Microbial Growth

1. 6 Microbial Growth

2. Microbial Growth • Microbial growth is the increase in number of cells, not cell size

3. Reproduction in Prokaryotes • Binary fission • Budding • Conidiospores (actinomycetes) • Fragmentation of filaments

4. Binary Fission Figure 6.11

5. Microbial Growth • Generation Time • Time required for a cell to divide (and its population to double) • E. coli: 20 minutes (under ideal conditions)

6. Figure 6.14

7. The Requirements for Growth: Physical Requirements • Temperature • Minimum growth temperature • Optimum growth temperature • Maximum growth temperature

8. The Requirements for Growth: Physical Requirements • Temperature • Psychrophiles: cold-loving microbes • Mesophiles: moderate temperature-loving microbes • Thermophiles: heat-loving microbes • Psychrotrophs: capable of growth between about 0O and 30OC • Cause food spoilage • Hyperthermophiles (extreme thermophiles): optimum growth temperature of 80OC or higher

9. Temperature Figure 6.1

10. Psychrotrophs Figure 6.2

11. The Requirements for Growth: Physical Requirements • pH • Most bacteria grow between pH 6.5 and 7.5 • Molds and yeasts grow between pH 5 and 6 • Acidophiles grow in acidic environments • Chemical buffers are included in growth media to neutralize acids and maintain proper pH

12. The Requirements for Growth: Physical Requirements • Osmotic pressure • Microorganisms obtain almost all their nutrients in solution from the surrounding water • Hypertonic environments, increase salt or sugar, cause plasmolysis (shrinkage of the cell’s cytoplasm) • Extreme or obligate halophiles require high osmotic pressure • Facultative halophiles tolerate high osmotic pressure

13. The Requirements for Growth: Physical Requirements Figure 6.4

14. The Requirements for Growth: Chemical Requirements • Carbon • Structural organic molecules, energy source • Chemoheterotrophs use organic carbon sources • Autotrophs use CO2

15. The Requirements for Growth: Chemical Requirements • Nitrogen • In amino acids and proteins • Most bacteria decompose proteins • Some bacteria use NH4+ or NO3– • A few bacteria use N2 in nitrogen fixation • Sulfur • In amino acids, thiamine and biotin • Most bacteria decompose proteins • Some bacteria use SO42– or H2S • Phosphorus • In DNA, RNA, ATP, and membranes • PO43– is a source of phosphorus

16. The Requirements for Growth: Chemical Requirements • Trace elements • Inorganic elements required in small amounts • Usually as enzyme cofactors

17. The Requirements for Growth: Chemical Requirements • Oxygen (O2) Table 6.1

19. The Requirements for Growth: Chemical Requirements • Organic growth factors • Organic compounds obtained from the environment • Vitamins, amino acids, purines, and pyrimidines

20. Culture Media and Obtaining Pure Cultures

21. Culture Media • Culture medium: Nutrient material prepared for the growth of microorganisms in a laboratory • Sterile: contains no living microorganism • Inoculum: microbes introduced into a culture medium to initiate growth • Culture: microbes that grow and multiply in or on a culture medium

22. Agar • Complex polysaccharide • Derived from algae • Used as solidifying agent for culture media in Petri plates, slants, and deeps • Generally not metabolized by microbes • Liquefies at 100°C • Solidifies ~40°C

23. Petri dish • Shallow dishes with a lid that nests over the bottom to prevent contamination

24. Culture Media • Chemically defined media: Exact chemical composition is known • Complex media: Extracts and digests of yeasts, meat, or plants • Nutrient broth (liquid form) • Nutrient agar (when agar is added)

25. A pure culture contains only one species or strain. • A colony is a population of cells arising from a single cell or spore or from a group of attached cells. • A colony is often called a colony-forming unit (CFU).

26. Streak Plate • The isolation method most commonly used to get pure cultures. • Sterile inoculating loop is dipped into a mixed culture • Streaked in a pattern over the nutrient medium • Last cells to be rubbed off the loop are far enough apart to grow into isolated colonies • Colonies can be picked up with an inoculating loop and transferred to a test tube of nutrient medium to form a pure culture Figure 6.10a–b

28. Streak Plate Figure 6.10a–b

29. Direct methods Plate counts Filtration MPN Direct microscopic count Dry weight Indirect methods Turbidity Metabolic activity Dry weight Measuring Microbial Growth

30. Direct Measurements of Microbial Growth Figure 6.19, steps 1, 3

31. Estimating Bacterial Numbers by Indirect Methods • Turbidity Figure 6.20