Bacterial Growth and Reproduction

Bacterial Growth and Reproduction

Bacterial Growth

  • Bacterial growth refers to the increase in the number of bacteria, not their size.
  • Bacteria reproduce through binary fission, resulting in two identical daughter cells (clones).

Generation Time

  • Generation time is the time it takes for a bacterial cell to divide.
  • The duration varies significantly among different bacterial species.
    • E. coli has a rapid generation time (e.g., every 10 minutes).
    • Mycobacterium tuberculosis has a much longer generation time (e.g., 12 hours).
  • Example: Starting with a single E. coli cell, after 8 hours, there can be a million bacterial cells.
    • After another 10 minutes, that 1 million can become 2 million.
  • Knowing the generation time helps determine the incubation period required for laboratory cultures.

Factors Affecting Bacterial Growth

  • Temperature:
    • Bacteria have optimal temperature ranges for growth.
    • Psychrophiles: grow best at cold temperatures.
    • Mesophiles: grow best at moderate temperatures (less than 4040 degrees).
    • Thermophiles: grow best at high temperatures (around 6565 degrees).
    • The growth rate is temperature-dependent, illustrated by growth curves for different bacteria types.
    • Bacteria that cause disease usually require a temperature of very close to 3737 degrees (36.8 to 37.2).
  • Water:
    • Most bacteria require a moist environment for reproduction.
    • The surrounding solution must be isotonic.
  • Osmotic Pressure:
    • Isotonic solutions: have the same solute concentration as inside the bacteria.
    • Hypertonic solutions: can cause bacteria to swell and burst due to osmosis.
    • Hypotonic water (pure water with no salts) can cause bacteria to swell and burst.
    • Osmosis: Water moves across a semi-permeable membrane from an area of high water concentration to an area of low water concentration until the concentration of solutes is equal on both sides.
  • Oxygen Requirements:
    • Aerobes: require oxygen for growth.
      • Obligate aerobes: Need oxygen to survive (e.g., Mycobacterium tuberculosis).
    • Anaerobes: do not require oxygen.
      • Obligate anaerobes: are poisoned by oxygen (e.g., Clostridium species like Clostridium perfringens and Clostridium tetani, causing gas gangrene and tetanus).
    • Facultative anaerobes: can grow with or without oxygen; they can switch between anaerobic and aerobic respiration (can make ATP and switch).
    • Microaerophiles: require oxygen at a lower concentration.
  • Chemical Factors (Nutrition):
    • Water, carbon, nitrogen, phosphorus, sulfur, trace elements, and minerals.

Bacterial Lifestyles

  • Most bacteria are free-living and can exist outside of a host cell.
  • Some bacteria are obligate intracellular parasites and must live inside a host cell.
    • Examples: Rickettsia, Chlamydia, and Mycoplasmas.
    • Chlamydia trachomatis infects human cervical cells.

Collecting Bacteria

  • When collecting samples, it's important to know if the bacteria are aerobic or anaerobic to ensure proper storage conditions (with or without oxygen).
  • Obligate aerobes accumulate at the top of the test tube (e.g., Pseudomonas aeruginosa)
  • Obligate anaerobes accumulate at the bottom of the test tube.
  • Facultative aerobes are kind of more at the top (e.g., Escherichia coli)
  • Microaerophiles accumulate a little bit down from the top of the test tube.

Pili and Genetic Exchange

  • Pili (fimbriae) are hair-like projections on the bacterial surface.
  • Pili can facilitate the exchange of genetic information between bacteria.
  • Some bacteria contain extra DNA called a plasmid (e.g., an F plasmid).
  • An F pilus connects two bacteria, allowing a copy of the plasmid to be transferred from the donor to the recipient.
  • This process can transfer antibiotic resistance genes, converting a non-resistant bacterium into a resistant one.