Notes on Dilution Plating, Concentration Calculations, and Plate Handling (D3–D5, N7, Endo)

Key Concepts

• Dilution and plating basics
  • We plate a known volume (e.g., 100 µL) from a diluted sample to estimate concentration of organisms in the original sample.
  • Concentration is typically reported as CFU per mL (CFU/mL) in the diluted sample, and then scaled back to the original sample using the dilution factor.
• Per mil vs per 100 µL reporting
  • The conversation emphasizes reporting concentration as per mL (CFU/mL) rather than per 100 µL. This aligns with standard practices where concentrations are given per mL (or per L).
  • The term “per mil” is used in the dialogue to denote CFU per mL for the diluted sample; the important idea is to convert to a per-mL basis before scaling to the original sample.
• Dilution factors and notation
  • D3, D4, D5 denote successive dilutions by factors of 10: Dk corresponds to a 10^(-k) dilution from the original sample.
  • When a plate is from dilution Dk, the original (undiluted) concentration is scaled by 10^k relative to the concentration measured in the plate’s diluted sample.
• Calculation workflow (summary)
  • Count colonies on a plate from a known plating volume V (mL).
  • Convert plate count N to concentration in the diluted sample: C_diluted = N / V.
  • Convert to per mL: V = 0.1 mL for 100 µL plating, so C_diluted = N / 0.1 = 10 N [CFU/mL].
  • Scale to the original sample: C0 = Cdiluted × 10^k, where k is the dilution exponent for the plate’s dilution (e.g., k = 5 for D5).
  • Example: If N = 13 and the plate came from D5 (k = 5), then C_0 = 13 / 0.1 × 10^5 = 130 × 10^5 = 1.3 × 10^7 CFU/mL.
• Plate counting strategy and data quality
  • Ideal plates show a countable number of colonies (roughly 20–50 is often optimal; too many or too few are less reliable).
  • In practice, some plates will have uneven distribution or clumps; choose the best plate(s) for calculation.
• Reproducibility and sampling bias
  • Differences in extraction (e.g., how well the sample was shaken) can cause large differences in colony numbers between plates.
  • Plan to repeat the experiment with more uniform sampling to reduce variation.

Concentration and Dilution Math

• Plate from a 100 µL aliquot yields a diluted-concentration in CFU/mL given by:
  • C_{ ext{diluted}} = rac{N}{V} = rac{N}{0.1} = 10N ext{ CFU/mL},
  • where N is the number of colonies counted on the plate and V = 0.1 mL is the plated volume.
• If the plate comes from a dilution of factor 10^k (i.e., Dk, where k = 3,4,5,…), the original concentration C0 is:
  • C0 = C{ ext{diluted}} imes 10^{k} = rac{N}{0.1} imes 10^{k} = N imes 10^{k+1} ext{ CFU/mL}.
• Worked example from the transcript
  • Given N = 13 colonies on a plate from 100 µL plating, plate from D5 (k = 5):
  • C_{ ext{diluted}} = rac{13}{0.1} = 130 ext{ CFU/mL},
  • $C_0 = 130 imes 10^{5} = 1.3 imes 10^{7} ext{ CFU/mL}.$
• Relationship among dilutions
  • If D5 has concentration C5, then for D4 (one step more concentrated) the concentration is: $C<em>4 = C</em>5 imes 10,$
  • for D3: $C<em>3 = C</em>4 imes 10 = C_5 imes 10^2,$
  • and so on, increasing by a factor of 10 for each step closer to the original sample.
• Practical reporting note
  • In standard practice, report CFU/mL for the diluted sample and then compute C0 using the dilution factor; notate the dilution used (e.g., D3, D4, D5) to allow back-calculation.

Plate Handling and Counting Strategy

• Plate layout and labeling
  • Count colonies on one plate, labeled by its dilution (e.g., D3, D4, D5) and by which sample (e.g., N7, Endo).
  • Use a dot to mark each colony on the butt side of the plate with a fine-tip marker to avoid smearing or interfering with lid placement.
  • Avoid counting from the lid side because the lid can move and contaminate the plate.
• Counting criteria
  • Look for plates with a countable number of colonies; avoid plates with too many to count (e.g., many colonies per field).
  • Two plates per sample are suggested (one for each dilution under consideration), and you should select the best plate(s) to count.
  • Desired counting target: roughly 20–50 colonies per plate when feasible; if necessary, use a plate with around 15–25 or up to ~50 colonies.
• Red-dot verification step
  • After selecting the best plate, place a red dot to indicate the plate that will be used for final calculation.
• Sample labeling and tracking
  • Use lab tape to label saved plates (two pieces of tape on either side of the plate) to designate the plate to keep for calculation.
  • Create a trash pile for plates that are not going to be counted due to abnormal growth or contamination.
  • Keep notes on which plates are kept for N7 and which are kept for Endo (and which plate is the best for calculation).
• Endo vs N7 planning
  • The session discusses working with two sample sets: N7 and Endo (Endo N7 plates), selecting the best plate from each for calculation.
  • Some Endo plates may be behind in growth and may not be usable today; plan to save representative Endo plates for next week if needed.
• Reproducibility considerations
  • Expect variability between plates, even for plates prepared on the same day; plan to repeat to obtain more uniform results.
  • For uniform sampling, discuss methods to ensure even extraction from the original vessel (e.g., thorough shaking, consistent suction technique).

