cocci (spherical)

• bacilli (rod-like)

• spirochetes (spiral)

• Some bacteria are variable in shape and said to pleomorphic.

• The shape of a bacteria is determined by its rigid cell wall.

• The microscopic appearance of a bacterium is one of the most important criteria used in identification.

Bacteria come in a wide variety of shapes

• The arrangement of bacteria is important. Example:

• Cocci in pairs (diplococci)• Cocci in chains (streptococci)• Cocci in clusters (staphylococci)

Bacteria Size

• Bacteria range in size from about 0.2 to 5 um.  

• The smallest bacteria (Mycoplasma) are the same size as a large virus.

• The longest bacteria rods are the same size as some yeasts and human red blood cells.

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CLASSIFICATION OF BACTERIA

• ON BASIS OF SHAPE

• ON BASIS OF ARRANGEMENT

• ON BASIS OF STAINING

• ON BASIS OF OXYGEN REQUIREMENT

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MORPHOLOGY OF BACTERIA(SHAPE)

• COCCI -- round• BACILLI-- rods• SPIROCHETES-- spiral• PLEOMORPHIC

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ARRANGEMENT OF BACTERIA

• CHAINS

• CLUSTERS

• PAIRS

• TETRADS

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STAINING OF BACTERIA

• GRAM’S STAINING

Gram positive

Gram negative

Gram variable

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ON BASIS OF OXYGEN REQUIREMENT

• Aerobes

• Anaerobes

• Facultative anaerobes

etc

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STRUCTURE OF BACTERIA

• ESSENTIAL STRUCTURES

• NON-ESSENTIAL STRUCTURES

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Essential structures

• Cell envelope:

Cell wall (except Mycoplasma)

Cell membrane

• Ribosome

• Nucleoid

• Mesosome

• Periplasm ( in gram’s negative bacteria)

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Non Essential structures

• Capsule

• Pilus

• Flagellum

• Spore

• Plasmid , Transposons

• Granule

• Glycocalyx

Bacterial Structure

Cell Structure

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CELL ENVELOPE

KEY WORDS Cell envelope

• Cell wall

• Cell membrane

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Cell envelope– The cell envelope: Consists

of cell membrane and cell wall plus an outer membrane if present.

– Most bacterial cell envelopes fall into two major categories: Gram positive and Gram negative based on Gram staining characteristics that reflect major structural differences between the two groups

22CytoplasmCytoplasm

Lipoteichoic acid Peptidoglycan-teichoic acid

Cytoplasmic membrane

GRAM POSITIVE CELL GRAM POSITIVE CELL ENVELOPEENVELOPE

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GRAM NEGATIVE GRAM NEGATIVE CELL ENVELOPECELL ENVELOPE

CytoplasmCytoplasm

Inner (cytoplasmic) membrane

Outer Membrane(Major permeability barrier) LipopolysaccharidePorin

Braun lipoprotein

Periplasmic space

Periplasmic binding proteinPermease

Cell Walls

Depending on Structure of their cell walls, the bacteria absorb either the purple dye or the pink dye.

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Prokaryotic cell

Gram +

Gram -

Cell wall

Cell (inner) membrane Outer membrane

Cell wall

Cell membrane

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Cell wall

• Cell wall consists of the peptidoglycan layer attached structures

(eg: capsule, pili etc)

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PEPTIDOGLYCANS ( murein/ mucopeptide)

– Multilayered in gram positive bacteria.

– Surrounds the bacterial cell membrane.

– Provides rigidity. It is huge (billions in molecular weight)

– Consists of a glycan (polysaccharide) backbone with peptide side chains.

– PG is found in all bacteria except Chlamydia and Mycoplasma.

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• Gram positive cell envelope:

Thick peptidoglycan with covalently bound teichoic acid. These negatively charged molecules concentrate metal ions from the surroundings. Lipoteichoic acid is primarily associated with the cell membrane.

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Gram negative cell envelope

• Outer membrane present ( lipopolysachride, lipoprotein &phospholipid) --- endotoxin

Lipid A --- toxic

Polysaccharide core

Outer polysaccharide -- antigenic• Thin peptidoglycan layer• No teichoic acid• Periplasmic space : b/w two membranes

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Gram negative cell envelope

• Porin protein

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GRAM POSITIVEGRAM POSITIVE

GRAM NEGATIVEGRAM NEGATIVE

CytoplasmCytoplasm

CytoplasmCytoplasm

Lipoteichoic acid Peptidoglycan-teichoic acid

Cytoplasmic membrane

Inner (cytoplasmic) membrane

Outer Membrane

LipopolysaccharidePorin

Braun lipoprotein

Peri

plas

mic

spa

ce

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CytoplasmCytoplasm

CELL MEMBRANE.

Cell membrane

•Oxidative phosphorylation (no mitochondria).•Synthesis of cell wall precursor.•Active transport•Synthesis of enzymes.

