Bacterial Growth Lecture 4

Bacterial Growth (Lecture 4)

• Lecture presented by Dr. Rasha Mokhtar Elnagar, specializing in Medical Microbiology and Immunology.

Chemical Growth Requirements

• Major elements:

  • Carbon: Autotrophs obtain carbon from inorganic substances (like CO2), while heterotrophs get it from organic compounds (sugars, lipids, proteins)
  • Nitrogen: Autotrophs use inorganic forms like nitrogen gas (N2) and ammonia (NH3); heterotrophs obtain nitrogen from organic compounds.

• Minor elements: Essential trace elements include Na, Ca, phosphorus, K, Mg, iron, and sulfur.

Physical Growth Requirements

• pH:
  • Most pathogenic bacteria thrive in neutral pH (6.8-7.4).
  • Acidophilic bacteria survive in acidic pH (3-5) (e.g., Helicobacter pylori, lactobacillus).
  • Alkaliphilic bacteria prefer alkaline conditions (up to 10.5) (e.g., Vibrio cholerae).
• Temperature:
  • Normal flora and many pathogens grow at or near 37°C (human body temperature).
  • Bacteria are classified by their temperature preference:
    • Psychrophilic: 0°C-20°C
    • Mesophilic: 20°C-44°C (most disease-causing bacteria)
    • Thermophilic: 50°C-80°C
• Osmotic pressure:
  • Most bacteria grow in isotonic (0.9% NaCl) and hypotonic solutions.
  • Hypertonic solutions dehydrate most bacteria, but some halophilic bacteria can tolerate high salt concentrations.
• Gaseous requirements:
  • Oxygen:
    • Strict aerobes require oxygen to grow (e.g., Mycobacterium, Pseudomonas)
    • Strict anaerobes cannot tolerate oxygen (e.g. Clostridium)
    • Facultative anaerobes can grow with or without oxygen (e.g., Escherichia coli)
    • Microaerophiles need low levels of oxygen (e.g., Helicobacter, campylobacter)
    • Aerotolerant anaerobes are not harmed by oxygen, but they don't need it to grow (e.g., Enterococcus faecalis)
  • Carbon Dioxide (CO2):
    • Most bacteria require low CO2 levels (0.03%).
    • Capnophilic bacteria need higher levels (5-10%) (e.g., Streptococcus pneumoniae, Neisseria, Brucella abortus).
  • Moisture: Water is essential for bacterial growth and metabolic reactions.

Bacterial Growth Curve

• Bacterial growth is the division of a bacterium into two daughter cells.
• The growth curve shows the relationship between microbial quantity and incubation time.
• Phases of growth:
  • Lag phase: No increase in number of living bacterial cells.
  • Exponential (log) phase: Exponential increase in the number of living bacterial cells.
  • Stationary phase: Plateau in the number of living bacterial cells; rate of cell division and death are roughly equal.
  • Decline phase: Exponential decrease in the number of living bacterial cells.

Bacterial Metabolism

• Aerobic respiration: Biochemical pathway that uses oxygen to produce energy (38 ATP).
  • Glycolysis produces pyruvic acid and a small amount of ATP
• Anaerobic respiration: Biochemical pathway that doesn't use oxygen, with different electron acceptors like nitrate or sulfate to generate energy.
  • Produces less ATP than aerobic respiration
• Fermentation: Biochemical pathway that doesn't use oxygen. Used by some bacteria and muscle cells to produce energy.
  • Produces a small amount of ATP from glucose.

Case Study Question (Page 17)

• A patient with a leg wound, showing thin dark fluid draining and Gram-positive, obligate anaerobic, spore-forming bacilli in the wound swab, most likely has an infection caused by Clostridium tetani.