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Modern concepts about bacterial cell

E-content for Programme: B.Sc.Botany (modern concept about bacterial

cell)

Prepared by Prof. Kanchan kumari ,

Department of Botany

S.U College, Hilsa

Patliputra university

email-kanchanancbio@gmail.com

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Approximate timing of major events in the history of life on Earth.

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ProcaryoteEucaryote

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Composite Bacterial Cell

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BACTERIAL DNA

• One circular chromosome packed to ”nucleoid”

• Plasmids: smaller circular DNA structures

• Genome size: 0.5 to 6 Mb (500 to 6000 genes)

- extrachromosomal DNA

- multiple copy number- genes for antibiotic-resistance- genes encoding toxins- multiply independently of cell division

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Cytoplasmic membrane

• Lipid bilayer • Semipermeable barrier• No sterols

Contains:• Structural proteins• Transport proteins• Electron transport system• Enzymes• Ion pumps• Flagellum anchor

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CYTOPLASM

Cell wall• rigid, protecting cell from osmotic lysis

• made up of PEPTIDOGLYCAN

Cell membrane

Bacteria have no inner cytoskeletonCell wall = outer skeleton

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NAG NAM

Basic components of peptidoglycan

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Structure of peptidoglycan

G = NAGM = NAM

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All bacteria have aPEPTIDOGLYCAN cell wall

Cell envelopes are different

basis for stain reaction developed byChristian Gram, 1884

BUT

*

*Exception: genus Mycoplasma

Gram positive G+Gram negative G-

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Gram positive cell wall

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TEICHOIC ACIDPolymer of phosphorylated sugar alcohols/glycerol

Ribitol based

Glycerol based

LIPOTEICHOIC ACIDfatty acid anchorin cell membrane

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Fluid Mosaic model (by Singer and Nicholson) of bacterial membrane structure

(under EM) Most of these proteins have one or more trans membrane (TM) segments,

and they interact closely with nearby lipids as well as other proteins. In addition, there

are peripheral membrane proteins that associate at the surface of the membrane and

lipid anchored proteins that are held into the membrane by covalently attached fatty

acids or lipids. hydrophilic moieties are always on the portions of glycoproteins and

glycolipids external to the membrane bilayer.

Like

hopanoids

Hydrophobic

-helix

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Gram negative cell wall

Lipopolysaccharide

Outer membrane

Periplasmic space

Cytoplasmic membrane

Cytoplasm

Peptidoglycan

Porin proteins

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Lipopolysaccharide = endotoxin

O-antigen

Core poly-saccharide

Lipid A isanchored inouter mem-brane (OM)

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IMPLICATIONS OF GRAM REACTION

• Diagnostically useful

• LPS in G- and TA in G+ activate complement &

are important for pathogenicity

• Sensitivity to antibiotics, lysozyme, complement

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Flagella and Motility

• Size-20 nm across and up to 15 to 20m long.

• Monotrichous-single and located at one end

• Amphitrichous- single flagellumat each pole

• Lophotricous-cluster of flagella at one or both ends

• Peritrichous-spread over the whole surface

• Flagella can rotate at rates as fast as 40 to 60 revolutions per second.

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Pseudomonas Proteus

Escherichia coli

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Flagella and Motility

• The mechanism of flagellar movement appears to be rotation; the hook and helical structure of the flagellum causes the flagellum to act as a propeller, thus driving the bacterium through its watery environment

• Counterclockwise rotation causes forward motion (called a run)

• Clockwise rotation disrupts forward motion (resulting in a tumble)

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FLAGELLA = locomotory organelles• project as long strand(s) from bacterial surface• embedded in cytoplasmic membrane• flagellin (protein) subunits• hollow tubular structure•move bacteria by propeller like action

Outer membrane

Peptidoglycan

Cytoplasmic membrane

THE FLAGELLUM ROTATOR MACHINE

M ringS ring

Hook filament

P-ring

L-ring

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Chemotaxis

• Chemotaxis is directed movement of bacteria either towards a chemical attractant or away from a chemical repellent

• The concentrations of these attractants and repellents are detected by chemoreceptors in the surfaces of the bacteria

• Directional travel toward a chemoattractant (biased random walk toward attractant) is caused by lowering the frequency of tumbles (twiddles), thereby lengthening the runs when traveling up the gradient, but allowing tumbling to occur at normal frequency when traveling down the gradient

• Directional travel away from a chemorepellent (biased random walk away from repellent) involves similar but opposite responses

• The mechanism of control of tumbles and runs is complex, involving numerous proteins and several mechanisms (conformation changes, methylation, and phosphorylation) to modulate their activity;

• despite this complexity chemotaxis is fast, with responses occurring in as little as 200 meters/second

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AXIAL FILAMENTS

• spirochetes, e.g., Treponema pallidum• similar function to flagella• run lengthwise along the cell• snake-like movement

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Pili = Fimbriae

• Short hair-like projections

• Aid in adhesion to host epithelium (e.g. cornea)

• Special (longer) F-pili for conjugation

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CAPSULE AND SLIME LAYER

• outside cell envelope

• ”capsule” well defined

• ”slime layer” or ”glycocalyx” not so defined

• usually polysaccharides

• often lost in vitro

• protective in vivo (important virulence factor)

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Encapsulated bacteria are less well phagocytosed

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BACTERIAL CELL DIVISION

Binary fission

• Chromosome attaches to membrane• DNA is replicated• Cell membrane + envelope invaginate• Daughter cells separate

NOTE:Plasmids replicate independently

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BACTERIAL ENDOSPORE

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BACTERIAL METABOLISM IN RELATION TO OXYGEN

Aerobic

Anaerobic

Facultative

Microaerophilic

Obligate aerobes require molecular oxygen

Obligate (strict) anaerobes can not grow in presence of oxygen

Facultative anaerobes grow both with and without oxygen (most bacteria)

Prefer reduced amount of oxygen

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SPORE FORMATION

Triggered at adverse conditions, such as lack of nutrients,dryness, heat, etc.

15% of the spore’s dry

weight consists of dipicolinic

acid complexed with

calcium ions. Calcium

dipicolinate stabilizes DNA

at high temperature

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WHAT IS A BACTERIAL SPORE?

•Dormant state designed for long-term survival

• Contains the whole genome

• Germinates when life

looks better again

•Contains calcium bound to dipicolinic acid

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ENDOSPORE CYCLE

Bacillus & Clostridium

Bacterial spores are NOT for

multiplication

To kill spores: autoclave 120°C, 20 min

Septum

formation

Lysis of sporangium

Germination of

endospore

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CLASSIFICATION OF BACTERIA

Kingdom Monera MoneraPhylum

Class

Order Eubacteriales Eubacteriales

Family Enterobacteriaceae Micrococcaceae

Genus Escherichia Staphylococcus

Species coli aureus

Type O157:H7 MRSA

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Binomial Nomenclature

Genussuggestive of morphology, discoverer, other characteristics(Streptococcus, Staphylococcus, Neisseria, Escherichia, Mycobacterium)

speciessuggestive of metabolic features, biochemical characteristics, disease(pyogenes, aureus, gonorrhoeae, coli, tuberculosis, anthracis)

More examples:Bordetella pertussis, Yersinia pestis, Haemophilus influenzaeLegionella pneumophila etc etc

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Helicobacter Pseudomonas

Staphylococcus Borrelia

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BACTERIA

• Variable morphology• Single circular chromosome• Unique type of cell wall

•Size 1-5 mmCoccus

Spiral

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Bacillus anthracis

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Staphylococcus aureus