Implicit factors

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

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

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

Grow Salmonella to mid-log phase Percent survival 0 2 4 6 hours 0 2 4 6 hours 0 2 4 6 hours 0 2 4 6 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. 1995. J. Bact. 177:4097

Adaptation to heat Bunning et al., 1990. Appl. Environ. Microbiol. 56:3216.

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

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

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?

Wikipedia

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

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) http://bioweb.usu.edu/microscopy/Research.htm http://genome.jgi-psf.org/strth/strth.home.html

Acidity pH Log CFU/g pH ST LB

Synergy between cultures From Walstra et al., 1999