VMC 211: General Veterinary Microbiology (1+1)

Physical conditions required for bacterial growth

PHYSICAL CONDITIONS REQUIRED

Temperature

• Optimum growth temperature is the temperature that allows for most rapid growth during a short period of time (12-24 hours).
• Psychrophiles: organisms which grow at 0-20°C.
• Mesophiles: they grow best at the temperature range of 25-40°C. All pathogenic bacteria are mesophiles. Most of them grow best at 37° C.
• Thermophiles: they grow best at temperature above 45°C. Growth range of many thermophiles extend in to mesophilic region. They are called facultative thermophiles. Other thermophiles which can not grow in the mesophilic region are called true thermophiles or obligate thermophiles or stenothermophiles.

Gaseous requirements

• Based on oxygen requirement bacteria can be divided in to
  • Aerobic bacteria
  • Anaerobic bacteria
  • Facultative anaerobic bacteria
  • Microaerophilic bacteria
  • Capinophilic bacteria
• Aerobic bacteria require oxygen for growth and can grow when incubated in an air environment (eg: 21 percent oxygen).
• Anaerobic bacteria do not use oxygen to obtain energy. Oxygen is toxic to them and can not grow in an air atmosphere. Some tolerate low levels of oxygen. They are called aerotolerant anaerobes. Others can not tolerate even low levels and may die up on brief exposure to air. They are called strict or obligate anaerobes.
• Facultative anaerobic bacteria do not require oxygen but may use it for energy production if it is available. They are not inhibited by oxygen. They grow well in air atmosphere as they do in the absence of oxygen.
• Microaerophilic bacteria require low levels of oxygen for growth but can tolerate the level of oxygen present in air atmosphere.
• Capinophilic bacteria require low level (5-10%) carbon dioxide for growth.

Oxygen toxicity

• Aerobic and facultative anaerobic bacteria have mechanisms that protect them against oxygen toxicity. Microaerphiles and anaerobes are deficient.
• Toxic derivatives of oxygen

O₂ + e^- → O₂ ^–

2O₂ ^– + 2H ⁺ → O₂ + H₂O₂

O₂ ^– + H₂O₂ → O₂ + OH^- + OH.

chelated iron

pH

• Most pathogenic bacteria grow best at a neutral or slightly alkaline pH(7.2-7.6)
• Based on optimum pH for growth bacteria can be classified as.
• Acidophiles pH 0 – 5.5 eg. Lactobacillus, Helicobacter pylori.
• Neutrophiles pH 55 – 8.0 most pathogenic bacteria.
• Alkalophiles pH > 8.5 –Vibrio cholerae and alcligens faecalis.
• Obligate acidophiles, some Thiobacillus species, actually require a low pH for growth since their membranes dissolve and the cells lyse at neutrality. Several genera of Archaea, including Sulfolobus and Thermoplasma are obligate acidophiles.
• Yeast and molds grow rapidly in acidic pH of 4-6.