Microbial Growth, Biofilms, and Culture Methods

Biofilms

Definition and Structure

Biofilms are complex communities of microorganisms encased in a self-produced matrix of extracellular polymeric substances (EPS) and attached to a surface. These structures provide protection and facilitate communication among cells.

• Biofilm: A community of microorganisms protected by a polysaccharide and protein matrix.

• Extracellular Polymeric Substance (EPS): A sticky matrix secreted by cells that supports biofilm structure and protects cells from harmful conditions such as UV light, chemical toxins, and antibiotics.

• Resistance: Genes for resistance can be transferred between organisms through DNA in the EPS.

Example: Dental plaque is a common biofilm found on teeth.

Steps of Biofilm Development

Biofilm development occurs in several stages:

1. Attachment: Cells adhere to a surface (often via fimbriae).

2. Colonization: Cells multiply and EPS is produced.

3. Maturation: Other cells attach and multiply as the EPS grows, forming a mature biofilm.

4. Dispersal: Cells detach to create new biofilms elsewhere.

Quorum Sensing

Quorum sensing is a bacterial process of detecting their own population density or the presence of other cells by sensing extracellular signaling molecules.

• Quorum Sensing: Bacteria detect and respond to the concentration of signaling molecules, which increases with cell density.

• When the concentration of signaling molecules is high, it triggers coordinated gene expression across the population.

Example: Quorum sensing regulates biofilm formation and virulence in Pseudomonas aeruginosa.

Pure Culture and Colony Formation

Pure Culture

In laboratory settings, bacterial and archaeal cells are often isolated, grown, and studied in pure cultures.

• Pure Culture: A population of cells containing only one species, grown from a single colony.

• Colony: A group of ~1 million cells required to start a pure culture, all descended from a single cell.

• Inoculum: A collection of cells added to a culture medium to start a new culture.

Microbial Growth in Closed Systems

Closed System (Batch Culture)

Cells grown in a lab with a fixed volume of liquid in a container or flask are in a closed system, where nutrients are limited and cannot support infinite growth.

Phases of Microbial Growth

Bacterial growth in a closed system occurs in four distinct phases:

1. Lag Phase: Cells from a colony synthesize enzymes required for cell growth.

2. Exponential (Log) Phase: Cells divide at a constant rate during active growth.

3. Stationary Phase: Nutrient levels are low; cell growth and death rates are approximately equal.

4. Death Phase: Number of viable cells declines as nutrients are depleted and waste accumulates.

Growth Rate: Plotted using a logarithmic scale for the number of cells.

Key Equations

• Exponential Growth: $N_t = N_0 \times 2^n$ Where $N_t$ = number of cells at time t, $N_0$ = initial number of cells, $n$ = number of generations.

Primary and Secondary Metabolites

Metabolite Production During Growth

The early and late stages of the log (exponential) growth phase can be monitored by the production of certain metabolites:

• Primary Metabolites: Produced during early log phase and used by the cell during normal growth (e.g., amino acids, nucleotides).

• Secondary Metabolites: Produced during late log and stationary phases; not required for cell survival but may provide advantages (e.g., antibiotics).

Practice Questions and Applications

• Biofilms: Are a community of microorganisms protected by a polysaccharide and protein matrix.

• Quorum Sensing: Allows bacteria to detect their own population density by sensing extracellular signaling molecules.

• Pure Culture: Consists of cells all descended from a single parent cell.

• Closed System: Bacterial cultures in a closed system will stop growing when nutrients are depleted.

• Growth Phases: The log phase is marked by vigorous cell division; the stationary phase by equal rates of cell production and death.

• Cell Number Increase: In a rapidly multiplying bacterial population, cell numbers increase logarithmically.

Summary Table: Phases of Microbial Growth

Phase	Main Characteristics
Lag Phase	Cells adapt to new environment, synthesize enzymes, no cell division
Exponential (Log) Phase	Rapid cell division, exponential increase in cell number, primary metabolite production
Stationary Phase	Nutrient depletion, waste accumulation, cell division rate equals death rate, secondary metabolite production
Death Phase	Cell death exceeds cell division, decline in viable cell number

Summary Table: Primary vs. Secondary Metabolites

Type	Produced During	Function	Examples
Primary Metabolites	Early log phase	Essential for growth	Amino acids, nucleotides
Secondary Metabolites	Late log/stationary phase	Not essential for growth; may provide competitive advantage	Antibiotics, pigments

Additional info: These notes are based on standard microbiology curriculum and expand on the provided study prep materials to ensure completeness and clarity for exam preparation.