Bacterial Growth Lecture 4

Bacterial Growth (Lecture 4)

• Lecture on bacterial growth, covering chemical and physical requirements, growth curve, and metabolism

Chemical Growth Requirements

• Major elements: Carbon and nitrogen
  • Autotrophs: Obtain carbon and nitrogen from inorganic substances, like CO2 and nitrogen gas (N2) or ammonia (NH3)
  • Heterotrophs: Obtain carbon and nitrogen from organic compounds like sugars, lipids, and proteins
• Minor elements: Na, Ca, phosphorus, potassium, magnesium, iron, and sulfur

Physical Growth Requirements

• pH:

  • Most pathogenic bacteria grow best at neutral pH (6.8-7.4)
  • Acidophilic bacteria (e.g., Helicobacter pylori, lactobacillus) thrive in acidic pH (3-5)
  • Alkaliphilic bacteria (e.g., Vibrio cholerae) prefer alkaline conditions (up to 10.5)

• Temperature:

  • Normal flora and most pathogens thrive near the human body temperature (37°C)
  • Psychrophiles: Grow 0°C - 20°C
  • Mesophiles: Grow 20°C – 44°C (bacteria that cause disease fall into this category)
  • Thermophiles: Grow 50°C–80°C

• Osmotic Pressure:

  • Most bacteria can grow in isotonic (0.9% NaCl) and hypotonic solutions
  • Hypertonic solutions cause shrinkage and death to bacteria.
  • Halophilic bacteria resist high salt concentrations

• Gaseous Requirements (O2 & CO2):

  • Obligate aerobes: Require oxygen to grow (e.g., Mycobacterium, Pseudomonas)

  • Obligate anaerobes: Die in the presence of oxygen (e.g., Clostridium)

  • Facultative anaerobes: Can grow with or without oxygen (e.g., Escherichia coli)

  • Microaerophiles: Grow best in low oxygen concentrations (e.g., Helicobacter)

  • Aerotolerant anaerobes: Do not use oxygen but can grow in its presence without being harmed (e.g., Enterococcus faecalis)

  • CO2 Requirements:

  • Carbon dioxide (CO2) required for some bacteria, while capnophilic bacteria need higher concentration (5-10%, e.g., Streptococcus pneumonia, Neisseria, Brucella abortus)

• Moisture: Water is essential for bacterial growth and metabolic reactions.

Bacterial Growth Curve

• Bacterial growth: Division of one bacterium into two daughter cells by simple binary fission. The growth curve represents the relationship between microbial cells and time of incubation
• Phases:
• Lag phase: No increase in the number of living bacterial cells (fixed, not increasing)
• Log (exponential) phase: Exponentialincrease in number of living bacterial cells (dividing rapidly)
• Stationary phase: Plateau in the number of living bacterial cells; rate of bacterial division is roughly equal to the rate of death
• Decline (death) phase: Exponential decrease in the number of living bacterial cells (dying rapidly),

Bacterial Metabolism

• Bacteria use different metabolic pathways to generate energy:

  • Aerobic respiration: Requires oxygen (38 ATP) 
  • Anaerobic respiration: Does not require oxygen (38 ATP)
  • Fermentation: Does not require oxygen (2 ATP)

• Specific examples like glycolysis, Krebs cycle, electron transport chain are associated to each of these pathways.

Case Study Question

• A 28-year-old man with a leg injury developed a wound infection. Evidence of anaerobic, spore-forming gram-positive bacilli was found, suggesting Clostridium tetani as the most likely cause of the infection.

This quiz focuses on the factors influencing bacterial growth, including the chemical and physical requirements necessary for different bacteria. It covers topics such as growth curves, metabolism, and specific needs like pH and temperature preferences for various bacterial types.