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Microbial Growth

Part B. 6. Microbial Growth. pp. 160-177. Fig. 6.11. Cell wall. DNA (nuclear area). Partially formed cross wall. Ch 6 Overview. Physical and chemical requirements for growth Various types of culture media Bacterial cell division Phases of bacterial growth. Distribution of Microbes.

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Microbial Growth

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  1. Part B 6 Microbial Growth pp. 160-177 Fig. 6.11 Cell wall DNA (nuclear area) Partially formed cross wall

  2. Ch 6 Overview • Physical and chemical requirements for growth • Various types of culture media • Bacterial cell division • Phases of bacterial growth

  3. Distribution of Microbes • Growth of a population • Increase in the number of cells • Not size • Each ‘spot’ on the agar is a colony • When visible • Cells number in the millions Bacterial colonies

  4. Requirements for Growth Physical and Chemical • Reasons for understanding the conditions necessary for growth: • To control growth of pathogens • To encourage growth of helpful microbes

  5. Physical requirements: • Temperature • pH • Osmotic pressure Chemical requirements: • Carbon, Nitrogen, sulfur, phosphorus, trace elements, organic growth factors • Oxygen

  6. Temperature • Classified in 3 groups • Psychrophiles (cold-loving) • Mesophiles (moderate-temp-loving) • Thermophiles (heat-loving) Mesophiles Thermophiles Psychrophiles Room T

  7. Minimum, Optimum, Maximum Growth Temperatures • Minimum: the lowest T that a species will grow • Optimum: best growth T • Maximum:highest T that growth is possible

  8. Most grow in a limited range • Maximum & minimum growth T about 30o C apart • Grow poorly at high & low T in their range Fig. 6.1

  9. Fastest reproduction = peak of curve • At either T extreme = rate drops • At T just above optimum = fastest drop Optimum Growth Rate Fig. 6.1

  10. Psychrophile • Can grow at Oo C • Optimal growth at ~15o C • But, very sensitive to higher T • Will not grow at ~25o C • Habitat: deep ocean • Pathogenesis: rarely causes food spoilage

  11. Psychrotroph A slight ‘variation’ of a psychrophile • Can grow at Oo C • Slightly higher optimal growth T ~ 20-30o C • Also, very sensitive to higher T • Will not grow above ~40o C • Much more common than psychrophiles • Pathogenesis: frequent cause of food spoilage

  12. Refrigeration • Most common food preservation method • (pathogenic) bacteria grow poorly • At the high and low T in their range • (even though) psychotrophs grow at ~20-30o C • Theygrow slowly • And are able to slowly degrade food

  13. Set T in Refrigerator at a Low Setting • To slow growth of • Most spoilage microbes • To prevent growth of • Most pathogenic species O F O C Fig. 6.2

  14. Mesophiles • Optimal growth ~ 37o C • Most common bacteria • Aerobic • Can be pathogenic • But also, can do work of industrial importance

  15. ‘Swiss’ Cheese • Produced by • Propionibacteria freudenreichii • Grows at 24o C • Ferments lactate to acetate, propionate & CO2 • Acetate, propionate give the cheese its flavor

  16. Thermophiles • Growth: ~50-60o C, and higher • Habitat: hot springs • H2S provides source of energy Boulder Spring Lower Geyser Basin Yellowstone Nat’l Park

  17. pH • Describes acidity or alkalinity of a solution • Most bacteria grow in a neutral environment • Between ~6.5 and 7.5 • Example: • Escherichia coli Fig. 2.7

  18. Fig. 2.7 ph below ~4 is acidic Preserves foods ..sauerkraut, pickles • Bacterial fermentation produces acids • Called acidophiles Helicobacter pylori Fig. 25.14

  19. Osmotic Pressure Solute = [NaCl] • The pressure [NaCl] is equal across the cell membrane Fig. 6.4

  20. Osmotic Pressure Refers to the concentration of NaCl Solute = [NaCl] • Inside a structure • And in the solution surrounding the structure Chapter 4, p. 93-4, Fig 4.18

  21. Isotonic Pressure Osmotic pressure [NaCl] concentration is equal across the membrane Fig. 6.4

  22. Hypertonic Pressure Osmotic pressure [NaCl] concentration is: • Higher in the solution surrounding the bacterium • Than inside the bacterium • Causes death of the bacterium

  23. Hypotonic Pressure Osmotic pressure [NaCl] concentration is: • Lower in the solution surrounding the bacterium • Than inside the bacterium • Also causes death Chapter 4, p. 93-4, Fig 4.18d

  24. Plasmolysis -Loss of Water- • Water leaves the cell • Enters the high solute [NaCl] • Causes shrinkage of the cell cytoplasm • Cell death • High salt used to preserve foods (or sugar)

  25. Chemical Requirements Microbes must acquire nutrients from its environment to grow & reproduce • In the lab, we provide nutritional needs • In culture media • Media must contain Water, carbon, nitrogen, minerals (S, P, Ca, Mg, Na, etc)

