1. Implicit factors

2. Implicit factors
Properties of the organisms and how they respond to their environment:
Growth rate and tolerance to stressors
Preadaptation: Heat shock, acid shock, etc.
Cold tolerance
Ecology

3. Salts/other osmolytes
Stress adaptation
Mixing ingredients
Exposure to:
High temperatures
Low temperatures
Drying
Limited nutrients
Salts/other osmolytes
Acids
Antimicrobials
And so on…
Raw materials
Final product

4. Effect of growth phase
Wen et al., Appl. Environ. Microbiol. 75:1581

5. Grow Salmonella to mid-log phase
Percent survival
hours
hours
hours
hours
ACID:
Grow Salmonella to mid-log phase
Subject to adaptation shown on X-axis
Expose to pH 3.3
Viable cell counts at 0, 2, 4, 6 hours
Lin et al J. Bact. 177:4097

6. Adaptation to heat
Bunning et al., Appl. Environ. Microbiol. 56:3216.

7. Cross-protection to stresses
Gruzdev et al., Appl. Environ. Microbiol. 77:1667

8. What is going on? One example: “Heat shock proteins”
Chaperones: prevent aggregation, assist in refolding of proteins
Under sublethal heat treatment, HSP chaperones are ~20% of total protein in Escherichia coli
Much still unknown about exact mechanisms of adaptation

9. Growth under suboptimal temperatures
Or under “cold shock”
Microorganisms dealing with:
Decreased rate of biochemical reactions
Decreased fluidity of membranes
How to react?
Pseudomonas grown at 25°C = 10 % unsaturated fatty acid
Pseudomonas grown at 5°C = 50% unsaturated fatty acid
Why?

10. Wikipedia

12. Fatty acid profiles
Mesophiles maintained at low temperatures contain more unsaturated (or other modified) fatty acid residues in the membrane than at higher temperature
As degree of unsaturation of fatty acids increases, the lipid melting point decreases allowing them to survive at low temperatures
Growth at low temperatures likely limited by both rate of biochemical reactions and ability to modify membrane

13. Yogurt: the role of microbial ecology
Lb. delbrueckii subsp. bulgaricus
Gram positive rods; often in chains
Moderate thermophile (42-45 oC)
Streptococcus thermophilus
Gram positive cocci; chains
Moderate thermophile (38-42 oC)

14. Acidity pH
Log CFU/g pH ST LB

15. Synergy between cultures
From Walstra et al., 1999