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BACTERIAL GROWTH

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Bacterial Growth

Pretend this jar of jellybeans is a colony of bacteria, made up of 256 (single-celled) bacteria

  • How many bacteria do you think were present in the first place to end up with 256 in the colony?
  • How many “generations” or divisions took place to reach 256?
  • How long do you think it would take to produce 256 bacteria?

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Bacterial Growth

  • Refers to an increase in cell number, not in cell size.

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  • Bacteria grow and divide by binary fission, a rapid and relatively simple process.

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Bacterial Growth: Binary Fission

  • Prokaryotes reproduce by simply splitting in two in a process called binary fission.

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  • Asexual reproduction
    • The DNA is copied and the cell divides into two identical cells.

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Recall

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Rates of Bacterial Growth

  • Bacteria grow exponentially:
    • When the cell divides, it becomes two daughter cells, which then divide themselves.
    • If each division takes 20 minutes, it’s easy to see how 256 can grow in just 2 hours and 40 minutes.

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Requirements for Growth

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1. Appropriate Temperature: Different microbes have different preferred temperature ranges.

      • Most pathogens grow between 5oC - 60oC.

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The thing about rice ….

Uncooked rice can contain spores of Bacillus cereus, bacteria that can cause food poisoning.

The spores can survive when rice is cooked. If rice is left standing at room temperature, the spores can grow into bacteria.

These bacteria will multiply and may produce toxins (poisons) that cause vomiting or diarrhoea.

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That’s why it’s important to eat rice once done cooking and store it within the hour in an airtight container in the fridge.

Rice can be kept in the fridge for one day.

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Requirements for Growth

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2. Appropriate pH:

  • Most bacteria prefer neutral pH (6.5-7.5).
  • Acidity inhibits most microbial growth and is used frequently for food preservation

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Requirements for Growth: Chemical Requirements

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3. Nutrients

    • Bacteria, like people, need food to grow
    • Carbon, Nitrogen, Sulfur, and Phosphorus
      • Structural backbones of all organic compounds.

4. Moisture

    • Bacteria grow best when they have plenty of moisture available

5. Oxygen

    • Some microbes need oxygen whereas others are able to survive in the absence of oxygen

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Bacterial growth curve - The number of cells in the population over time (in a liquid medium).

Generation Time: Time required for a cell to divide, and its population to double.

Generation time varies considerably:

  • E. coli divides every 20 minutes.
  • Most bacteria divide every 1 to 3 hours.

Growth of Bacterial Cultures

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Scientific Notation

  • Scientists need a short way of expressing the large numbers (in this case, of bacteria)
  • Therefore, numbers of bacteria in foods are often expressed using scientific notation.
    • For example, 256 bacteria in a food would be expressed as 2.56 x 102 in scientific notation.

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  • How would you express the following using scientific notation?

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A. 456

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B. 17,000,000

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Growth of Bacterial Cultures

Representation of Bacterial Growth:

We can express the number of cells in a bacterial generation as 2n, where n is the number of doublings that have occurred.

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Representation of Bacterial Growth:�Four Phases of Bacterial Growth Curve

Lag phase : bacteria adjusting to the environment

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Exponential growth phase : growth happens so rapidly that it is best described in logarithmic terms

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Stationary phase : bacteria runs out of nutrients or when bacteria produces waste that limits growth. Plateau signifies that birth rate = death rate

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Death phase : bacteria dying greater than that being created

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*not in your notes - you will need to find space to include this

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Food Preservation:�Limiting Bacterial Growth

  • At Home
    • Canning
    • Pickling
    • Refrigerating
    • Freezing
    • Smoking

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  • Industry
    • Pasteurization
    • UV irradiation
    • Vacuum packing

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Questions

1. Fred was making a macaroni salad, but he couldn’t find a spoon so he used his hands to mix in the dressing. There were four S. aureus bacteria on his hands, which contaminated the salad.

Fred then took the salad to a picnic, where it was left out, unrefrigerated for two hours. How many S. aureus grew in the salad? Was this salad safe to eat?

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(Note: generation time for S. aureus is 20 min, and it only takes 100 bacteria to make someone sick)