Redone Practical 1 Study Guide for Practical

Describing colony morphology

When examining your plate, look closely at single, well-isolated colonies and document the following features:

• Form: The overall shape of the colony when viewed from above. Common descriptions include:

  • Circular: Unbroken, round edges.

  • Irregular: Non-uniform, indented edges.

  • Punctiform: Tiny, pinpoint-sized colonies.

  • Rhizoid: Root-like, spreading growth. [1, 2, 3]

• Elevation: The "profile" of the colony when viewed from the side. Options include:

  • Flat: Barely raised above the surface.

  • Raised: Slightly elevated.

  • Convex: Dome-shaped.

  • Umbonate: Raised with a distinct, higher center (like a nipple).

• Margin: The edge of the colony. Is it smooth or rough?

  • Entire: Smooth, even edge.

  • Lobate: Lobe-like projections.

  • Undulate: Wavy edges.

  • Filamentous: Thread-like, fuzzy edges.

• Color & Optical Properties: Note the distinct color (e.g., white, yellow, red, pink). Additionally, note if the colony is opaque (blocks light) or transparent (clear). [1]

• Texture & Consistency: How the colony appears structurally.

  • Mucoid: Slimy and glistening.

  • Dry: Rough or matte surface.

  • Shiny: Smooth and glossy. [1]

Describing bacteria under the microscope

When describing bacteria under a microscope, microbiologists look for three primary characteristics: cell shape (morphology), cellular arrangement (how they group together), and staining properties (gram-positive or negative). Using the high-power oil immersion (100x) objective, you will describe bacteria by combining these distinct features.

cocci

Spherical or oval.

bacilli

Rod-shaped.

spiral

: Curved shapes, which include spirilla (wavy), spirochetes (corkscrew), or vibrios (comma-shaped). [1]

single

Cells randomly scattered individually

dilpo

Cells attached in pairs (e.g., Diplococci).

strepto

Cells arranged in long, bead-like chains.

staphylo

Cells in irregular, grape-like clusters (e.g., Staphylococci).

tetrad

Groups of four cells forming a square. [1, 2, 3, 4, 5]

gram-positive

Appear dark purple/blue due to a thick peptidoglycan cell wall.

gram-negative

Appear pink/red due to a thinner peptidoglycan wall and high lipid content. [1, 2, 3, 4]

common errors in asceptic transfer

• Using a hot loop: Inserting an uncooled, glowing loop into your parent culture causes a hissing sound, kills the bacteria, and creates infectious aerosols. When inserted into agar, it melts and buries the culture. [1, 3, 4, 5]

• Not flaming tube mouths: Forgetting to pass the opening of a broth or slant tube through the Bunsen burner flame before and after transfer allows airborne contaminants (like fungi or dust) to enter the sterile medium. [1, 2]

• Improper cap handling: Placing test tube caps flat on the laboratory bench instead of holding them with your pinky finger exposes the sterile interior of the cap to surface contaminants. [1, 2]

• Leaving plates fully open: Taking the lid entirely off an agar plate during an open-plate transfer maximizes exposure to settling airborne microorganisms. [1, 2]

• Reaching across the bench: Dragging your hand or tools over open containers increases the risk of dropping dust and skin microbiota directly into your sterile samples. [1]

common errors in making a streak plate

• Failure to sterilize loop between quadrants: Carries too many bacteria into subsequent quadrants, resulting in a heavy, dense streak across the whole plate with no isolated colonies. [1, 2, 3, 4, 5]

• Not dragging through the previous quadrant: Results in growth only in the initial quadrant because no bacteria were transferred to the next section. [1, 2, 3]

• Sterilizing the loop before obtaining a culture sample but before streaking: Kills any bacteria picked up, leading to no growth on the plate. [1, 2, 3]

• Gouging the agar: Tearing the agar surface instead of lightly streaking it creates erratic growth and prevents proper colony isolation. [1, 2]

• Improper plate orientation during incubation: Storing plates lid-up allows condensation to drip onto the agar, smearing and contaminating the colonies. [, 2]

• Airborne contamination: Leaving the plate uncovered for too long allows environmental microbes to settle, resulting in colonies growing randomly outside of the streak line

common errors in making a simple stain

• Overheating during heat-fixing: Passing the slide through the Bunsen burner too many times boils the water inside the bacteria and destroys their structural shape or causes them to burst. [1, 2]

