mic

• Environmental factors affecting growth of bacteria

  • Gas requirement (oxygen)
  • Temperature
  • Moisture
  • Hydrogen ion concentration (pH)
  • Light
  • Osmotic pressure
  • Atmospheric pressure
  • Mechanical and sonic vibration

  
• Gas requirement (oxygen)


  According to oxygen requirement, bacteria are divided into the following groups: Aerobes, Facultative anaerobes, Obligate anaerobes, Micro-aerophilic
• Identifying aerobes, facultative anaerobes, and obligate anaerobes


  Growing them in test tubes of thioglycollate broth
• Aerobes


  • Need oxygen because they cannot ferment or respire anaerobically, gather at the top of the tube where the oxygen concentration is highest
• Facultative anaerobes


  • Can grow with or without oxygen because they can metabolise energy aerobically or anaerobically
• Obligate anaerobes


  • Are poisoned by oxygen, so they gather at the bottom of the tube where the oxygen concentration is lowest
• Micro-aerophilic


  • Need oxygen because they cannot ferment or respire anaerobically, however they are poisoned by high concentrations of oxygen, they gather in the upper part of the test tube but not the very top
• Aerobes


  Aerobic bacteria that grow only in the presence of oxygen, example: pseudomonas, bacillus
• Facultative anaerobes


  Bacteria that can live with or without oxygen, example: vibrio, E. coli, salmonella
• Obligate anaerobes


  Bacteria that can live only in the absence of oxygen, example: clostridium
• Micro-aerophilic bacteria


  Bacteria that grow well with relatively small quantity of oxygen, example: haemophilus
• Optimum temperature


  The temperature range at which bacteria grow best, in humans the pathogenic bacteria optimum temperature range is between 30°C and 37°C

  
• Groups of bacteria as regards temperature of growth


  • Psychrophilic (0°C to 25°C)
  • Mesophilic (25°C to 45°C)
  • Thermophilic (more than 45°C)

  
• Water is quite essential for the growth of bacteria, organism like Neisseria gonorrhea and Treponema palladium die almost at once on drying, however, Mycobacterium tuberculosis and staphylococcus aureus survive for quite a long time even in drying

  
• Hydrogen ion concentration (pH)


  Most pathogenic bacteria grow best at pH between 6.2-7.6, lactobacillus grow at acidic pH, Vibrio cholerae grow at alkaline pH
• Darkness is usually favorable for the growth of most of the bacteria, direct light exposure shortens survival of bacteria
• Bacteria are usually resistant to changes of osmotic pressure
• Atmospheric pressure


  Bacteria growth is affected by atmospheric pressure, most bacteria live at normal atmospheric pressure and are not affected by minor changes in it, some bacteria like very high atmospheric pressure (Barophiles) like in oil wells and deep oceans

  
• Bacteria have tough cell wall, the shaking and the exposure to ultrasonic vibration may cause rupture or disintegration of cell wall