Unit 1: Cell Counting - Subjective Questions

Definition: A Neubauer chamber, also called a hemocytometer, is a specialized glass slide containing a precisely ruled counting grid of known dimensions and depth.

Primary use: It is used to determine the concentration of cells in a suspension by counting cells present in a known volume.

• The chamber has a calibrated depth, usually .
• The grid consists of squares with known areas.
• The number of cells counted is converted into cells per milliliter using the chamber volume and dilution factor.
• It can be used for relatively large yeast cells and, with suitable precautions, bacterial cells.

A Neubauer chamber contains a central ruled platform that is lower than the surrounding platforms. When a special coverslip is placed correctly, a chamber of known depth is formed.

• The ruled area is divided into a large grid measuring .
• The central large square is divided into smaller squares.
• The depth between the coverslip and the ruled surface is generally .
• The volume above any square is calculated as:
  
• The grid allows cells to be counted systematically and prevents random or overlapping counts.

The principle is based on counting cells in a known volume of a well-mixed suspension.

• A small volume of cell suspension is introduced into the chamber by capillary action.
• Cells settle within the chamber depth and are observed under a microscope.
• Cells in selected squares are counted according to a predetermined boundary rule.
• The average number of cells per square is calculated.
• This value is converted into cells per milliliter using the chamber volume and dilution factor.

The general formula is:

The method assumes that the cells are evenly distributed and that the chamber is filled uniformly.

The following steps should be followed:

1. Mix the yeast culture thoroughly to obtain a uniform suspension.
2. If the culture is too concentrated, prepare a known dilution using sterile diluent.
3. Clean the Neubauer chamber and special coverslip with appropriate laboratory tissue.
4. Place the coverslip over the ruled area so that Newton's rings appear, indicating proper contact.
5. Mix the diluted yeast suspension again immediately before loading.
6. Touch the pipette tip to the edge of the coverslip and allow the chamber to fill by capillary action.
7. Avoid overfilling, underfilling, and formation of air bubbles.
8. Allow the cells to settle briefly before counting under the microscope.

Bacterial cells may be counted by loading a diluted bacterial suspension into a Neubauer chamber and observing the selected grid squares under a high-power microscope.

Procedure:

• Mix the bacterial suspension thoroughly.
• Prepare an appropriate dilution.
• Load the chamber without bubbles.
• Count cells in selected squares using a consistent boundary rule.
• Calculate the concentration using the chamber volume and dilution factor.

Difficulties:

• Bacteria are very small and may require oil-immersion or phase-contrast microscopy.
• Individual cells may be difficult to distinguish from debris.
• Bacterial clumps can produce an underestimation of the actual cell number.
• Motility may make counting difficult.
• Uneven distribution and low contrast can reduce accuracy.

Therefore, bacterial counting with a Neubauer chamber is generally a direct total-cell count rather than a viable-cell count.

Let:

• be the average number of cells counted in one selected square.
• be the dilution factor.
• be the area of the selected square in square millimeters.
• be the chamber depth in millimeters.

The volume counted is:

Since:

The volume in milliliters is:

The concentration in the original sample is therefore:

Thus, the general formula is:

First calculate the average count:

The concentration is:

Therefore:

Answer: The original yeast suspension contains approximately cells per milliliter.

Mixing is essential because cells tend to settle at the bottom of the container or form aggregates during handling.

• A well-mixed suspension gives a more representative sample.
• It distributes cells uniformly throughout the liquid.
• It reduces variation between different counting areas.
• It prevents the first or last portion of the sample from containing an unusually high or low cell concentration.
• Gentle mixing is preferred because vigorous shaking may damage cells or create bubbles.

The suspension should be mixed immediately before loading and, when necessary, between replicate chamber fillings.

Boundary rules are used to avoid counting the same cell twice when it lies on the edge of a square.

A common rule is:

• Count cells touching the top and left boundaries.
• Do not count cells touching the bottom and right boundaries.
• Apply the same rule to every square counted.
• Count a cell as one unit unless the protocol specifically requires counting budding cells separately.
• Exclude cells outside the selected square.

Using a consistent boundary rule improves accuracy and allows results from different squares or replicate counts to be compared.

Dilution reduces the concentration of cells to a level suitable for accurate microscopic counting.

Importance of dilution:

• Highly concentrated samples contain too many cells, causing overlap and inaccurate counting.
• A suitable dilution separates cells and makes individual cells easier to identify.
• It allows several grid squares to contain a countable number of cells.
• The dilution factor must be recorded and included in the final calculation.

