Serial dilutions in microbiology
Here you can find questions and answers about serial dilutions and how the Dilucup® System can benefit your laboratory.
Serial dilutions in microbiology
Serial dilution in microbiology is the stepwise dilution of a microbial sample (usually by a constant factor like 1:10) to progressively lower the concentration of microorganisms so that they can be plated and counted (CFU) within a measurable and reliable range.
How the Dilucup® System can benefit you:
The Dilucup® System replaces traditional dilution tubes and vortex mixers with ready-to-use sterile dilution cups and a dedicated shaker. This streamlines the workflow, reduces handling errors and cross-contamination, and ensures consistent, standardized decimal dilutions across users and laboratories.
Decimal dilutions are 10-fold (1:10) serial dilutions, expressed as 10⁻¹, 10⁻², 10⁻³, etc., commonly used in microbiology to reach manageable microbial concentrations.
How the Dilucup® System can benefit you:
The Dilucup® System is specifically designed for decimal dilution workflows.
Each Dilucup® contains 9 mL of ready-to-use sterile diluent. Each time you add 1 mL to a Dilucup®, you are performing a 1:10 decimal dilution.
To make serial dilutions, simply pipette 1 mL from Dilucup® to Dilucup® until the desired concentration is achieved.
Perfect for streamlining routine microbiological testing.
Serial dilutions are used whenever the original sample is too concentrated to give meaningful, countable results.
In microbiology, serial dilutions are useful when the culture is so dense that plating it directly would give a lawn or too many colonies to count. The counts need to have a readable range, typically 30 to 300 colonies; therefore, the sample must be diluted to achieve this range.
How the Dilucup® System can benefit you:
The Dilucup® System allows you to easily and quickly make decimal dilutions.
You can perform between 1 and 12 dilutions per sample with the Dilugent® Shaker or the Dilushaker III.
- Transfer a known volume from your sample (for example 1 mL) into a known volume of diluent (9 mL).
- Mix thoroughly.
- Then transfer 1 mL from that dilution into a new diluent recipient.
- Repeat the process until you reach the desired dilution level.
How the Dilucup® System can benefit you:
Performing serial dilutions with the Dilucup® System is very easy. Dilucups are ready-to-use cups containing 9 mL of sterile diluent.
Simply place the cups on the shaker, open them, and begin diluting your sample.
The shaking is activated when you pass your hand over the sensor or when you press the foot-pedal.
The Dilucup® System is faster, more reproducible, easier to run, and much more ergonomic than dilution tubes and the vortex mixer.
Typically: pipettes and sterile tips, dilution tubes with sterile diluent, vortex mixer, petri dishes or petrifilms and incubators.
If diluents need to be prepared in the laboratory, this requires many more instruments and materials such as chemicals, scales, an autoclave, bacterial strains for quality control, etc.
Dilucup® replaces separate tubes by a tray of ready-to-use diluent cups, reducing the need for multiple materials, instruments and setup steps, thus simplifying inventory and preparation. With the Dilucup® System, you only need a Dilugent® Shaker or Dilushaker III, Dilucups, pipettes and tips, petri dishes, and a incubator to make serial dilutions and be able to count the microorganisms.
Typical errors include wrong diluent volumes, skipping and mislabelling tubes, poor solution mixing, incorrect volume taken by the pipette, reuse of pipette tips, using contaminated diluents and miscalculating dilution factors.
How the Dilucup® System can benefit you:
Dilucup® offers numerous benefits that contribute to minimize errors in serial dilutions.
The diluent volumes in Dilucup® are highly accurate, typically with a deviation of less than 0.5% from 9 mL.
The cup-by-cup blister design and the LED lights show which cup is being processed, minimizing the possibility of confusion.
Vortex agitation by the Dilugent® Shaker or Dilushaker III ensures thorough mixing of the dilution.
Dilucup® cups are sterile and should only be opened immediately before use, minimizing the risk of cross-contamination.
Finally, but importantly, dilutions made with Dilucup are decimal (1:10), making calculations very simple.
It is very important to have very accurate diluent volumes otherwise, the error in each dilution will accumulate and give a final incorrect result.
Use calibrated pipettes, proper mixing, sterile tips for each transfer, clear labeling and consistent technique.
How the Dilucup® System can benefit you:
Dilucup® features highly accurate diluent volumes, with a standard deviation typically of less than 0.5% from 9 mL, ensuring reliable microbiological counts.
Uniform and consistent agitation and a standardized method enhance repeatability across users and laboratories.
Partially yes.
Serial dilutions with Dilucup® System are semi-automated and standardized: its pre-filled dilution cups and ergonomic workflow bring many advantages of automation (speed, reproducibility and reduced hands-on error).
Calculations
Mix 1 part sample with 9 parts diluent (e.g. 1 mL sample + 9 mL diluent) to obtain a 10⁻¹ (1:10) dilution.
How the Dilucup® System can benefit you:
Dilucup® typically has 9 mL of diluent, so adding 1 mL of the sample to be analysed inside a Dilucup® automatically provides a 1:10 dilution. Easy and without complicated procedures.
Do 4 consecutive 1:10 dilutions using Dilucup® (10⁻¹ → 10⁻² → 10⁻³ → 10⁻⁴).
You only need 4 Dilucups and a shaker. Dilutions are easy to perform with minimal equipment, and the initial microorganism concentration is simple to calculate.
CFU/mL = (Number of colonies on plate) ÷ (plated volume in mL × dilution factor).
Example: 150 colonies from 1 mL of a 10⁻⁴ dilution → 150 ÷ (1 × 10⁻⁴) = 1.5 × 106 CFU/mL.
How to calculate CFU/mL with the Dilucup® System:
Because Dilucup® is filled with 9 mL, it provides 1:10 (decimal) dilutions. To find the number of colonies in the initial sample, simply multiply by 10 for each dilution, which is the same as to divide by 1/10.
For example, if you obtained 70 CFU/mL in the Dilucup® number 5 (10-5 dilution), you have to multiply 70 CFU/mL · 10 · 10 · 10 · 10 · 10 = 7 · 106 CFU/mL of original sample.
Serial dilute your sample and plate 1 mL from the Dilucups that you believe have a suitable range of microorganisms to be read. Incubate the plates for the required time and count the colonies.
Choose plates with 30–300 colonies, then calculate:
Total count (CFU/mL or CFU/g) = colonies ÷ (plated volume × dilution factor)
How the Dilucup® System can benefit you:
Dilutions performed with Dilucup® are always 1:10, so it's very easy to calculate the concentration of microorganisms in the initial sample.
Since the plated volume is 1 mL, you only need to divide the number of CFU on the plate by the dilution factor.
For example, if 1 mL inside the Dilucup® number 5 contained 83 CFU, the concentration of microorganisms in the sample is:
If the original sample was liquid and was diluted directly with Dilucup®:
83 CFU ÷ 10-5 = 83 CFU x 105 = 8,3 x 106 CFU/mL of original sample
If the original sample was solid and a prior dilution (10-1) had to be done in a stomacher bag:
83 CFU ÷ 10-6 = 83 CFU x 106 = 8,3 x 107 CFU/mL of original sample
Dilucup® System: your ally for easy and reliable enumerations
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