O’Donnell Bacterial Metabolism

Where is a bacterial growth curve observed?

In cultures in a lab

• Incubated in a closed vessel

• Single batch of medium (liquid with nutrients)

Four phases of bacterial growth

1. Lag

2. Exponential (Log)

3. Stationary

4. Death

Lag Phase of bacterial growth

Growth curve is flat as bacteria adjusts to the new environment

Cell synthesizing new components to replenish spent materials and adapt to new medium or other conditions

Varies in length

• Can be very short/absent

Exponential (Log) Phase of bacterial growth

Constant growth rate

Population is most uniform

• Chemically and physically speaking

Stationary Phase of bacterial growth

Total number of viable cells remains constant

• Metabolically active cells stop reproducing

• Cell division = death rate

  • Growth curve plateau

Why does bacterial growth decrease during the stationary phase?

Nutrient limitation

Limited oxygen availability

Toxic waste accumulation

Critical population density reached

How do bacteria respond to the stationary phase of growth?

Starvation responses

• Morphological changes

  • Decreased size and nucleoid condensation

• Production of starvation proteins

  • Chaperones, DNA protection, etc.

• Long-term survival

  • Increased virulence

During which phase of growth are bacteria the most pathogenic?

Stationary phase

Death phase of bacterial growth

Decline in apparently viable cells

Three possibilities for bacteria cells:

1. Cell death (lack of nutrients)

2. Apoptosis of some cells

3. “Viable but non-culturable”

What does it mean for a bacteria to be viable but non-culturable?

Alive but not growing

Shutting down as much as possible and waiting until conditions are better

• Survivors resuscitate upon change in environment

• Can be years for some bacteria

General pH requirement for pathogenic bacteria

Generally grow well in conditions found in the human body

Optimal pH of 7.2

• Exceptions: bacteria that live in the stomach such as H. pylori prefer a low pH environment

No matter what the pH of the environment is, bacteria must maintain a _____ internal pH

Neutral

Type of bacteria that prefers a temperature around the average human body temperature

Mesophiles

How do siderophores allow bacteria to compete with the host for iron?

Our bodies use transferrin to capture free iron from the circulation

Bacteria release siderophores that compete with transferrin to capture the available iron

Bacteria with effective siderophores are _____

More pathogenic

Ex. Tuberculosis

• Has one of the strongest siderophores

Obligate Aerobe

Bacteria that needs oxygen to survive

Facultative Anaerobe

Bacteria that can live with or without oxygen, but prefers when oxygen is present

Aerotolerant Anaerobe

Bacteria that does not care about oxygen, can live with or without it

Obligate Anaerobe

Organism that cannot live in the presence of oxygen, as it is toxic

Microaerophile

Bacteria that likes oxygen but has a small range of oxygen concentration where they can exist (~2%-10%)

Why is oxygen toxic to some types of bacteria?

Oxygen is easily reduced to toxic products

• Superoxide radical (O₂•⁻)

• Hydrogen peroxide (H₂O₂)

What enzymes protect bacteria from the toxic products of oxygen?

Superoxide dismustase (SOD)

• 2 O₂•⁻ + 2 H⁺ → H₂O₂ + O₂

Catalase

• 2 H₂O₂ → 2 H₂O + O₂

SOD and catalase contents of bacteria types

Obligate aerobe

• SOD and catalase

Facultative anaerobe

• SOD and catalase

Aerotolerant anaerobe

• SOD

Obligate anaerobe

• Neither

Microaerophile

• SOD and sometimes low levels of catalase

General goal of bacterial metabolism

Build new bacteria using whatever is available in the environment

Energy

Capactiy to do work/cause change

Chemical Work

Synthesis of complex molecules

Transport Work

Nutrient uptake, waste elimination, osmotic balance

Mechanical Work

Locomotion

Movement of internal structures

Three common pathways for catabolism of glucose in microorganisms

• Glycolysis

• Pentose phosphate pathway

• Entner-Doudoroff pathway

  • Only a few bacteria; Pseudomonas

What types of bacteria perform glycolysis?

Both aerobic and anaerobic

Aerobic Respiration

Glycolysis + oxidative phosphorylation

Pyruvate feeds into the citric acid (or Krebs) cycle

• O₂ is terminal electron acceptor

Anaerobic Respiration

Glycolysis + fermentation

• Pyruvate feeds into many different fermentation pathways

• End products are short-chain alcohols or fatty acids (Ex. ethanol or lactic acid)

• Type of pathway and endproduct depends on the type of bacteria

Obligate Aerobe Growth Characteristics

Glycolysis + oxidative phosphorylation

Produce more ATP

Grow quickly (24-48 hours in lab)

Obligate Anaerobe Growth Characteristics

Glycolysis + fermentation

Produce less ATP

Grow slowly (longer than a week in some cases)

Facultative Anaerobe Growth Characteristics

If O₂ is present, they will use aerobic respiration to generate more ATP

• Can grow quickly with O₂

Differences between eukaryotic and prokaryotic DNA polymerase

Eukaryotic DNA polymerase is slow and careful, and it stops to correct most errors (50-100 base pairs per second)

Prokaryotic DNA polymerase is fast and sloppy, and it tends to make more mutations, which it does not always fix

• This is not always bad, as “trying out” many mutations can sometimes give antibiotic resistance

How do the processes of transcription and translation in bacteria differ from eukaryotic cells?

Since there is no membrane separating transcription and translation, these processes are couple and happen at the same time.

• Involves the formation of a polysome, or polyribosomal complex, where many ribosomes attach to the same mRNA as it is transcribed

• Allows protein synthesis to happen quickly

Process of cell wall synthesis

1. NAM is synthesized in the cytoplasm while attached to UDP

2. NAM binds to bactoprenol in the cell membrane

3. In the cell membrane, NAM and NAG are attached to each other

4. Bactoprenol acts as a “shuttle” carrying sugars out of the cell by flipping to the outside of the cell wall

5. Outside of the cell membrane, NAM and NAG are attached to the growing peptidoglycan chain

6. Bactoprenol moves back to the inner membrane

Primary goal of bacteria

To survive and spread (NOT to infect/kill us, this is just the result)

Function of autolysin in bacterial growth/division

Cleave peptidoglcan to allow for cell division

Remove old cell wall fragments for maintenance

How is autolysin involved in antibiotic therapy?

Autolysins are always on

• This means that cell wall inhibitors can prevent new pieces from being made while autolysins continue to remove old pieces, leading to the death of the cell