  26. Water (80-85% weight of cells) • Carbon (50% wt/cell) • To get C, use organic molecules (proteins, carbohydrates, lipids) or CO2 • Nitrogen (14% wt/cell) • Digest protein into amino acids • Use N for protein, NA synthesis

  27. Minerals • Sulfur (amino acids, thiamine, biotin) • Phosphorus (DNA, RNA, ATP, membranes) • K, Mg, Ca (cofactors for enzymes) • Trace elements (usu enzyme cofactors) • Iron, copper, molybdenum, zinc • Organic growth factors for some bacteria • p 169

  28. Oxygen Classification based on oxygen requirements • Obligate aerobes • Facultative anaerobes • Obligate anaerobes • Aerotolerant anaerobes • Microaerophiles

  29. 1. Obligate Aerobes • Microbes that use oxygen • Produce more energy from nutrients • Than microbes that do not use oxygen • Obligate aerobes require oxygen to live Examples: Bacillus, Serratia, Pseudomonas, Vibrio

  30. 2. Facultative Anaerobes • Oxygen is poorly soluble in water • Therefore, many bacteria have evolved to live in the absence of water • Can use oxygen when it is present • But, in absence, use fermentation or anaerobic respiration • Produces less energy Examples: Escherichia coli, Shigella, Salmonella

  31. 3. Obligate Anaerobes • Microbes unable to use oxygen for energy yielding reactions • Most are killed by oxygen Example: Clostridium (botulism, tetanus)

  32. 4. Aerotolerant Anaerobes • Cannot use oxygen for growth • But, can tolerate it • Posses enzyme SOD which neutralizes oxygen • Mostly ferment carbohydrates to lactic acid Example: Lactobacillus

  33. 5. Microaerophile • Microaerophiles are aerobic • They require oxygen • But, only in LOW [O2]’s • Sensitive to superoxide free radicals • They do not have the SOD enzyme Example: Micrococcus

  34. O2 and Enzymes Aerobes, facultative anaerobes & aerotolerant anaerobes must have the enzyme: Superoxide dismutase (2O2– + 2H+ O2 + H2O2) Peroxide is toxic to cells See Lab experiment 14

  35. And aerobes & facultative anaerobes must have either: Catalase (2H2O2 2H2O + O2) Peroxidase (H2O2 + 2H+ 2H2O) See Lab experiment 14

  36. Bacterial Cell Division • Called binary fission • A single cell divides into 2 cells • A few species divide by ‘budding’ Fig. 6.11

  37. Streak Plate Method Pure cultures are obtained by the streak plate method Fig. 6.10a,b; Ch 6, p 173-4

  38. Phases of Bacterial Growth • Generation time • Time required for a cell to divide • Or, a population to double • Most bacteria have a generation time of • One to three hours • Others require more than 24 hours

  39. Logarithmic Representation of Bacterial Populations Bacterial division occurs according to a logarithmic progression • 2 cells, 4 cells, 8 cells, 16 cells, 32 cells… (yx) Fig. 6.12 b

  40. Bacterial Growth Curve

  41. Q’s • Which of these terms describes a microorganism that grows at human body temperature (37oC)? • Thermophile • Psychrophile • Psychrotroph • Mesophile • Hyperthermophile • During log phase, bacteria are: • Dividing at fastest rate • Preparing to divide • Dying exponentially • Dying and dividing in equal numbers

  42. Q’s • Which of these terms describes microbes that do not use oxygen, but can grow if it is present? • Obligate aerobe • Obligate anaerobe • Facultative anaerobe D. Microaerophile E. Aerotolerant anaerobe • Which of these is an example of an organic growth factor? • Glucose • Oxygen • Biotin D. Sodium chloride E. Agar

  43. Appendix

  44. Bacterial Growth Curve - Phases • Lag phase: • Little or no change in the number of cells, but metabolic activity is high • Log phase: • Bacteria multiply at the fastest rate possible under the conditions provided • Stationary phase: • An equilibrium between cell division and death occurs • Death phase: • Deaths exceed the number of new cells formed

  45. Q’s • Shrinking of the plasma membrane in response to osmotic loss of water is called: • Bacteria that spoil food in the refrigerator are most likely: • Thermophiles • Psychrotrophs • Mesophiles • psychrophiles • The _____ _____ _____ is the most common technique used to obtain pure cultures of microbes.

  46. Q’s • Organisms such as Clostridium lack oxygen-detoxifying enzymes, thus they are: • Obligate aerobes • Obligate anaerobes • Facultative anaerobes • Microaerophiles • Aerotolerant anaerobes • Which enzyme catalyzes the reaction: • H2O2 + 2H+ 2H2O • Catalase • Oxidase • Peroxidase • Superoxide dismutase

  47. Q’s • Which growth phase is also called the exponential growth phase? • Lag phase • Log phase • Stationary phase • Death phase

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