• Making a thick, dense smear: Applying too much bacteria creates piled-up cell clusters where light cannot pass. This makes it impossible to distinguish individual bacterial morphology or arrangement. [1, 2, 3]

• Forgetting to heat-fix: Failing to pass the slide through the flame leaves bacterial proteins un-coagulated. When you apply water to rinse the stain, the bacteria will wash away entirely. [1, 2, 3]

• Heating the wet smear: Trying to dry the slide by heating it over the flame instead of letting it air-dry causes the smear to boil, completely destroying cell shape. [1, 2]

• Incorrectly staining with acidic dyes: Basic stains (positively charged chromophores like crystal violet or methylene blue) adhere to negatively charged bacteria. If you accidentally use an acidic/anionic dye (like Congo red), the stain will be repelled by the bacteria, leaving the cells clear and stained only in the background

errors with antibiotics resistance lab/kirby bauer method

1. Inoculum Density Issues

• Too many bacteria: Over-inoculating the plate creates a lawn that is too dense. This can reduce the size of the zones of inhibition, causing a susceptible bacterium to appear resistant.

• Too few bacteria: A sparse lawn causes zones to artificially enlarge, leading to false susceptibility.

• Correction: Always compare your bacterial suspension to a \(0.5\) McFarland Standard using a densitometer to ensure accurate cell density. [1, 2, 3, 4]

2. Media and Agar Problems

• Incorrect agar depth: The Mueller-Hinton agar must be poured to a uniform depth of roughly \(4 \text{ mm}\). If it is too deep, the antibiotic diffuses slower and creates a smaller zone. If too shallow, the drug diffuses further, creating a larger zone. [1]

• Wrong agar type: Using standard nutrient agar instead of Mueller-Hinton can cause multiple errors and high discrepancies in zone measurements. [1]

• 3. Disk Placement & Contact

  • Improper spacing: Placing antibiotic disks closer than \(24 \text{ mm}\) (center-to-center) or too close to the edge of the plate causes overlapping zones and distorted edges, making measurement difficult. [1]

  • Not pressing disks flat: If the disk doesn't make full contact with the agar surface, or if it moves after placement, the antibiotic cannot diffuse evenly, resulting in irregular zone shapes. [1, 2]

  • 4. Incubation Conditions

    • Incorrect time or temperature: Incubating outside the standard \(35^{\circ }\text{C}\) for \(16\text{--}18\) hours alters both bacterial growth rates and antibiotic stability.

    • Wrong atmosphere: Incubating plates in a \(CO_{2}\) atmosphere decreases the pH of the agar. This pH shift invalidates the drug’s effectiveness for certain antibiotics. [1, 2]

• 5. Measurement Errors

  • Reading the wrong zones: Failing to cross-reference the measured zone diameter (in millimeters) with the correct Zone Size Interpretive Chart for that specific antibiotic and bacterial species. [1, 3, 4]

  • Including hazy zones: If there is a hazy zone inside a larger clear zone, only the completely clear inner zone should be measured; growth in the hazy zone indicates resistance. []

purpose of antibiotics resistance lab/kirby bauer method

to determine the susceptibility or resistance of pathogenic bacteria to various antimicrobial drugs. By measuring the zones of inhibition, this standardized test helps clinicians choose the most effective antibiotic to treat an infection. [1, 2]

Core Concepts of the Kirby-Bauer Method

• Disk Diffusion Technique: Small paper disks impregnated with known concentrations of antibiotics are placed on an agar plate (usually Mueller-Hinton agar to allow uniform diffusion). [1, 2]

• Zone of Inhibition: As the antibiotic diffuses into the agar, it creates a concentration gradient. If the bacteria are susceptible, a clear, circular zone of inhibition (an area where bacteria cannot grow) will form around the disk. [1, 2, 3, 4]

• Standardization: The results are only valid if the procedure is highly standardized. This involves controlling the depth of the agar, bacterial density (often using a McFarland Turbidity Standard), and incubation time. [1, 2]

procedure for antibiotic resistance lab/kirby baur method

Prepare the Inoculum

Inoculate the Agar and make a bacterial lawn

Apply Antibiotic Disks

Incubate

Observe and Measure: Inspect the plate for clear zones of inhibition where the bacteria failed to grow. Use a metric ruler or sliding caliper to measure the complete diameter of each zone in millimeters

Interpret Results: Compare your measured diameters to a standard CLSI Zone Size Interpretive Guide. This will classify whether your bacteria are (S), (I), or (R) to each antibiotic tested

expected results for antibiotic resistance lab/kirby baur method

To determine your expected results, measure the full diameter (including the disc) of the clear ring where bacteria did not grow: [1]

• Susceptible (S): Large zone of inhibition indicating the antibiotic successfully halted bacterial growth.

