Pure Culture Techniques, Microscopy, and Staining Lecture Notes

What constitutes a pure culture?

A pure culture contains a single microbial species.

Where are mixed microbial cultures typically found?

Environmental and patient samples almost always contain a mix of microbes.

What is the primary aim of pure culture techniques?

To dilute bacterial samples to separate individual cells that can grow into isolated colonies when plated.

Which type of growth media is often used to isolate microbes from a mixed culture?

General purpose media.

Why are solid media often preferred for obtaining pure cultures?

Solid media allow for the separation of individual cells into distinct colonies.

What is agar?

Agar is a solidifying agent derived from seaweed.

Why is agar used as a solidifying agent in microbiology?

Microbes are unable to metabolize agar.

How is culture handled in the pour plate method?

The culture is diluted in molten agar ($>40^{\circ}C$) and then poured into a sterile petri dish.

At what temperature is the agar in a pour plate allowed to solidify?

$$40^{\circ}C$$

Where can colonies be observed in a pour plate after incubation?

On top of the agar, embedded in the agar, and underneath the agar.

What type of dilution is used in the spread plate isolation method?

A broth culture is diluted in a series of tubes, known as serial dilution.

Why is the specific spread plate procedure described in the transcript considered non-quantitative?

The dilution is performed using an inoculating loop.

Where do colonies form in the spread plate method?

Colonies will only form on the surface of the agar.

What is the Isolation Streak Plate Technique: 3-Zone Method used for?

It is a method to separate a single species from a mixed species population.

What is the primary streak?

It is the initial plating of the sample across the surface of the agar to isolate mixed species.

What characterizes a secondary streak or subculture of white colonies?

It results in a pure culture.

In the 3-Zone Method, how do the sizes of the zones relate to one each other?

Zone I is the smallest, and each zone increases in size up to Zone IIIB.

Which zone in a streak plate is intended for obtaining isolated colonies?

Zone IIIB.

What is the purpose of overlapping zones in the streak plate technique?

It helps distribute bacteria evenly to achieve isolation of all organisms present.

What is the main purpose of staining bacteria?

To provide contrast, making it easier to view bacteria using a bright-field microscope.

What three characteristics of a bacterial sample can be identified using staining?

Morphology, size, and arrangement.

How is a simple stain defined?

One dye is used to stain all cells present.

What is a differential stain?

A stain that detects differences between organisms through the use of many dyes and reagents.

What is a structural stain used for?

To confirm structural characteristics of cells.

What are cationic dyes?

Basic dyes with positively charged chromophores.

Name two examples of cationic dyes.

Methylene Blue and Crystal Violet.

What are anionic dyes?

Acidic dyes with negatively charged chromophores.

Name three examples of anionic dyes.

Acid Fuchsin, Congo Red, and Nigrosin.

What is the result of a negative stain?

Transparent cells appear against a colored background because the stain does not penetrate the cell.

Why is the distortion of cells minimized in a negative stain?

The cells are not heat fixed.

Which specific anionic dye is used for negative staining?

Nigrosin.

Why is Nigrosin repelled by the bacterial cell?

Both the dye and the bacterial cell surface have a net negative charge.

How does a simple stain using Crystal Violet work?

The positive charge of the dye interacts with the negative charge on the bacterial cell wall to color the cell surface.

When preparing a smear from broth, how much culture is transferred?

$1$ loopful.

What must be done differently when preparing a smear from a slant or plate compared to broth?

$1$ loopful of water must be added to the slide first.

How long is a slide heat fixed over a microincinerator?

For approximately $10\,s$.

What tool should always be used when holding the slide over the microincinerator?

A clothespin.

Why is a mixture of colony growth and water spread to maximum thinness on a slide?

To aid in drying and to facilitate viewing a thin layer of cells later.

What are the two requirements for carrying a microscope correctly?

Carry it with two hands: one hand on the handle and the other supporting the base.

What materials are used to clean oil off the $100\times$ objective?

Lens cleaner and a kimtech wipe.

What is the first step in microscope storage regarding the stage?

Lower the stage as low as possible.

Which objective lens should be in position when storing the microscope?

The $4\times$ objective.

What are the three core principles of microscopy?

$1.$ Illumination, $2.$ Magnification, and $3.$ Resolution.

What is the role of the condenser in bright-field microscopy?

It directs light towards the objective lens.

What is the function of the iris diaphragm?

It adjusts the diameter of the cone of light so that it just fills the objective lens.

What is the magnification of the ocular lens?

$$10\times$$

List the four objective lenses available on the microscope.

$4\times$, $10\times$, $40\times$, and $100\times$.

How do you calculate total magnification?

Ocular magnification $\times$ objective magnification.

What is the definition of resolution (resolving power)?

The smallest distance between two objects which can be seen as separate.

What is the resolving power of the light microscopes used in the lab?

Approximately $0.2\,\mu m$.

What magnification objective is the oil immersion lens?

$$100\times$$

Why is immersion oil necessary for the $100\times$ objective?

Oil has the same refractive index as glass and stops the refraction of light between the specimen and the lens.

What is the formula for calculating resolution ($d$)?

$$d(\text{Resolution}) = \frac{\lambda}{2NA}$$

In the resolution formula, what does $\lambda$ represent?

The wavelength of light used for illumination in nanometers ($nm$).

What does "NA" represent in microscopy?

Numerical Aperture, which refers to the light gathering capacity of the lens.

What is "empty magnification"?

Magnification without resolution, which provides little to no information.

What is the morphological term for spherical bacteria?

Cocci.

What is the morphological term for rod-shaped bacteria?

Bacilli.

What are comma-shaped bacteria called?

Vibrios.

What characterizes the morphology of Spirochetes?

Spiral shapes.

What is the morphology of Vibrio cholerae?

Comma shaped.

What is the morphology of Spirillum volutans?

Rigid and spiral.

What does the term "pleomorphic" mean in bacterial morphology?

Organisms that can occur in different shapes, such as club-shaped.

What is the morphology of Corynebacterium diphtheriae?

Club-shaped.

Who developed the Gram stain and when?

Hans Christian Gram in the mid 1880s.

What was Hans Christian Gram's original goal?

He was trying to differentiate bacteria causing pneumonia from lung tissue.

What did Hans Christian Gram find when cleaning samples with ethanol?

Some bacterial cells would not retain the purple crystal violet stain.

What is the most widely used and important stain in bacteriology?

The Gram stain.

Based on what cell wall component