Morphology & Cell Biology of Bacteria

Siti Sarah Jumali (ext 2123)

Room 3/14

sarah_jumali84@hotmail.com

The basic morphology of a cell

the

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

Characteristics of Lipid bilayer• Semi-permeable

• Consists of : 1) hydrophilic

2) 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.

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

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-acetylmuramic acid is a peptide chain of three to five amino acids.

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

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 periplasmic space between the cytoplasmic and 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

Peptidoglycan in Gram positive bacteria

• Linked by polypeptides

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 bacterial cell walls

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 characterize using standard microbiological techniques (e.g. Gram stain - if you gram stained an AFB the result would be an abnormal gram positive organism, which would indicate further testing), though they can be stained using concentrated dyes, particularly when the staining process is combined with heat. Once stained, these organisms resist the dilute acid and/or ethanol-based de-colorization procedures common in many staining protocols—hence the name 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

Polar Monotrichous flagellum

Vibriocholerae

Amphitrichous flagella arrangement

Spirillum

Lophotrichous flagella arrangement

Spirillum

Structure of flagella in gram –ve and +ve bacteria

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

Chemotaxis

Chemotaxis

Pili

• Pilus (singular)• Tiny hollow projections• Used to attach bacteria to surfaces• Not involved in movement

1. Long conjugation

2. Short attachment pili (fimbriae)

E. coli (14,300X)

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

• A microbial community that usually forms slimy layer or hydrogel on a surface

• Bacteria attracted by chemicals via quorum sensing• Composed of populations or communities of

microorganisms adhering to environmental surfaces.• The microorganisms are usually encased in extracellular

polysaccharide that they synthesize.• Can be found in sufficient moisture is present. • Their development is most rapid in flowing systems

where adequate nutrients are available

Biofilms

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

• Share nutrients• Shelter from harmful

factors

Thoughts for the day..

• What is the biofilm that forms on teeth called?

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

Questions?

Functions Of The Bacterial EnvelopeFunction Component(s)   Structural Rigidity    All.

   Packaging Of Internal Contents    All.

   Permeability Barrier   Outer membrane or plasma membrane.

   Metabolic Uptake   Membranes 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.