Cell Wall

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CYTOPLASM• MesosomeCytoplasmic membrane invagination.Cell division-----DNA binding site. Ribosomes70S in size (50S & 30S)Protein synthesis. Nucleoid• No nuclear membrane.• DNA---Single, circular, about 2000 genes.• No introns in DNA, no mitotic spindle, no

nucleolus, no histones

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Non Essential structures

• Capsule

• Pilus

• Flagellum

• Spore

• Plasmid , Transposons

• Granule

• Glycocalyx

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Capsules and slime layersCapsules and slime layers (Glycocalyx)(Glycocalyx)

• well defined: capsulewell defined: capsule• outside cell envelopeoutside cell envelope• usually polysaccharide (in B. anthracis– D usually polysaccharide (in B. anthracis– D

glutamate)glutamate)• Gives virulance (antiphagocytic).Gives virulance (antiphagocytic).• Antigenic (vaccine formation).Antigenic (vaccine formation).• Adherence.Adherence.• Quellung reaction (Identification).Quellung reaction (Identification).2.2. not well defined-- slime layer or glycocalyxnot well defined-- slime layer or glycocalyxo Adherence Adherence o S.mutans (Plaque formation)S.mutans (Plaque formation)

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FLAGELLAFLAGELLA• For motility – long & whip likeFor motility – long & whip like• Subunits—FlagellinSubunits—Flagellin• Respond to food/poison –chemotaxisRespond to food/poison –chemotaxis• NeverNever present in cocci. present in cocci.• Use ATPs – proton motive forceUse ATPs – proton motive force• Number and location of flagella.Number and location of flagella.

(Peritrichous, monotrichous, (Peritrichous, monotrichous, lophotrichous, axial). lophotrichous, axial).

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• Flagella – embedded in cell membraneembedded in cell membrane– project as strandproject as strand– Flagellin (protein) subunitsFlagellin (protein) subunits– move cell by propeller like actionmove cell by propeller like action– *axial filament*axial filament– *flagellar antigen *flagellar antigen

Flagella

A-Monotrichous; B-Lophotrichous;C-Amphitrichous; D-Peritrichous;

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Axial filamentsAxial filaments

– spirochetes spirochetes

– similar function to flagellasimilar function to flagella

– run lengthwise along cellrun lengthwise along cell

– snake-like movement snake-like movement

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Pili (fimbriae)Pili (fimbriae)

•Short , hair-like projections of the cellShort , hair-like projections of the cell•Protein--pilinProtein--pilin•Mostly on gram-ve bacteriaMostly on gram-ve bacteria•adhesion to host epitheliumadhesion to host epithelium•Two types– ordinary piliTwo types– ordinary pili

sex pilisex pili•sexual conjugation (sex pili)sexual conjugation (sex pili)

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Endospores (spores)Endospores (spores)• Metabolically dormant cell Metabolically dormant cell • Produced when adverse conditions eg starved – Produced when adverse conditions eg starved –

sporulation – bacterial DNA, cytoplasm, cell sporulation – bacterial DNA, cytoplasm, cell membrane, peptidoglycans, water & keratin- like coatmembrane, peptidoglycans, water & keratin- like coat

• Resistant to adverse conditions Resistant to adverse conditions - high temperatures (not killed by boiling), - high temperatures (not killed by boiling), - radiation, dehydration- radiation, dehydration- organic solvents- organic solvents

• Killed by autoclaving (121 C for 30 min) Killed by autoclaving (121 C for 30 min) • contain dipicolinic acidcontain dipicolinic acid• Bacillus (central)Bacillus (central) and and Clostridium (terminal)Clostridium (terminal)• Survival for many yearsSurvival for many years

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CYTOPLASM

Granules– Storage of nutrients– Stained with dyes

Transposons Jumping genes – within DNA or between DNAs of

bacteria, plasmids & bacteriophages Pieces of DNA No independent replication. Can be more than one in 1 DNA. For toxins, enzymes, antibiotic resistance etc. 4 domains– inverted repeats, transposase, repressor,

resistance.

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PlasmidsMultiple copies in numberExtra-chromosomal DNA, double

stranded, circularCan be incorporated in DNACoding pathogenesis and antibiotic

resistance, heavy metal & U-V light resistance, toxins, pili etc.

Replication independent of bacterial chromosome.

Transmissable & Non transmissable.

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Making Wall-less formsMaking Wall-less forms• Result from action of:Result from action of:

– enzymes lytic for cell wall (eg lyzozyme)enzymes lytic for cell wall (eg lyzozyme)– antibiotics inhibiting peptidoglycan biosynthesisantibiotics inhibiting peptidoglycan biosynthesis

• Wall-less bacteria that don’t replicate:Wall-less bacteria that don’t replicate:– spheroplasts (with outer membrane) gram - vespheroplasts (with outer membrane) gram - ve– protoplasts (no outer membrane) gram +ve. protoplasts (no outer membrane) gram +ve.

• Wall-less bacteria that replicateWall-less bacteria that replicate– L forms ( relative resistance to antibiotics)L forms ( relative resistance to antibiotics)

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Naturally Wall-less GenusNaturally Wall-less Genus

• MycloplasmaMycloplasma

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The Cell EnvelopeThe Cell Envelope

Gram PositiveGram Positive Gram NegativeGram Negative

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GRAM STAIN

• Gram positive

• Gram negative

• Gram variable

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Procedureof Gram Staining

• Developed by Christian Gram

• In 1884

• 4 steps:Stain with Crystal voilet (primary stain)Then pour Gram,s iodine (mordant)DecolourizePour Safranin (counter stain)

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Gram negativeGram negative Gram positiveGram positiveHeat/DryHeat/Dry

Crystal violet stainCrystal violet stain

IodineIodine FixFix

Safranin stainSafranin stain

AlcoholAlcohol dede-stainstain

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Gram stain morphologyGram stain morphology

• ShapeShape

– cocci (round)cocci (round)

– bacilli (rods)bacilli (rods)

– spiral or curved (e.g. spirochetes)spiral or curved (e.g. spirochetes)

• Single or multiple cellsSingle or multiple cells

– clusters (e.g. staphylococci)clusters (e.g. staphylococci)

– chains (e.g. streptococci) chains (e.g. streptococci)

• Gram positive or negativeGram positive or negative

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