Microbial Cultivation Techniques

Cultivation of Microorganisms

• Cultivation is the process of growing organisms by providing suitable environmental conditions.
• Organisms need energy (e.g., metabolic energy) to synthesize molecules and maintain chemical gradients across their membranes.
• They also require nutrients in accessible forms.
• Factors like nutrients, pH, temperature, aeration, salt concentration, and ionic strength of the medium need to be controlled.
• Essential metabolic energy is generated through fermentation, respiration, or photosynthesis.
• Cultivation methods involve choosing a suitable medium and isolating a pure bacterial culture.

Fundamental Goals in Microbial Cultivation

• Raising a culture of a specific species.
• Determining the quantity and type of organisms in a sample.
• Isolating a specific organism from a natural source.

Isolation of Microorganisms in Pure Culture

• A single cell needs to be isolated and cultivated to ensure all progeny are isolated as well.
• Several methods are used, including plating methods and dilution methods to achieve this.

Plating Method

• If a small number of cells are placed on a gelled medium (e.g., agar), each cell will develop into a distinct colony.
• Agar is a suitable gelling agent for most microbe media.
• Agar is an acidic polysaccharide extracted from red algae.

Dilution Method

• The dilution method reduces the density of microorganisms in a sample via serial dilutions.
• Diluted samples can be plated or inoculated.
• The method isolates single cells, ensuring each grows into a pure colony.
• Suspensions are serially diluted, and samples are plated or streaked on solid media.

Survival of Microorganisms in Nature

• Microbial populations in the biosphere tend to remain relatively constant.
• Growth is counterbalanced by death.
• Microbial survival depends on maintaining a living population pool and successful competition for resources.

Meaning of Growth

• Growth involves an increase in an organism's size or mass.
• In single-celled microorganisms, this means an increase in the number of individual organisms in the population.

The Bacterial Growth Curve

• The growth curve displays the population dynamics of bacteria in a closed system.
• Distinct phases exist (Lag Phase, Log/Exponential Phase, Stationary Phase, Death Phase).
• The curve describes changes in the number of living cells over time, given a fixed amount of nutrients and environmental conditions.

The Lag Phase

• Bacteria adapt to the new environment and prepare for division.
• There's little to no cell division happening during this phase, but metabolic activity increases.
• Duration is dependent on factors like nutrient availability and organism type.

The Exponential Phase (Log Phase)

• Cells divide at a constant rate, generating exponential population growth.
• Each cell generation time doubles the population size.
• Bacteria are highly sensitive to environmental changes and antibiotics during this phase.

The Stationary Phase

• Growth rate slows as nutrient depletion and waste accumulation occur.
• The number of new cells equals the number of dying cells, leading to a stable population size.
• Metabolic adjustments occur.

The Death (Decline) Phase

• Nutrients are depleted.
• Toxic waste products accumulate, causing an increase in dead cells.
• Cells die at an exponential rate, and some can form spores or remain dormant.
• Cellular structures break down.