Microbiology Lab Techniques and Bacterial Growth

Vocabulary-style flashcards covering microscopy techniques, staining, culture media, colony morphology, growth measurement, and bacterial growth dynamics.

Bright-field microscopy

A light microscopy method where light passes through the specimen to produce an image; best for stained or prepared slides.

Phase-contrast microscopy

A technique that converts phase shifts in light passing through a transparent specimen into brightness differences, allowing visualization of living cells without staining.

Differential-interference-contrast (DIC) microscopy

A contrast-enhancing method using polarized light to produce high-contrast, pseudo-3D images of transparent specimens.

Dark-field microscopy

Illumination from the side so only scattered light enters the objective; improves visualization of unstained or motile organisms.

Simple stain

Staining with a single basic dye to color cells and increase contrast for visualization.

Negative stain

Uses an acidic dye that is repelled by cell walls, leaving cells clear on a dark background; often used for capsules.

Acid-fast stain

Stain (carbol-fuchsin) used to identify acid-fast organisms like Mycobacteria; red cells resist decolorization.

Acid-fast organisms

Bacteria with high lipid content in their walls that retain certain stains after acid-alcohol treatment.

Fluorescence microscopy

A light microscope technique that uses fluorescent chemicals to visualize specimens, with detection based on emitted light.

Excitation wavelength

Wavelength of light absorbed by a fluorophore to reach an excited state.

Emission wavelength

Wavelength of light emitted by a fluorophore after excitation.

Fluorescent antibodies (immunofluorescence)

Antibodies labeled with fluorophores that bind to specific targets, causing them to glow under fluorescence microscopy.

Transmission electron microscopy (TEM)

Electrons pass through ultrathin sections of the specimen to reveal internal cellular structures.

Scanning electron microscopy (SEM)

Electrons scan the surface of a specimen to produce detailed three-dimensional images of external features.

Pure culture

A culture containing only a single microbial species.

Complex (undefined) medium

Nutrient-rich medium with unknown exact chemical composition, often including extracts.

Synthetic (defined) medium

Medium with known chemical composition and defined carbon source.

Selective medium

Medium containing ingredients that inhibit some organisms while allowing others to grow.

Differential medium

Medium that contains indicators to distinguish organisms by biochemical properties.

Mannitol Salt Agar (MSA)

Selective for Staphylococcus due to high salt; differential for S. aureus via mannitol fermentation turning the medium yellow.

MacConkey agar (MAC)

Selective and differential for Enterobacteriaceae; bile salts/crystal violet inhibit Gram-positives; lactose fermentation yields pink/red colonies with neutral red indicator.

Colony

Visible cluster of bacteria on solid media, presumed to arise from a single cell.

Colony morphology

The physical appearance of colonies on solid media, useful for distinguishing species.

Complex medium

Nutrient-rich medium with unknown composition, often containing extracts.

Synthetic (defined) medium

Medium with known chemical composition and exact constituents.

Streak-plate isolation method

Technique to spread cells on an agar surface to obtain discrete colonies for isolation.

Optical density (OD)

Turbidity-based measure of culture density using a spectrophotometer.

OD600

Optical density at 600 nm; conventionally correlates with cell concentration (e.g., ~1x10^9 cells/mL for E. coli, varies by organism).

CFU/mL

Colony-forming units per milliliter; estimate of viable cell concentration from plating.

Inoculation loop

Tool used to transfer small amounts of culture to plates or media for inoculation.

Urine culture cutoff

In UTIs, ~10^5 CFU/mL suggests infection; healthy samples usually <10^4 CFU/mL.

Bacterial growth cycle

The sequence of growth phases: lag, log (exponential), stationary, and decline (death).

Lag phase

Period after inoculation when cells adapt and do not divide yet.

Log phase (exponential growth)

Phase of rapid, constant-rate cell division; cells are most vigorous.

Stationary phase

Growth halts due to nutrient depletion or waste accumulation; metabolism continues at reduced rate.

Decline/Death phase

Population declines as resources are exhausted and waste accumulates.

Generation time (doubling time)

Time required for a bacterial population to double; g = t/n where n is the number of generations in time t.

Growth rate constant (k)

k = n/t = log10(Nt/N0) / 0.301t; used to describe rate of exponential growth.

Nt and N0

Nt is the cell count after n generations; N0 is the initial cell count.

Ribosome

Molecular machine for protein synthesis in cells; visible in electron microscopy.

Nucleoid

Region in bacterial cells where the chromosome is located, not surrounded by a membrane.