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Culture Dilution Preparation SOP for QC Microbiology

Last reviewed 19 Oct 2026 · 6 min read

Contents
  1. 1. Purpose and scope
  2. 2. Responsibilities and related procedures
  3. 3. Preparation and serial dilution
  4. 3.1 Before starting
  5. 3.2 Enrichment step
  6. 3.3 Serial dilution sequence
  7. 4. Enumeration and selection
  8. 5. Use, holding period and disposal
  9. 6. Example record fields
  10. Frequently asked questions
  11. Does USP <61> define the 10–100 CFU/mL target in this SOP?
  12. Why does the dilution table include a 10⁻⁶ step before 10⁻⁷?
  13. Can one prepared culture be used for environmental monitoring, MLT and media-fertility tests?
  14. How long are batch-related culture records retained?
  15. References

In short: This illustrative SOP organizes preparation and recording of microbial culture dilutions for QC laboratory work. Use the approved organism-specific method and reference material instructions; the medium, incubation, target-count and holding-time values below are example in-house values and need local verification before use.

1. Purpose and scope

This example provides a framework for preparing serial dilutions from an approved working culture in a pharmaceutical microbiology laboratory. It is not a universal culture-maintenance or positive-control method. Adapt it to the organism, intended test, approved culture source, media and local quality procedures.

USP-NF General Chapter <61> describes quantitative enumeration of mesophilic bacteria and fungi that grow under aerobic conditions. Its public chapter page does not establish the target concentration, enrichment conditions or storage period for the in-house culture suspension described here.

  • Analyst: follow the approved culture and test method, prepare and identify dilutions, and record the work as it is performed.
  • Microbiology supervisor or designee: confirm the method and acceptance criteria are approved, review results and handle any unexpected growth or method failure under local procedures.
  • Quality assurance: oversee the approved document and training process.

These are example role assignments to adapt under the site’s quality system.

See also:

3. Preparation and serial dilution

3.1 Before starting

Confirm the culture identity, approved working-culture source, intended test and applicable controlled method. Check that the required media, diluent, tubes and pipettes are approved and within their applicable status. Work using the aseptic controls in the laboratory’s microbiology procedures.

Example in-house criterion (not a pharmacopoeial requirement): this example SOP refers to a weekly transferred working-culture slant. The transfer schedule and culture passage limits are local controls; do not infer them from this example.

3.2 Enrichment step

This example SOP says to inoculate a loopful from the working slant into 10 mL of “Nutrient Agar” for bacteria and fungi, then incubate the resulting suspension. Example in-house criterion (not a pharmacopoeial requirement): this medium, volume and enrichment approach require confirmation in the approved method before use.

Example in-house criterion (not a pharmacopoeial requirement): this example SOP gives these incubation conditions:

Culture groupExample temperatureExample duration
Bacteria32.5 ± 2.5°C24–48 hours
Fungi22.5 ± 2.5°C24–72 hours

3.3 Serial dilution sequence

Use the sterile diluent, transfer volume, mixing procedure and dilution depth defined in the approved method. Example in-house criterion (not a pharmacopoeial requirement): this example SOP specifies 0.9% sterile saline and a 1 mL transfer into 9 mL of diluent at each step. That is a ten-fold step by volume. Label each tube with the organism, dilution, date and other identifiers required by the local procedure.

StepTransferRelative dilution
11 mL of starting suspension into 9 mL diluent10⁻¹
21 mL of 10⁻¹ into 9 mL diluent10⁻²
31 mL of 10⁻² into 9 mL diluent10⁻³
41 mL of 10⁻³ into 9 mL diluent10⁻⁴
51 mL of 10⁻⁴ into 9 mL diluent10⁻⁵
61 mL of 10⁻⁵ into 9 mL diluent10⁻⁶
71 mL of 10⁻⁶ into 9 mL diluent10⁻⁷

The final step is labelled 10⁻⁷ because each 1 mL into 9 mL transfer adds one ten-fold step, so 10⁻⁶ comes before it. Confirm the dilution scheme and labels against the approved method before adoption.

