Morphology of bacteria

Course: B.Sc. MicrobiologySem II

Sub: BacteriologyUnit II

• In microbiology, the term morphology means cell shape. Several morphologies are known among prokaryotes.

The basic morphology of a cell

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1. The Cell Membrane

• Phospholipid bilayer: 2 surface layers of hydrophilic

of polar head and inner layer of hydrophobic

nonpolar tail.

• Peripheral proteins: Function as enzyme scaffold for

support and mediator for cell movement

• Integral proteins: disrupting lipid bilayer. Other types

known as transmembrane protein

• Glycoprotein: protein attached to the carbohydrate

• Glycolipid: Lipid attached to carbohydrate

Membrane structureFluid mosaic model

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Characteristics of Lipid bilayer• Semi-permeable

• Consists of : 1) hydrophilic2) hydrophobic

• Main function is to:– Protect cell (as outercovering)– Keep the things that are on the

inside of a cell inside, and keep what things are outside the cell on the outside.

– It allows some things, under certain situations, to cross the phospholipid bilayer to enter or exit the cell.

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Function of the Cell Membrane• Protective outer covering for the cell.

• Cell membrane anchors the cytoskeleton (a cellular 'skeleton' made of protein and contained in the cytoplasm) and gives shape to the cell.

• Responsible for attaching the cell to the extracellular matrix (non living material that is found outside the cells), so that the cells group together to form tissues.

• Transportation of materials needed for the functioning of the cell organelles without using cellular energy.

• The protein molecules in the cell membrane receive signals from other cells or the outside environment and convert the signals to messages, that are passed to the organelles inside the cell.

• In some cells, the protein molecules in the cell membrane group together to form enzymes, which carry out metabolic reactions near the inner surface of the cell membrane.

• The proteins help very small molecules to get transported thru cell membrane, provided, the molecules are traveling from a region with lots of molecules to a region with less number of molecules.

The cell membrane

• Lies between two dark line

• Prokaryote have less sterol as compared to the eukaryote, causing rigid structure

• Less-sterol wall in prokaryote is Mycoplasma

Lipid bilayer of plasma membrane

Taken with TEM

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2. The bacterial cell wall

• Has peptidoglycan

• Marks the difference between gram +ve and gram –ve bacteria

Peptidoglycan• Polymer of disaccharide• Also known as murein,• is a polymer consisting of sugars

and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria (but not Archaea), forming the cell wall.

• The sugar component consists of alternating residues of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Attached to the N-acetylmuramicacid is a peptide chain of three to five amino acids.

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3. Cell wall of Gram negative and positive

Gram negative Gram positive

Thin peptidoglycan Thick peptidoglycan

Has outer membrane Teichoic acid

Periplasmic space

Tetracycline sensitive Penicillin sensitive

4 ring basal body 2-ring basal body

Disrupted by lysozyme Disrupted by endotoxin

Gram positive and Gram negative

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A, Gram positive bacterium thick peptidoglycan layer contains teichoic and lipoteichoic acids.

B, Gram negative bacterium thin peptidoglycan layer and an outer membrane that contains lipopolysaccharide, phospholipids, and proteins. The periplasmicspace between the cytoplasmicand outer membranes contains transport, degradative, and cell wall synthetic proteins. The outer membrane is joined to the cytoplasmic membrane at adhesion points and is attached to the peptidoglycan by lipoprotein links.

Gram positive and Gram negative bacterial cell walls

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Peptidoglycan in Gram positive bacteria

• Linked by polypeptides

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Gram positive bacterial cell wall

• Teichoic acid

- Lipoteichoic acid links to plasma membrane

- Wall teichoic acid links to peptidoglycan

• May regulate movement of cations

• Polysaccharides provide antigenic variation

Gram negative bacteria outer membrane

• Lipopolysacharides, lipoproteins, phospholipids

• Forms the periplasm between the outer membrane and the plasma membrane

• Protection from phagocytes, complement and antibiotics

• O-polysaccharide antigen e.g. E. coli O157:H7

• Lipid A is an endotoxin

• Porins (proteins) form channel through membrane

The Gram stain Mechanism

• Crystal violet-iodine crystals form in cell

• Gram positive

- Alcohol dehydrates peptidoglycan

- CV-I crystals do not leave

• Gram negative

- Wall is lesser or

- Wall is made up of pseudomurein (lack NAM and D-amino acids)

Acid-fast organisms are difficult to characterizeusing standard microbiological techniques (e.g.Gram stain - if you gram stained an AFB the resultwould be an abnormal gram positive organism,which would indicate further testing), though theycan be stained using concentrated dyes, particularlywhen the staining process is combined with heat.Once stained, these organisms resist the dilute acidand/or ethanol-based de-colorization procedurescommon in many staining protocols—hence thename acid-fast.