• Intermediate (I): Moderate zone size suggesting the antibiotic may be effective at higher doses or in specific body locations.

• Resistant (R): No zone, or a very small zone indicating the bacterium is immune to the drug's effects. (Any colony growing right up to the edge of the disc means automatic resistance). [1, 2, 3]

ELISA errors

Preparation and Pipetting Errors

Pipetting Inconsistencies: Failing to change tips between samples can result in cross-contamination. Failing to properly mix reagents also results in an uneven distribution. [1, 2]

2. Washing and Incubation Issues

Insufficient Washing: Leaving unbound antibodies or antigens in the wells results in high background noise, false positives, and high variability (CV) between replicates. [1, 2]

cross-contamination between wells

purpose of an ELISA

(Enzyme-Linked Immunosorbent Assay) is to detect and quantify specific antigens (like viral proteins, bacteria, or toxins) or specific antibodies produced in response to an infection. [1, 2, 3, 4]

The assay relies on highly specific antigen-antibody interactions to produce a measurable, often colorimetric, signal. [1, 2]

Common Applications

• Diagnosing Infectious Diseases: Screening patient blood samples for antibodies to pathogens like HIV, Lyme disease, and various viral/bacterial infections.

• Detecting Pathogens and Toxins: Identifying harmful microbes or toxins directly in food, water, or clinical samples.

• Vaccine Efficacy: Checking if a patient has successfully developed antibodies after receiving a vaccine. [1, 2, 3, 4, 5]

ELISA procedure

• The Target is Trapped: The sample (like a drop of blood) is placed into a small well on a plastic plate. The bottom of the well acts like a sticky trap for the specific target molecule you are looking for. [1, 2, 3, 4]

• The Wash: The well is washed out to remove everything that didn't stick, leaving only the target molecule. [1, 2]

• The Detector is Added: A "detector antibody" is added. This antibody acts like a scout; it specifically locks onto your target molecule. This scout antibody is physically attached to a special reporter enzyme. [1, 2, 3, 4, 5]

• The Color Change: A chemical substrate is added to the well. The reporter enzyme interacts with this chemical, causing it to change color (usually from clear to blue or yellow). [1, 2, 3]

ELISA expected results

• Color Change: The scout found the target. The darker or more intense the color, the more of the target substance is present in the sample.

• No Color Change: The target was not in the sample, meaning the detector had nothing to attach to. [1, 2, 3, 4, 5]

Regardless of whether you are looking for an antibody or an antigen, the final step is adding a substrate. If the enzyme-labeled detection antibody is present, it will react with the substrate to produce a color change (or a fluorescent/luminescent signal). The intensity of this signal is measured using a spectrophotometer; darker or brighter wells indicate a higher concentration of the target molecule

antigen detection ELISA

• What it targets: The actual presence of the pathogen (e.g., proteins or surface markers of a virus/bacteria) in the patient's sample.

• Testing Format: Commonly a Sandwich ELISA. Laboratory plates are coated with "capture antibodies" specific to the target. When the patient's sample is added, these antibodies grab onto the antigen. A second, enzyme-linked "detection antibody" is then added, which binds to the antigen to create a "sandwich."

• Clinical Meaning: Indicates an active, current infection.

• Examples: Rapid influenza tests, HIV antigen tests, and detecting specific bacterial toxins. [1, 2, 3, 4]

sandwich ELISA

a highly sensitive and specific immunoassay used to detect and quantify antigens (such as pathogens or proteins) in a sample. The target antigen is "sandwiched" between two primary antibodies—a bound capture antibody and an enzyme-conjugated detection antibody—producing a measurable colorimetric signal. [1, 2, 3]

antibody detection ELISA

• What it targets: The immune system's proteins (e.g., IgG, IgM) produced in response to a past or present infection or a vaccination. [1, 2, 3]

• Testing Format: Commonly an Indirect ELISA. The lab coats the testing plate with a known, purified antigen from the pathogen. The patient's blood serum is added; if the patient has encountered that pathogen, their specific antibodies will stick to the antigen. A secondary enzyme-linked antibody is then added to bind to the patient's antibodies. [1, 2, 3, 4, 5]

