Physiology of Bacterial Growth

Lecture 1

Prokaryotes in general

• mirco-organsims

• no nucleus

• single-celled (although some exceptions)

Age

• Oldest cellular life

• 3.8 billion years ago

Where found

All ecosystems

• deep-sea thermal vents

• dust particles

• On organisms:

  • muutualistically

  • pathogen

Archaea

Inhabit only extreme environemnts

• but some are found in many environmnets

Bacterial cell stucrure

Size: 2-5 micrometers

Essential bacterial structural elements

1. Nucleaur body- single circular chromosome

   • 2000 genes

2. Cytoplasms

   • Metabolic acitivity

   • ribosomes etc

3. Protoplast/ cell membrane

   • major barrier between cell and environment

4. Cell wall

   • petidoglycan

   • confer shape

   • osmotic protection

Non essential extra stutures

1. Granules

   • storage polymers

   • made of polyphosphate or glycogen

2. Photosynthetic membrane

   • some do photosynthesis

   • with bacteriochlorophyll (like plants)

     • or

   • Arachea use bacteriorhodopsin proton pump

3. Capsule

   • polymer sugar or amino acids

   • surround cell

4. Pili

   • protein filaments 0.1 micrometre long

   • attachment

5. Flagellum

   • protein filament

   • 3-20 micromemtres

   • long for mobility

Flagellum motion how

Driven by H+ or Na+ driven rotary motor

Kinetics of Bacterial growth

Binary fission

• divides symmetrically

• two daughter cells

Optimal conditions:

• exponential increase

• doubling time= 10-20 mins

• 10²2 cells in 24 hours

Bacterial growth graph

1. Lag: adapting to environement

2. Log phase: exponential growth

3. Stationary: stops due to lack of nutrients or toxic waste

4. Death: exponential loss of viable cells

Nutrient acquisition in bacteria

1. Active transport

   • low molecular weight solutes across membrane

   • E.g lactose H+ symport or phosphate H+ symport

2. Release enzymes

   • degrade polymeric substances

   • so small enough to be transported in

3. Release toxins

   • plants and animal pathogens

   • Increase nutrient availability

4. Adaptive reponses

   • chemotaxis- moving towards nutrient sources

5. Induction of high affinity transport systems

6. Novel thing of getting iron in: Siderophores

Nutrient requirements for bacteria

1. Carbon: energy source

2. Hydrogen: organic nutrients

3. Phosphorus: Pi

4. Nitrogen: NH4+ or amino acids

5. Sulphur: SO42-

6. Ions: K+,MG2+ and Cl- and trace Fe2+, (MoO4)2-

Acquisition of Fe2+

• Free iron in environments is low: 10^-17

• release siderophores

  • e.g enterobactin

  • Bind iron with high affinity

  • then acitively transported back into cell

  • iron released for use

How can permeability of bacterial cell envelope differ?

Can have Gram positive and Gram-negative

• permeability is different for each

• found with differential staining

Gram-positive bacteria

• 50 layers of peptidoglycan

• interspersed with teichoic acids

Permeability:

• freely permeable to molecules of < 1000 Da

  • Most stuff gets through

Only limited by rate of diffusion

Gram-negative bacteria strucutre

• Outer and inner membrane with cell wall inbetween

• Creates na additional compartments: periplasm

  • space between outer and cell wall

• Cell wall thinner:

  • 3-5 sheets of peptidoglycan

Gram negative advantages

Advantage over gram positive:

• limit access of delertious mocleues

  • antibiotics and detergenes

• extra compartment: periplasms

Gram negative permeability how?

Outermembane specific proteins

• e.g Porins

  • Trimeric with water filled pore of 0.8-1.3 nm diameter

  • allows hydrophobic moelcules of <700 Da

  • No ion-gradeint linked active transport

Example strucutre of porin

OmpF and OmpC

Role of peptidoglycan

• Structural component

• Maintain shape

• osmotic protectant

Evidence for peptidoglycan role

1. Purified peptidogclycan reatins shape of cells

2. Treatment of bacteria with lysozyme

   • cleaves peptidoglycan off

   • when put in low hypotonic solution:

     • lysis

Challenge to bacteria growth

Must expand the cell walls

But

Grow in dilute aqueous environments

• hypotonic environment: under high osmotic pressure

How get over this problem: how grow?

1. Autolysins

   • always breaking down the outside layers

2. Biosynthetic enzymes

   • crosslink new peptiglycan as inner layers

   • incorporated in realxed state

3. Eventually gets to the outside layers

How penecillin works

Inhibits acitivity of cross-link enzymes:

• no more cell wall made

• bacteria eventually autolyse all cell wall off

Grow Peptidoglycan in Gram-positive rod- shaped bacterium STEP 1

1. • Enxtention of linear petidoglycan between two rigid poles

   • incorporated in relaxed sate in inner surface

   • outer layers cleaved

   • Internal osmotic pressure pushes poles apart

Result: Linear growth of the wall

Grow Peptidoglycan in Gram-positive rod- shaped bacterium STEP 2

2. • synthesis of double thickness cross wall

   • constricts protoplast membrane

   • divides cytoplasm in two

Grow Peptidoglycan in Gram-positive rod- shaped bacterium STEP 3

3. -

   • Cleavage of the wall before it is fully cross-linked

   • internal osmotic pressure of the cell pushes out the cross wall

   • forms a hemispherical pole

Grow Peptidoglycan in Gram-positive rod- shaped bacterium STEP 4

4. Final cross-linking of the poles

   • to form rigid structures

NB: this is only part of the cell division process

• DNA rep

• DNA seg

• cytosolic and envelope components

Ways bacteria are unicellular BUT can work together

1. Quorum sensing

2. Rapid Electrical signalling

Quorum sensing

• Release chemical signals

  • homoserine, lactone

• Monitor own Population density

  • Take co-ordinated action together only when a critical cell density is reached

Why have Quorum sensing?

Controls toxin and effector production by pathogentic bacteria

• tell when big enough to be able to sufficiently overwhelm e.g plant defences

Rapid electrical signalling

• form communities of biofilms

  • e.g tooth decay

• What they do?

  • ensure enven distribution of food supply

  • electrical signalling mediated by K+ channels in plasma membrane

Rapid electrical signalling how work- glutamic acid

1. Cells at edge find glutamate

2. cells at centre lack glutamate

3. These activate K+ ion extrusion

4. Change in trans-membrane voltage around neighbouring cells

5. … which activates YugO channels

6. expanding wave of increased K+ created

7. inhibits glutamate uptake by edge cells

8. More to the centre

This can then reverse as the edge cells get hungry- oscillations

OVERALL: even distribution of resource