Experimental Plan for Today

• What to do now
  • Measure and count colonies on multiple plates, then choose the best plate for N7 and the best plate for Endo to perform forward calculations.
  • If a plate has too many colonies to count, or too few, designate it as trash and select another plate.
  • Mark saved plates with two pieces of tape; designate trash plates clearly.
• Data collection goals
  • Aim to have 20–50 colonies on the counted plate whenever possible.
  • Record the counted N for each plate used in calculations and the corresponding dilution factor k.
• Replating and sample preparation
  • Refill or re-plating as needed with fresh plates (at least 12 fresh plates noted in the plan).
  • Ensure consistency in plating volume and dilution steps to minimize variability.
• Contingencies and follow-up
  • If Endo plates are sparse, save at least one Endo plate for comparison next week.
  • If a plate shows contamination, discard and replace with a clean plate.

Uniformity, Pipette Checks, and Quality Control

• Pipette accuracy check with a dye test
  • Prepare blue dye in water to visualize and compare the volume delivered by different pipettors.
  • Use 100 µL pipettors to draw up 100 µL from each device into a common tube and visually inspect if volumes are consistent across pipettors.
  • Use small 0.2 mL tubes (200 µL) for the dye test to easily compare drop volumes.
  • If any pipettor is consistently off (e.g., delivering notably more or less than 100 µL), address calibration or replacement.
• Suspected sources of variability
  • Shaking intensity and uniformity at the start of sample preparation can cause large differences in colony counts between plates.
  • Differences in how much sample is drawn up from the same vessel can lead to clumping vs. liquidy extraction, affecting counts after plating.
• Plate handling to minimize contamination
  • Two key moments that risk contamination: when the bottle is open and when the plate is being poured or handled.
  • Best practices described:
  • Keep the bottle open for as short a time as possible; pour quickly and return the lid promptly.
  • When removing the lid, tilt the bottle and avoid exposing the bottom to the air for long durations.
  • If using a flame, perform in a controlled manner to create an updraft that reduces contamination; verify local lab protocol for flame use.
• Safety and PPE reminders
  • Wear gloves and safety glasses when handling plates and samples.
  • If there is a risk of lid falling off or leaks, avoid direct contact with skin; wash hands after handling.
  • Avoid inhaling odors; work in a well-ventilated area.
• Contamination assessment and disposal
  • Inspect plates for contamination; discard contaminated plates.
  • Keep track of which plates are kept vs discarded using clear labeling with tape.
• Lab workflow and organization
  • Plan for additional supplies (e.g., more tubes, more plates, more tape) and coordinate re-stocking on campus facilities days.
  • The instructor mentions autoclave checks and stockroom replenishment to ensure availability of replating materials.

Practical Notes and Philosophies from the Session

• Emphasis on reproducibility and good lab practices
  • Uniform extraction and consistent plating techniques are critical for reliable results.
  • Small procedural differences (pipetting, shaking, timing) can lead to large differences in colony counts and calculated concentrations.
• Real-world lab pragmatics
  • Plates can smell; handle with care and dispose of waste appropriately.
  • Avoid over-handling plates to reduce disruption to colonies and potential contamination.
• Reflections about next steps
  • Plan to repeat the experiment with improved uniformity and to collect more robust data across multiple plates (at least 12 fresh plates this session).
  • Maintain a plan to compare N7 and Endo results, with clear criteria for selecting the best plate for final calculations.
• Final reminders
  • Don’t rush the process to keep safety and accuracy in focus.
  • If you’re unsure about a step, pause, and confirm the protocol before proceeding.

Summary of Key Formulas in One Place

• Plated-volume concentration (diluted sample):
  • C{ ext{diluted}} = rac{N}{V} ext{ with } V = 0.1 ext{ mL} ightarrow C{ ext{diluted}} = rac{N}{0.1} = 10N ext{ CFU/mL}.
• Original concentration from a Dk plate:
  • C0 = C{ ext{diluted}} imes 10^{k}
    ightarrow C_0 = rac{N}{0.1} imes 10^{k} = N imes 10^{k+1} ext{ CFU/mL}.
• Example (N = 13, k = 5):
  • $C_0 = 13 imes 10^{6} = 1.3 imes 10^{7} ext{ CFU/mL}.$