If part of culture is mixed with parts of diluent, the dilution is , and the dilution factor is .

The original concentration is calculated as:

A Neubauer chamber provides a fast estimate of total cell concentration, whereas the plate-count method estimates viable cells. The two values may differ because some cells can be alive but unable to form colonies under the selected culture conditions.

Yeast cells are generally identified using their characteristic size, shape, refractive appearance, and cellular structures.

• Yeast cells are usually larger and more regularly shaped than small particles of debris.
• Budding cells may show a smaller daughter cell attached to a parent cell.
• Focusing through different microscope planes helps confirm that an object is a cell.
• Phase-contrast or suitable staining can improve visibility.
• Debris is often irregular, variable in shape, and lacks a consistent cellular outline.
• A viability stain may help distinguish intact living cells from damaged or dead cells, depending on the stain used.

Consistent identification criteria should be established before counting all squares.

Replicate counts involve counting multiple squares or preparing multiple chamber loadings from the same sample.

Significance:

• They reveal whether cells are evenly distributed.
• They reduce the effect of random counting variation.
• They allow calculation of an average value.
• They help identify errors caused by bubbles, incomplete filling, or clumping.
• They provide a measure of precision, such as the range or standard deviation.

If replicate counts differ greatly, the sample should be remixed and the chamber loading procedure checked before reporting the result.

Common errors include:

• Poor mixing: Causes uneven cell distribution and unreliable counts.
• Incorrect dilution: Produces a proportional error in the final concentration.
• Improper coverslip placement: Changes the chamber depth and counted volume.
• Air bubbles: Displace the suspension and reduce the usable counting area.
• Overfilling or underfilling: Causes an unknown chamber volume.
• Counting boundary cells inconsistently: May cause cells to be counted twice or omitted.
• Cell clumping: Leads to underestimation because several cells may be recorded as one particle.
• Counting too few squares: Increases random error.
• Dirty chamber or coverslip: Obscures cells and introduces background particles.
• Failure to include the dilution factor: Produces an incorrect original concentration.

Cleaning:

• Rinse the chamber and coverslip with distilled water or the recommended cleaning solution.
• Dry them with lint-free tissue.
• Avoid scratching the ruled surface.

Loading:

• Place the special coverslip correctly over the counting grid.
• Mix the sample gently.
• Introduce a small drop at the coverslip edge by capillary action.
• Check that the chamber fills completely without bubbles or leakage.

Focusing:

• Begin with the low-power objective to locate the grid.
• Move to a higher-power objective for cell identification and counting.
• Adjust the fine focus and illumination for clear cell outlines.
• Count only after cells have settled and the grid boundaries are clearly visible.

Clumping causes multiple cells to appear as one unit, resulting in an underestimate of the actual cell concentration.

Ways to reduce clumping:

• Mix the suspension gently but thoroughly before loading.
• Use an appropriate diluent or buffer.
• Avoid excessive culture age or overgrown cultures when possible.
• Use gentle pipetting to disperse aggregates.
• For yeast, suitable treatment may include mild mechanical dispersion, provided it does not damage the cells.
• For bacteria, appropriate dispersing agents may be used if compatible with the experiment.
• Examine the suspension microscopically before counting.

If clumps remain, the result should be reported with a note about this limitation.

The dilution factor is:

Using the formula:

Substitute the values:

Therefore:

Answer: The original bacterial suspension contains approximately cells per milliliter.

The number of cells counted is meaningful only when the volume from which they were counted is known.

The volume is calculated as:

where is the square area and is the chamber depth.

• If the chamber depth is larger than expected, the actual volume contains more cells than assumed, so the calculated concentration may be overestimated.
• If the depth is smaller than expected, the calculated concentration may be underestimated.
• An incorrect square area produces a similar proportional error.
• Correct coverslip placement is important because it establishes the calibrated depth.

Accurate knowledge of these dimensions is therefore essential for converting a cell count into cells per milliliter.

A viability stain can distinguish living cells from dead or damaged cells while using the Neubauer chamber.

• The stain is mixed with the yeast suspension before loading.
• Living cells may exclude the dye and remain unstained.
• Dead or membrane-damaged cells may take up the dye and become colored.
• Total cells and stained cells can be counted separately.
• Viability percentage is calculated as:
  

The exact interpretation depends on the stain and organism. Staining time, concentration, and exposure conditions must be standardized because they can influence the result.

In both cases, accurate dilution, mixing, chamber filling, boundary rules, and replicate counts are essential.