Chapter 2 Microbial Cell Observation: Microscopy, Staining, and Cell Morphology

Who first revealed the existence of microorganisms?

Antonie van Leeuwenhoek

What is the resolution of the human retina?

About 150 µm

What is the difference between detection and resolution?

Detection is the ability to determine the presence of an object; resolution is the smallest distance between two objects that allows them to be seen as separate.

What is magnification in microscopy?

An increase in the apparent size of an image.

What is the size range of eukaryotic microbes?

2-2000 µm

What is the size range of prokaryotic microbes?

0.2-10 µm

What is the size range of viruses?

5-1000 nm

What are the common shapes of bacteria?

Bacilli (rods), Cocci (spheres), Spiral

What arrangement do cocci bacteria form when they divide in one plane?

Diplococci and streptococci

What are the arrangements of bacilli bacteria?

Single bacilli, Diplobacilli (pairs), Streptobacilli (chains)

What are the three types of spiral bacteria?

Vibrios (curved rods), Spirilla (helical), Spirochetes (helical and flexible)

What is the difference between monomorphic and pleomorphic bacteria?

Monomorphic bacteria maintain a single shape; pleomorphic bacteria can have many shapes.

What is light microscopy (LM)?

A technique that resolves images of individual bacteria by their absorption of light.

What is scanning electron microscopy (SEM)?

A type of electron microscopy that produces a 3-dimensional image.

What is transmission electron microscopy (TEM)?

A type of electron microscopy that produces a 2-dimensional image.

What is wet mount preparation?

A method where microbes are placed in a drop of water on a slide with a coverslip.

What is fixation in microscopy?

The process of making cells adhere to a slide in a fixed position.

What is the purpose of staining in microscopy?

To give cells a distinct color and enhance detection and resolution.

What is a simple stain?

A stain that adds dark color specifically to cells, like methylene blue.

What is a differential stain?

A stain that colors one kind of cell but not another, such as the Gram stain.

What does the Gram stain differentiate between?

Gram-positive and Gram-negative bacteria.

What is the cell wall structure of Gram-positive bacteria?

Thicker, with 3-20 layers of peptidoglycan.

What is the cell wall structure of Gram-negative bacteria?

Thinner, with 1-2 layers of peptidoglycan, more porous.

What is the application of the Gram stain in medicine?

It provides valuable information for the treatment of diseases.

What is an acid-fast stain used for?

To stain mycolic acids in Mycobacterium species.

What is the purpose of a spore stain?

To detect endospores of Bacillus and Clostridium.

What is a negative stain?

A stain that colors the background to make transparent components like capsules more visible.

What are antibody stains used for?

To identify bacteria or specific molecular components of cells.

The smallest distance between two objects that allows them to be distinguished as separate entities is defined as _____.

Resolution

At what specific measurement is the resolution limit of the human retina established?

Approximately $150 \mu m$.

Which specific portion of the retina contains the highest density of light-absorbing photoreceptor cells?

The fovea.

How does 'detection' differ from 'resolution' in microscopy?

Detection is the ability to determine an object's presence, while resolution is the ability to distinguish its internal detail or separate objects.

What is the typical size range for prokaryotic microbes, such as bacteria and archaea?

0.2 to 10 µm.

What is the typical size range for eukaryotic microbes, including protozoa and algae?

2 to 2000 µm.

Standard metric unit: 1 nanometer (nm) is equal to what fraction of a meter?

10^-9.

Standard metric unit: 1 picometer pm is equal to what fraction of a meter?

10^-12.

Which morphological term describes bacteria that are rod-shaped?

Bacilli.

Which morphological term describes bacteria that are spherical?

Cocci.

A bacterial arrangement consisting of a cubic configuration of eight cells is known as _____.

Sarcinae

What division pattern results in the formation of 'staphylococci' grape-like clusters?

Division in multiple planes.

What is the primary difference between 'spirilla' and 'spirochetes' regarding their physical structure?

Spirilla have rigid bodies, whereas spirochetes have flexible bodies.

Bacteria that maintain a single, consistent shape are categorized as _____.

Monomorphic

Identify two genera of bacteria that are characterized as being genetically pleomorphic.

Rhizobium and Corynebacterium.

What is the main limitation of using a 'wet mount' preparation for microbial observation?

Minimal contrast between the cell and background, as well as rapid sample dehydration.

How does a temperature-controlled flow cell improve upon the standard wet mount technique?

It prevents overheating while continuously supplying fresh medium to enable extended observation of living microbes.

During fixation, what chemical process allows the bacteria to adhere to the glass slide?

Heat or methanol denatures proteins, exposing amino acid side chains that bind to the glass surface.

What structural features of dyes like methylene blue are responsible for the absorption of visible light?

Conjugated double bonds or aromatic rings.

Why do cationic dyes, such as crystal violet, effectively bind to bacterial cell surfaces?

The positive charge of the dye attracts the negatively charged phosphoryl groups in the membrane phospholipids.

What is the primary objective of a 'differential stain' in microbiology?

To distinguish between different kinds of cells based on specific chemical or structural traits.

Identify the four main reagents used in the Gram stain procedure in their correct order.

Crystal violet, Iodine, Ethanol (decolorizer), and Safranin.

In a Gram stain, what is the role of iodine?

