Food: what sorts of microbes live there, and how?

1. Food: what sorts of microbes live there, and how? How do we know? How do we find out? What are the factors affecting microbial growth?

3. Food as an “ecosystem” • Environment and the organisms living in it • Intrinsic factors • pH, water level, nutrients • Extrinsic factors • Temperature, presence of oxygen, “competition” • A food can have multiple environments • The environments can change

4. See Fig. 2.1 in textbook Source: Microbiology, a Human Perspective. Nester et al, 7th edition.

5. Detection and enumeration methods are artificial • Climate control • Microbial growth media (liquid or solid) • Isolation/pure culture techniques • Enumeration methods may require dilution or enrichment

6. Physiological processes can complicate measurement Nature.com • VNC (viable but nonculturable) • Cells may be injured but capable of recovery • Cells can go into a protective dormant state (so they don’t actually die) • Staining and microscopy techniques can be used instead

8. Intrinsic factors: pH • What is pH? • Hydrogen ion concentration in a solution • Pure water is “neutral” (pH7) • pH of acid lower than 7, base higher than 7 • Most cells have internal pH around 7

9. alkaline-ion-water.com

10. Significance of food (environmental) pH • Most foodborne pathogens cannot grow at pH of less than 4 (very acidic environment). They either die or cannot grow • Fungi and lactic acid bacteria can grow at low pH • “Successful” organisms have specific processes to adapt to acid pH (genetic) • Effect of “wrong” pH on microbes?

11. Intrinsic factor: water activity (aw)A few words about water • All living things require water • Cohesion • High specific heat • Density of ice • Solvent

12. What is water activity? • How do you measure it? • Ratio of vapor pressure of food to that of pure water (number will always be between 0 and 1) • Number represents availability of water (e.g., for microbial growth) • Actual moisture content of foods can vary • What is the aw of different foods? • Can you change the aw? • What sorts of organisms grow at different aws?

13. Aw ranges for different foods • 0.10-0.20 • Cereals, crackers, salt, sugar • <0.60 • Noodles, honey, chocolate • 0.60-0.85 • Jams and jellies, nuts, dried fruits, dry cheeses • 0.85-0.93 • Fermented sausages, dried cured meats, maple syrup • 0.93-0.98 • Evaporated milk, bread, fresh sausage, processed cheese, fruit juice • 0.98-0.99 • Fresh meat, fruits, vegetables, milk, eggs

14. Can you change the aw? Sure! • Hydrating • Freezing; drying • Adding solutes • Treatments to control microbial growth might be needed if aw is very high • Adding preservatives • Antimicrobial treatment, e.g., Pasteurization • Adjusting pH

15. Growth patterns of microbes: a growth curve Decreased by lower aw Increased by lower aw

16. Aw requirements for foodborne microbes • Most Gram-negative microbes require a high aw (more than 0.93) • Staphylococcus can grow at a low aw but won’t produce toxin • Spore formers are less sensitive • Molds (especially) and yeast can grow at a lower aw than bacteria • See Table 2.6 for actual values

17. Extrinsic factors: Temperature Cigna.com foodsafety.suencs.com

18. Extrinsic factors, continued • Microbes vary in their capacity to grow at different temperatures • Access to oxygen (aerobes, anaerobes, etc.) • Hurdle concept: to use multiple measures to control microbial growth • Increase shelf life • Improve quality of food • We’ll get back to this

19. A few words about microbial metabolism • Living organisms generate ATP to do work (convert one form of energy to another) • Series of chemical reactions that move electrons around • Some organisms use oxygen as a terminal electron acceptor (aerobes) • Some cannot use oxygen but use something else (anaerobes, fermenters) • Some can use either (facultative anaerobes)

20. Comparison of different types of respiratory pathways

21. Fermenters and their end products

22. So what? • Aerobes can extract the most energy from a molecule of glucose. End products are CO2 and water • Anaerobes obtain less energy and produce different end products • Fermenters produce many useful end products • Microbes can metabolize many molecules that other organisms cannot • Which type of respiration occurs in foods?

23. A few words about spores • Bacterial spores • Also termed “endospores” • Protect bacteria from environmental stresses • Only certain bacterial genera produce endospores • Some sporeformers produce toxins • Fungal spores • Reproductive structures • Also serve protective function • Some sporeformers are important food spoilage organisms

24. Special problems with sporeformers • Extremely heat- and dryness- resistant • Canning processes for low-acid foods have been standardized to prevent botulism and spoilage • Sporeformers are indicator organisms to test autoclaves!

25. Important sporeformingfoodborne pathogens • Clostridium botulinum • Anaerobe; produces botulism toxin • Clostridium perfringens • Can survive cooking temperatures and produces toxins in the “danger zone” (40-140oF) • Bacillus cereus • Can produce toxins that cause diarrhea and vomiting

26. Why does a bacterium form an endospore? • “Vegetative” cell senses starvation conditions • Spore is formed internally, and then vegetative cell degrades • Calcium is required • “Germination” occurs after exposure to heat or certain chemicals • Viable cells have germinated from spores that were thousands (even millions) of years old

27. Summary • To learn about food microbiology, you need to consider: • The wide variety of growth environments found in food • Intrinsic and extrinsic factors that affect microbial growth (and how they can be manipulated) • The diverse capacity for metabolism found within the microbial world • The capacity of microbes to adapt to changing environments