Microbial Nutrition and Growth: Key Concepts and Growth Dynamics - CH 6

Microbial Nutrition and Growth

Microbial Nutritional Types

Microorganisms require various sources of energy and carbon for growth. Their nutritional classification is based on how they obtain these essential resources.

• Photoheterotrophs: Organisms that use light as an energy source but obtain carbon from organic compounds. Example: Rhodobacter species.

• Photoautotrophs: Organisms that use light for energy and carbon dioxide as a carbon source. Example: Cyanobacteria.

• Chemoheterotrophs: Organisms that obtain both energy and carbon from organic molecules. Example: Most bacteria, fungi, and protozoa.

• Chemoautotrophs: Organisms that use inorganic chemicals for energy and carbon dioxide as a carbon source. Example: Nitrifying bacteria.

Oxygen Requirements

Microbes differ in their need for oxygen, which affects their metabolism and habitat preference.

• Aerobic: Require oxygen for growth and survival.

• Anaerobic: Grow in the absence of oxygen.

  • Obligate anaerobes: Must be in an oxygen-free environment; oxygen is toxic to them.

  • Facultative anaerobes: Can grow with or without oxygen; they adjust their metabolism based on the environment.

Factors Affecting Microbial Growth

Temperature

Temperature is a critical factor influencing microbial metabolism and growth rates. Microbes are classified based on their optimal temperature ranges:

• Psychrophiles: Grow best at low temperatures (typically 0–20°C). Found in cold environments like polar regions.

• Mesophiles: Prefer moderate temperatures (20–45°C). Most human pathogens are mesophiles.

• Thermophiles: Thrive at high temperatures (45–80°C). Common in hot springs and compost heaps.

pH

The acidity or alkalinity of the environment affects enzyme activity and microbial survival.

• Acidophiles: Grow optimally at low pH (acidic conditions).

• Neutrophiles: Prefer neutral pH (around 7); most human-associated microbes are neutrophiles.

• Alkaliphiles: Thrive in basic (alkaline) environments (pH > 8).

Microbial Growth Curve

Binary Fission

Most bacteria reproduce by binary fission, a process in which a single cell divides into two identical daughter cells. This leads to exponential population growth under optimal conditions.

Phases of the Growth Curve

When microbes are cultured in a closed system (batch culture), their population growth follows a characteristic curve with distinct phases:

• Lag Phase: Cells adjust to the new environment; little or no cell division occurs.

• Log (Exponential) Phase: Rapid cell division and population growth; cells are most metabolically active.

• Stationary Phase: Growth rate slows as nutrients become limited and waste accumulates; the rate of cell division equals the rate of cell death.

• Death Phase: Cells die at an exponential rate due to depletion of nutrients and accumulation of toxic products.

Example: Bacterial Growth in Broth Culture

When Escherichia coli is inoculated into nutrient broth, it exhibits a lag phase as it adapts, followed by rapid growth (log phase), then a stationary phase as nutrients run low, and finally a death phase as conditions deteriorate.

Additional info: The mathematical expression for exponential growth during the log phase is:

$N_t = N_0 \times 2^n$

where $N_t$ is the population at time t, $N_0$ is the initial population, and $n$ is the number of generations.