MICROBIAL GROWTH - BIOL3420 Study Notes

MICROBIAL GROWTH

Goals and Objectives

• Students should understand bacterial reproduction and how bacteria grow in laboratory conditions.

• After this lecture, students should be able to:

  • Describe binary fission in bacteria.

  • Explain generation time and exponential growth of bacteria in optimal conditions.

  • Perform calculations related to bacterial exponential growth.

  • List phases in bacterial population growth and explain main characteristics of these phases.

Key Concepts

• Microorganisms grow when required nutrients and environmental factors are provided.

• Bacterial growth can be observed in two main ways:

  • Increasing cell size.

  • Increasing the number of cells in a population.

• The majority of bacteria reproduce by binary fission.

Rate of Population Growth

• Generation (Doubling Time):

  • Definition: The time required for a complete binary fission cycle.

  • Average generation time for bacteria is typically between 30-60 minutes.

  • Examples of Generation Times:

  • Escherichia coli: 20 minutes.

  • Mycobacterium leprae: 10-30 days.

• Exponential Growth Pattern:

  • Growth pattern in bacterial populations follows an exponential model, expressed as:

  • 2^n

  • Where n represents the number of generations.

• Cell Count Calculation:

  • The number of cells in a bacterial population can be calculated using the formula:

  • Nf = (Ni) imes 2^n

  • Where:

    • N_f = number of cells in the population

    • N_i = starting number of cells

    • n = number of generations

Calculation Problem

• Problem Statement: A total of 100 Escherichia coli cells was inoculated in media and grew under optimal conditions. How many bacterial cells are present in the culture after two hours if the generation time of Escherichia coli is 20 minutes?

  • Calculation Steps:

  • Determine the number of generations in 2 hours:

    • 2 hours = 120 minutes

    • n = rac{120 ext{ minutes}}{20 ext{ minutes/generation}} = 6

  • Use the cell count formula:

    • N_f = (100) imes 2^6 = 100 imes 64 = 6400

  • Result: There will be a total of 6400 Escherichia coli cells in the culture after two hours.

Population Growth Curve

• Phases of Bacterial Growth:

  • Lag Phase:

  • Cells adjust to a new environment.

  • Cells enlarge, but the number of cells does not increase significantly.

  • Exponential (Log) Phase:

  • The number of cells increases exponentially as cells reach the maximum rate of cell division.

  • This phase lasts as long as conditions remain optimal.

  • Stationary Phase:

  • The number of cells remains stable as the rate of death balances the rate of multiplication.

  • This stability may occur due to depleted nutrients and the presence of toxic metabolic products in the environment.

  • Death (Decline) Phase:

  • The number of viable cells decreases as the cells can no longer multiply in a toxic environment devoid of nutrients.

  • Cells die at an exponential rate, but some viable cells may survive despite the conditions.