C4 Microbes in Food

Question 1

What are the phases in which primary metabolites and secondary metabolites are produced?

Answer 1

Primary metabolites: Log phase
Secondary metabolites: Stationary phase

Question 2

State the suitable mode of fermentation for producing primary metabolites and secondary metabolites respectively.

Answer 2

Batch: Secondary metabolites
Fed-batch and continuous: Primary metabolites

Question 3

Describe the factors affecting the growth rate of microorganisms in the LAG PHASE.

Answer 3

Growth in lag phase:
- Physiological stress from osmotic pressure
- Chemical composition: transfer of bacteria into different carbon sources leads to longer lag phase
- Age of inoculum: inoculum in exponential growth phase will have shorter lag phase in fresh medium
- Concentration of inoculum: Higher concentration leads to lower dilution shock.
- Viability and morphology of the inoculum: Healthy cells have lower lag phase

Question 4

Describe the growth of microorganisms in detail in the stationary phase.

Answer 4

In the stationary phase, the available limiting nutrients depletes. However MOs are still metabolically active and metabolizes intracellular storage compounds and utilize nutrients released from lysed cells.

Question 5

What are the DIRECT techniques for studying microbial populations?

Answer 5

Dry weight: Determines weight of living and dead cells. Time consuming, suspended non-cellular components may cause error.

Cell count: Direct microscopic count using a counting grid. Rapid. Not suitable for filamentous MOs, cannot distinguish btwn living and dead cells, not suitable for low concentrations

Plate count:
Pour plate: More difficult to count than spread plate, high temp of molten agar
Spread plate.
Requires 1-2 days of incubation, single colony may not be formed by single cell only.

Question 6

What are the INDIRECT techniques for studying microbial populations?

Answer 6

ATP Bioluminescence: Determines the conc of viable MOs via the ATP released. Luciferase-luciferin complex generates a photon of light for each ATP. The method is rapid and sensitive but not suitable for colored samples.

Turbidimetry, spectrometry, CO2 prod rate or Oxygen uptake rate.

Question 7

What are the factors to be considered before deciding on the method for assessing MO populations?

Answer 7

• Degree of accuracy and sensitivity required
• Duration of analysis
• Type of microorganisms
• Properties of culture media

Question 8

What are the environmental factors that can affect microbial growth.

Answer 8

(SHORT PP)
- Solutes and water activity: Xerophiles (Low Aw), Halophiles (High salt), Osmophiles (High sugar)
- Hydrostatic pressure: Barophilic and barotolerant MOs.
- Oxygen: Aerobes and Anaerobes
- Radiation
- Temperature: Thermophiles and psychrophiles
- pH: Acidophiles and alkalophiles

Question 9

What are the factors that contribute to MOs’ resistance towards radiation?

Answer 9

• Bacterial endospores
• Protective pigments (e.g., carotenoids)
• Effective repair mechanisms (e.g., S. cerevisiae, E. coli)