Microbial Growth: Physical & Chemical Requirements

• Lecture 6 discusses the physical and chemical requirements for microbial growth.
• The physical and chemical requirements for microorganisms are essential for their growth.

Learning Objectives

• Know the different types of physical and chemical requirements for microorganisms.
• Understand the importance of each requirement.
• Understand the classification of microbes according to each physical requirement.

Microbial Growth

• Requirements for growth of microorganisms can be studied by physical and chemical requirements.

Physical Requirements

• Physical requirements include: pH, temperature, oxygen concentration, and osmotic pressure

pH

• pH is the negative logarithm of the hydrogen ion concentration.
• Most bacteria prefer neutral pH, around 6.5-7.5.
• Molds and yeast can grow in a wider pH range, but prefer pH between 5 and 6.

Classification by pH

• Neutrophiles grow optimally between pH 5.5 and 8.
• Acidophiles grow optimally between pH 0 and 5.5.
• Alkalophiles grow optimally between pH 8 and 10.5.
• Extreme Alkalophiles grow best at pH 11.5 or higher.

Temperature

• Microbes are classified into several groups based on their optimum (preferred) temperature ranges.
• Temperature has a profound effect on microorganism viability because enzyme-catalyzed reactions are sensitive to temperature.
• Microorganisms exhibit distinct cardinal temperatures: minimal, maximal, and optimal growth temperatures.

Temperature Classifications:

• Psychrophiles, like Flavobacterium psychrophilum, can grow well at 0°C, with optimal growth at 15°C or lower, and usually not above 20°C.
• Mesophiles, like Staphylococcus aureus, Streptococcus pyogenes, and E. coli, have minimal growth at 15 to 20°C, optimal growth between 20 to 45°C, and a maximum growth temperature of about 45°C or lower.
• Thermophiles have minimal growth around 45°C, with an optimum between 55-65°C.
• Hyperthermophiles exhibit minimal growth around 55°C, with an optimum between 80-110°C.

Oxygen Concentration

• Microorganisms differ in their requirements for molecular oxygen and other atmospheric gases like carbon dioxide.
• Organisms using molecular oxygen produce more energy from nutrients than anaerobes.
• Microorganisms can be classified based on their oxygen requirements.

Oxygen Classifications:

• Obligate aerobes are completely dependent on atmospheric O2 for growth, examples of these are Pseudomonas, a common nosocomial pathogen.
• Obligate anaerobes grow only in the absence of oxygen, and are inhibited or killed by the presence of O2; an example is Clostridium, bacteria that cause tetanus and botulism.
• Facultative anaerobes grow in the presence of oxygen but can also grow even in the absence of O2; examples include E. coli, Staphylococcus, and many intestinal bacteria.
• Aerotolerant anaerobes do not require O2 to grow but can grow equally well in the presence of it; Lactobacillus carries out fermentation regardless of the oxygen presence.
• Microaerophiles require lower levels of oxygen (2 to 10%) for growth and are damaged by normal atmospheric levels of O2 (20%); an example is Campylobacter.

Osmotic Pressure

• Halophiles require moderate to large salt concentrations such as Halobacterium, most bacteria found in oceans.

Osmotic Classifications:

• Halophiles require moderate to large salt concentrations; ocean water contains 3.5% salt; an example is Halobacterium, a bacteria found in oceans.
• Extreme or Obligate Halophiles require very high salt concentrations of (20 to 30%); e.g. Bacteria in Dead Sea
• Facultative Halophiles do not require high salt concentrations for growth, but tolerate 2% salt or more.
• Most bacteria require an isotonic or hypotonic environment.

Chemical Requirements

• Chemical requirements of microbes include: carbon source in medium, minerals such as nitrogen, sulfur, phosphorous, and trace elements, and growth Factors.

Lecture on physical and chemical factors crucial for microbial growth, including pH, temperature, oxygen concentration, and osmotic pressure. Covers microbial classifications based on pH preferences like neutrophiles, acidophiles, and alkalophiles.