• Clinical Meaning: Indicates prior exposure, immunity, or a developing (convalescent) immune response. Because it takes time to mount an antibody response, this is typically not used for detecting very early acute infections. [1, 2, 3, 4, 5]

• Examples: Serology tests for Lyme disease, Rubella immunity tests, and past-infection COVID-19 antibody panels. [1, 2]

errors while performing gram stain

Failure to use fresh culture

Over-decolorizing (Left alcohol on too long): Dissolves the lipid outer membrane of Gram-negative cells and leaches the crystal violet-iodine complex out of Gram-positive cells. Result: Both Gram-positive and Gram-negative bacteria will appear pink (Gram-negative)

Under-decolorizing (Did not leave alcohol on long enough): The decolorizer is not given enough time to wash the crystal violet complex out of the thin peptidoglycan walls of Gram-negative bacteria. Result: Both Gram-positive and Gram-negative bacteria will appear purple. [1, 2]

Over-heating during heat-fixation: Applying too much heat or heating an insufficiently dry slide ruptures cell walls and destroys cell morphology. This prevents cells from retaining any stain accurately

Failure to add Iodine (Mordant): The iodine is responsible for binding to the crystal violet and forming an insoluble complex that is trapped in the thick peptidoglycan of Gram-positive cells. Result: Both Gram-positive and Gram-negative cells will wash clear during decolorization and end up stained pink by the safranin. [1, 3, 4, 5]

Smear is too thick: Applying too much bacterial inoculum causes clumps of cells to pile on top of one another. The thick layers resist penetration and proper washing of the reagents, causing everything to look like a mix of faint red and deep purple

Over-washing between steps: Rinsing the slide too aggressively with a stream of water can wash off the bacterial cells from the slide entirely before you are finished staining

purpose of a gram stain

The primary purpose of the Gram stain in microbiology is to differentiate bacteria into two main groups (Gram-positive and Gram-negative) based on the structural differences in their cell walls, particularly the thickness of the peptidoglycan layer

procedure for gram stain

smear bacteria on slide following proper sterile technqiues and air dry

heat fix smear

Primary Stain (Crystal Violet)

Mordant (Gram’s Iodine)

Decolorizing Agent (Acetone-Alcohol)

Counterstain (Safranin): Flood the smear with safranin for 1 minute and rinse gently with water. Blot dry carefully using bibulous paper. Gram-negative cells will take up the pink/red counterstain, while Gram-positive cells will remain deep purple. [1, 2, 3, 4, 5]

expected results for gram stan

The expected results for the exercise are that Gram-positive bacteria will appear purple, and Gram-negative bacteria will appear pink or red.

capsule stain errors

1. heat-fixing

you have to make sure you heat fix very gently! Unlike the other stains… you have to be careful... heat-fixing is usually skipped and complete air drying is favored. But in our class, we do fix it. So you have to be careful not to put it under the fire too long or The result: You will see a false negative or severely distorted bacterial morphology.

3. Making the Smear Too Thick [1, 2]

• The Error: Applying too much culture to the slide or failing to spread the stain thinly enough.

• Why it’s a problem: Thick, clumped bacteria will block the light and make it nearly impossible to differentiate individual cells and their surrounding halos.

• The result: The background will be uneven or overly dark, causing the stain to be unreadable.

• How to avoid it: Use a small amount of bacteria and a spreader slide to draw the sample into a uniformly thin layer across the slide

4. Misreading Halos (False Positives/Negatives)

• The Error: Misidentifying artifacts as capsules. [1, 2]

• Why it’s a problem: Shrinkage of a cell due to drying can create a clear ring around the cell that looks like a capsule, while background tears can mimic halos. [1, 2]

• The result: False positives or failure to recognize a true capsule. [1, 2]

• How to avoid it: A true capsule is a smooth, uniformly distinct halo around a clearly stained cell. Review Quizlet or check Lab Tests Guide for visual examples and MCQs to hone your diagnostic skills. [1, 2, 3, 4]

purpose of a capsule stain

The primary purpose of a capsule stain is to identify and distinguish the gelatinous outer layer (capsule) surrounding certain bacteria from the bacterial cell itself. It is a critical differential stain in microbiology because it helps confirm a bacterium's potential to cause disease (virulence). [1, 2, 3]

how a capsule stain works

A capsule stain is a specialized differential staining technique used in microbiology to visualize the gelatinous, protective outer layer (capsule) surrounding certain bacterial cells. Because capsules are water-soluble and non-ionic, they cannot be heat-fixed and do not absorb standard dyes

Because the capsule lacks a charge, it does not bind to either the acidic background stain or the basic cell stain. As a result, the capsule remains completely clear. Under a microscope, you will see a stained bacterial cell surrounded by a clear, uncolored zone (the capsule), all resting against a dark background.

capsule stain procedure

Place one drop of Nigrosine on a slide.

