Microbiology Culture Media Quiz

Culture media provide the nutritional needs of microorganisms, including carbon (C), nitrogen (N), phosphorus, and trace elements.
Defined media contain precise amounts of highly purified chemicals.
Complex media (undefined) contain highly nutritious substances.
Selective media contain compounds that selectively inhibit the growth of certain microbes, encouraging the growth of a specific microbe.
Differential media show changes in colony size or color, or form gas bubbles and precipitates, allowing for discrimination between microbes.
Enrichment media encourage the growth of a particular microbe in a mixed culture.

Media can be classified by physical state, chemical composition, and functional type.
Physical states include liquid, semisolid, and solid.
- Liquid media do not solidify above freezing and tend to be free-flowing.
- Semisolid media exhibit a clot-like consistency.
- Solid media are firm, allowing for discrete colony growth, and are used for isolating and culturing microbes.
  - Two types exist: liquefiable (reversible) and non-liquefiable.

Synthetic media are chemically defined and contain pure organic and inorganic compounds.
Complex media contain at least one ingredient that is not chemically definable (e.g., extracts from plants or animals). They lack an exact formula.

Plate count method: measures colony-forming units (CFUs) per unit volume after incubating a sample on a solid medium.
Turbidity measurement (spectrophotometry): measures the cloudiness of a liquid culture, proportional to cell density using optical density (OD) or absorbance values.
Impedance microbial growth analyzer: measures changes in electrical impedance due to microbial growth and metabolism.
Flow cytometry: measures individual cells in a liquid sample using lasers and detectors, tagging cells with fluorescent dyes, and determining size, complexity, and viability.

Biomass determination involves direct measurement of microbial biomass, using dry weight, protein content, or other cellular components. Harvesting and drying cells are required prior to measurement.

Methods for viable cell counting include plate counting, most probable number (MPN) method, and fluorescent dyes. Note that MPN usually involves statistical estimation.

Quantitative PCR (qPCR) measures DNA amount, providing an indirect measure of microbial growth.

Monitoring metabolic processes, such as carbon dioxide production, provides an indirect measure of microbial growth.

Lag phase: recovering from no-growth period, actively creating macromolecules.
Log phase (exponential): maximum growth rate.
Stationary phase: nutrients depleted; wastes accumulate, growth rate equals death rate.
Death phase: death rate (decline rate) exceeds growth rate.

Microbes increase via binary fission resulting in exponential increases in numbers.
After n generations, the number of cells from one parent cell is 2ⁿ.
Generation time is the time for a single cell to produce two daughter cells.

Placing a sample on a sterile medium providing appropriate nutrients for growth.
Selection of proper medium & sterility critical.
Some microbes require a live organism or tissue as inoculum medium.

Setting proper growth conditions via an incubator (e.g., adjusting temperature and gas content).
Incubation produces a culture, visible microbial growth, in or on the medium.

The consequence of inoculation & incubation is isolation.
On solid media, observation of separate colonies and on broth media observation of turbidity helps.
Sub-culturing may be needed for further isolation.

Macroscopically observe culture characteristics (color, texture, size of colonies).
Microscopically observe stained slides of the culture for cell shape, size, and motility.