4. Enumeration and selection

Plate an aliquot from the selected dilution using the approved medium, plating method, inoculum volume and incubation conditions for the organism and intended application. Record the plate identity, dilution, colony count, calculation basis and result in the controlled form. USP <61> is an enumeration method for aerobic mesophilic bacteria and fungi in its stated scope; it does not by itself validate this example as an environmental-monitoring, microbial-limit-test or media-fertility positive-control preparation.

This example SOP calls for pour plating 1 mL onto Nutrient Agar for both bacteria and fungi, counting plates with visible growth, calculating an average, and selecting a dilution said to provide 10–100 CFU/mL. These are example method details and require approval for the intended application.

Example in-house criterion (not a pharmacopoeial requirement): the target in this example is 10–100 CFU/mL. Define the applicable countable range, plate method and whether the target refers to CFU/mL in the suspension or CFU recovered per plate in the approved method.

5. Use, holding period and disposal

Use the selected dilution only for the approved purpose and according to that test’s inoculum instructions. Example in-house criterion (not a pharmacopoeial requirement): this example SOP says to use 1 mL of the prepared culture for environmental-monitoring positive control, microbial limit testing and media-fertility testing. Each application may have its own approved preparation and inoculum requirements; do not assume one volume is suitable for all three.

Example in-house criterion (not a pharmacopoeial requirement): this example SOP says to retain the selected tube for daily positive-control use for one week from preparation and then discard it under the media-disposal SOP. It does not specify the storage conditions or demonstrate the suspension’s stability. Follow only a validated or otherwise approved holding period and storage condition.

Record culture preparation, dilution, enumeration, use and disposal in the approved forms. Retain GMP records associated with a product batch for at least 1 year after the expiry date of that respective batch; under EU GMP Part I Chapter 4 §4.11, batch documentation is kept 1 year after expiry or 5 years after Qualified Person certification, whichever is longer. Retain longer when local law or the marketing authorisation requires it. 21 CFR 211.180(a) sets a one-year-after-expiry period for covered production, control or distribution records specifically associated with a drug-product batch.

6. Example record fields

The controlled record may include:

  • organism name or code, culture source, working-culture identifier and passage information required by the approved method;
  • medium and diluent identity and lot details;
  • preparation date and time, preparer, dilution sequence and tube identifiers;
  • plating method, medium, volume, incubation conditions, colony count and calculation;
  • selected dilution, intended use, storage condition, expiry or use-by time, and disposal record as defined by the approved method; and
  • reviewer and any deviation or follow-up reference.

Frequently asked questions

Does USP <61> define the 10–100 CFU/mL target in this SOP?

No. The public USP <61> page describes an enumeration test for aerobic mesophilic bacteria and fungi. The 10–100 CFU/mL figure is an example in-house target and needs confirmation against the approved test method.

Why does the dilution table include a 10⁻⁶ step before 10⁻⁷?

Each 1 mL transfer into 9 mL diluent makes another ten-fold step. Going from 10⁻⁵ to 10⁻⁷ would skip a ten-fold step, so the example includes the intervening 10⁻⁶ tube. Confirm the approved sequence in your own method.

Can one prepared culture be used for environmental monitoring, MLT and media-fertility tests?

Use the organism, preparation, volume and acceptance requirements in each approved method. This example SOP gives 1 mL for all three uses, but that volume has not been confirmed for any specific method.

At least 1 year after the respective product batch expires. If EU GMP applies, keep batch documentation for 1 year after expiry or 5 years after Qualified Person certification, whichever is longer; retain longer if local law or the marketing authorisation requires it.

References

  • United States Pharmacopeia. USP-NF General Chapter <61>, “Microbiological Examination of Nonsterile Products: Microbial Enumeration Tests,” 2024 public chapter page. Official USP page. The public page describes quantitative enumeration of aerobic mesophilic bacteria and fungi; it does not confirm the site-specific values above.
  • European Commission. EudraLex Volume 4, Part I, Chapter 4, “Documentation,” §§4.11–4.12, revision 1, effective 30 June 2011. Official Chapter 4 PDF.
  • United States Government. 21 CFR §211.180(a), “General requirements,” current eCFR text accessed 19 October 2026. eCFR.

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