It acts as a mordant, binding with crystal violet to form a large complex that increases stain retention.

Why do Gram-negative bacteria lose the crystal violet-iodine complex during the decolorization step?

Their thin peptidoglycan layer (1 to 3 layers) and porous outer membrane fail to trap the complex when exposed to ethanol.

What color do Gram-positive bacteria appear after a successful Gram stain, and why?

Purple, because their thick peptidoglycan layer retains the crystal violet-iodine complex.

What is the specific duration required for the ethanol wash in the Gram stain to ensure accuracy?

10 seconds.

Why are Gram-negative bacteria generally more resistant to antibiotics like penicillin than Gram-positive bacteria?

The antibiotic cannot easily penetrate the lipopolysaccharide outer membrane layer of Gram-negative cells.

The acid-fast stain is primarily used to identify members of which bacterial genus?

Mycobacterium.

What specific biochemical component is targeted by the carbolfuchsin dye in an acid-fast stain?

Mycolic acids.

Which dye is used in a spore stain to bind specifically to the endospore coat?

Malachite green.

What is the primary purpose of a 'negative stain' such as India ink?

To color the background rather than the cells, making transparent components like capsules visible.

The Giemsa stain is a mixture of methylene blue, eosin, and _____.

Azure B

How do leukocyte nuclei appear when treated with a Giemsa stain?

Purple.

In immunofluorescence, what is typically attached to the antibody for visualization?

A fluorophore (fluorescent molecule).

What is the fundamental difference between 'direct labeling' and 'indirect labeling' in antibody staining?

Direct labeling uses a primary antibody with a marker, while indirect labeling uses a secondary antibody to detect the primary antibody.

Which type of microscopy reveals internal structures by passing an electron beam through a thin specimen?

Transmission electron microscopy (TEM).

Which microscopy technique is best suited for generating high-resolution 3D images of a specimen's surface?

Scanning electron microscopy (SEM).

What technique is used to build computational models of individual molecules like proteins by detecting X-ray interference patterns?

X-ray crystallography.

Microscopy that exploits small differences in the refractive indices of cell components is called _____.

Phase-contrast microscopy

Under a bright-field microscope, how is the specimen typically perceived by the observer?

As a dark object against a light-filled field.

What is the typical diameter of a red blood cell as cited in the size comparison diagrams?

$8 \mu m$.

What is the size of Prochlorococcus marinum, one of the smallest known bacteria?

0.4 µm.

What happens to the side chains of bacterial proteins when they are denatured during fixation?

They are exposed and adhere to the glass slide.

In the context of bacterial arrangements, 'streptococci' form _____.

Chains

Which bacterial shape is described as looking like a curved rod?

Vibrio.

According to the lecture, what is the consequence of leaving ethanol on a Gram-positive sample for too long?

The cells will appear pink instead of purple, leading to a false Gram-negative result.

Which differential stain is essential for diagnosing tuberculosis and leprosy?

Acid-fast stain.

In bacterial endospores, what is the biological function of the spore state?

Survival under adverse environmental conditions (it is not a reproductive structure).

Atomic question: Which type of microscopy uses light absorption as its primary principle of resolution?

Bright-field (Light) microscopy.

What size range corresponds to viruses, which typically require an electron microscope for visualization?

5 to 1000 nm.

Which microbial group has cells large enough to see internal organelles under a light microscope?

Eukaryotic microbes.

If a bacterium is described as an 'oval that looks much like a coccus,' it is a _____.

Coccobacillus

What structural component of the bacterial cell wall consists of sugar chains cross-linked by peptides?

Peptidoglycan.

Congo red and India ink are common reagents used in which staining technique?

Negative staining.

Which microscopy technique targets objects smaller than 100 nm but larger than individual atoms, effectively bridging the gap between TEM and X-ray crystallography?

Atomic force microscopy.

What causes the 'unresolved' state in imaging two close points?

The distance between the two points is less than the resolution limit of the observer.

During simple staining, why is the smear air-dried before fixation?

To ensure the bacteria remain attached to the slide during the subsequent liquid staining and washing steps.

How does crystal violet interact with Gram-negative cells initially, before decolorization?

It stains the cells purple reversibly.

Which bacterial genus is specifically mentioned as a common target for the endospore stain?

Bacillus or Clostridium.

What is the primary visual difference between yeasts observed with and without methylene blue?

Stained yeasts show clearer definition of size and shape compared to the low-contrast unstained cells.

In simple staining, methylene blue is applied to the slide for approximately _____.

1 minute

A helical and flexible bacterial shape is called a _____.

Spirochete

Which microscopy technique specifically detects 'unresolved objects' via light scattering?

Dark-field microscopy.

What is the result of division in three perpendicular planes in cocci?

Sarcinae.

Why is the Gram stain considered medically important?

It provides information for antibiotic selection based on cell wall composition.

What metric unit is equal to 1^-3 meters?

Millimeter (mm).

Which type of electron microscope is used to see ribosomes on messenger RNA?

Transmission electron microscopy (TEM).

In direct labeling antibody stains, the enzyme or fluorophore is attached to the _____.

Primary antibody

Define 'monomorphic' as it applies to bacterial morphology.

The genetic trait of maintaining a single, consistent cell shape.

Which specific light-sensitive nerve cells are found in the fovea of the human eye?

Cones.