2. Add 3-5 loops of broth culture to the nigrosine.

3. Use a clean slide to push the mixture across the first slide, creating a thin film

4. Air-dry the slide.

5. Very gently heat fix.

6. Stain the slide with crystal violet for 1 min.

7. Rinse, carefully blot dry.

8. View with 1000X magnification.

capsule stain expected results

In a capsule stain, you should expect to see the bacterial cells stained dark and the background stained dark or colored. The capsule appears as a clear, unstained "halo" or zone of exclusion surrounding the cell against this dark background

how a simple stain works

Stains are salts where the colored portion (the chromogen) carries a specific electrical charge: [1]

• Basic Stains (Positive Charge): Dyes like crystal violet, methylene blue, and safranin are the standard for simple stains. Because bacterial cells have a slight negative charge on their surface, the positively charged dye is strongly attracted to the cell. [1, 2, 3]

• Acidic Stains (Negative Charge): Dyes like nigrosin or eosin are negatively charged. They are repelled by the cell and color the background instead, leaving the bacterial cells transparent (often called a negative stain)

how the Kirby-Bauer Method Works

• Diffusion and Gradient Creation: Small paper disks impregnated with standard concentrations of specific antibiotics are placed on an agar plate uniformly inoculated with a pure bacterial culture. As the plate incubates, the antibiotic molecules dissolve into the water content of the agar and diffuse radially outward. This sets up a concentration gradient: the concentration of the drug is highest immediately next to the disk and decreases exponentially as the distance from the disk increases. [1, 2]

• Bacterial Growth: The bacteria in the culture begin to multiply and grow uniformly across the entire surface of the agar. [1, 2]

• Zones of Inhibition: Where the concentration of the antibiotic remains high enough to interfere with bacterial life processes (such as cell wall synthesis, DNA replication, or protein synthesis), the bacteria cannot grow. This creates a circular, microbe-free halo around the disk called the Zone of Inhibition. [1, 2, 3, 4, 5]

how an ELISA works

All ELISA methods rely on "immobilization" (binding a target molecule to a solid plastic surface) and "detection" (using an enzyme attached to an antibody to produce a visible signal). If the target molecule is present, a colorimetric substrate is added which the enzyme converts into a colored product. The intensity of the color directly correlates with the amount of target substance in the sample

how a gram stain works

1. Crystal Violet (Primary Stain)

• What it does: The purple dye stains all bacteria present on the slide.

• The mechanism: Both gram-positive and gram-negative cell walls take in the dye. [1, 2]

2. Gram's Iodine (Mordant)

• What it does: Iodine is added to act as a mordant.

• The mechanism: It binds to the crystal violet to form a large, insoluble chemical complex (the crystal violet-iodine complex) inside the cell. [1, 2, 3, 4]

3. Alcohol or Acetone (Decolorizer)

• What it does: A solvent is used to wash the slide. [1, 2]

• The mechanism: This is the critical differentiating step.

  • Gram-Positive: The alcohol dehydrates the thick peptidoglycan layer, causing it to shrink and trap the large dye-iodine complex inside. The cells remain purple.

  • Gram-Negative: The alcohol dissolves the outer lipid-rich membrane and makes the thinner peptidoglycan layer porous. The dye-iodine complex washes out, leaving these cells colorless. [1, 2, 3, 4, 5]

4. Safranin (Counterstain)

• What it does: A contrasting pink or red dye is applied.

• The mechanism: The already purple gram-positive cells are unaffected by this lighter dye. The decolorized gram-negative cells absorb the safranin and appear pink or red. [1, 2, 3]

Summary of Results

• Gram-Positive: Appear dark purple under the microscope due to the thick peptidoglycan layer.

• Gram-Negative: Appear pink to red because the decolorizer washes out the purple stain, allowing the pink counterstain to show. [1, 2]