Atypical Cell Wall

• Acid-fast cell wall

- e.g gram positive

- Waxy lipid (mycolic acid) bound to peptidoglycan

- Mycobacterium

- Nocardia

Atypical Cell Wall

• Mycoplasmas

- Lack cell walls

- Sterols in plasma membrane

• Archaea

-Wall-less or Wallas of pseudomurein (lack Nam and D-amino acids)

Cell Wall-less Forms

• Few bacteria are able to live or exist without a cell wall.• The mycoplasmas are a group of bacteria that lack cell wall.• Mycoplasmas have sterol-like molecules incorporated into their

membranes and they are usually inhabitants of osmotically-protected environments.

• Mycoplasma pneumoniae is the cause of primary atypical bacterial pneumonia, known in the vernacular as "walking pneumonia". For obvious reasons, penicillin is ineffective in treatment of this type of pneumonia. Sometimes, under the pressure of antibiotic therapy, pathogenic streptococci can revert to cell wall-less forms (called spheroplasts) and persist or survive in osmotically-protected tissues. When the antibiotic is withdrawn from therapy the organisms may regrow their cell walls and reinfect unprotected tissues.

Damage to the cell wall

• (Gram –ve): Lysozyme digests disaccharide in peptidoglycan

• (Gram +ve): Penicillin inhibits peptide bridges in peptidoglycan

• Protoplast is a wall-less cell

• Spheroplast is a wall-less gram positive cell

- Protoplasts and spheroplasts are susceptible to osmotic lysis

• L forms are wall-less cells that swell into irregular shapes

External structures

• Many bacteria have structures that extend beyond or surround cell wall

I. Flagella and pili extend from the cell membrane through the cell wall and beyond

II. Capsules and slime layers surround the cell wall

4. Bacterial Cell Surface Structures

• Arrangements of Bacterial Flagella

1. Monotrichous: Bacteria with a single flagellum located at one end (pole)

2. Amphitrichous: Bacteria with 2 flagella one at each end

3. Peritrichous: Bacteria with flagella all over the surface

4. Atrichous: Bacteria without flagella

5. Cocci shaped bacteria rarely have flagella

Structure of flagella in gram –ve and +ve bacteria

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Chemotaxis

• Bacteria move away or towards subtances that are present in the environment through a nonrandom process

1. Positive chemotaxis: movement towards attractants (nutrients)

2. Negative chemotaxis: movement away from the repellent

Pili

• Pilus (singular)

• Tiny hollow projections

• Used to attach bacteria to surfaces

• Not involved in movement

1. Long conjugation

2. Short attachment pili (fimbriae)

Glycocalyx

• Capsule & Slime Layer

• Used to refer to all polysaccharide/polypeptide containing substances found external to cell wall

1. Capsule

2. Slime layers

3. All bacteria at least have thin small layer

5. Glycocalyx

Capsule• More firmly attached to the cell wall. • Have a gummy, sticky consistency and

provide protection & adhesion to solid surfaces and to nutrients in the environment.

• Bacteria that possess capsules are considered encapsulated, and generally have greater pathogenicity because capsules protect bacteria, even from phagocytic white blood cells of the immune system.

• The adhesive power of capsules is also a major factor in the initiation of some bacterial diseases.

Slime Layer

• A glycocalyx is considered a slime layer when the glycoprotein molecules are loosely associated with the cell wall. Bacteria that are covered with this loose shield are protected from dehydration and loss of nutrients.

Capsule

• Protective structure outside the cell wall of the organism that secretes it

• Only certain bacteria are capable of forming capsules

• Chemical composition of each capsule is unique to the strain of bacteria that secrete it

• Encapsulated bacteria are able to evade host defense mechanism (phagocytosis)

Slime Layer

• Less tightly bound to the cell wall and is usually thinner than the capsule

• Protects the cell against drying, traps nutrients and binds cells together (biofilms)

Biofilms

• Biofilm usually begins to form when free swimming bacterium attaches to a surface

• Share nutrients

• Shelter from harmful factors 10

Case Study

• Patients with indwelling catheters received contaminated heparin with Pseudomonas fluorescens

• Bacterial numbers in contaminated heparin were too low to cause infection

• 84–421 days after exposure, patients developed infections

Delayed Bloodstream Infection Following Catheterization

Functions Of The Bacterial EnvelopeFunction Component(s)

Structural Rigidity All.

Packaging Of Internal Contents All.

Permeability BarrierOuter membrane or plasma

membrane.

Metabolic UptakeMembranes and periplasmic

transport proteins, porins, permeases.

Energy Production Plasma membrane.Adhesion To Host Cells Pili, proteins, teichoic acid.

Immune Recognition By Host All outer structures.

Escape From Host Recognition Capsule, M protein.

Antibiotic Sensitivity Peptidoglycan synthetic enzymes.

Antibiotic Resistance Outer membrane.Motility Flagella.Mating Pili.Adhesion Pili.

ReferencesBooks:Biology of microorganisms By M. T. Madigan, J. M. Martinko, D. A. Stahl and D. P. Clark

Images:1. http://3.bp.blogspot.com/_BwlbMaa50jo/TI1aRa_uoPI/AAAAAAAAHZI/CRr9tao2htE/s160

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