From CFU to Genome Copies: Reference Materials for PCR-Based Mycoplasma Validation

Mycoplasma testing is a critical quality control step for cell-based products, biopharmaceuticals, and Advanced Therapy Medicinal Products (ATMPs). Since classical mycoplasma culture methods are labor-intensive and associated with long time-to-result, many laboratories have adopted PCR-based methods to enable faster and highly sensitive mycoplasma detection.

With recent pharmacopoeial updates, PCR-based mycoplasma testing is now supported by a unit that is better aligned with molecular detection: genome copies (GC). This shift raises an important question for many laboratories: Which mycoplasma standard should be used for method validation: CFU-based or GC-based reference material?

New era for PCR-based mycoplasma testing

With the introduction of the United States Pharmacopeia (USP) chapter <77> and the revision of the European Pharmacopeia (EP) chapter 2.6.7 (12.2), PCR-based mycoplasma testing is now supported by a more appropriate unit: genome copies (GC). This is an important development for molecular laboratories, as genome copies directly reflect the target detected by PCR methods. While the established ≤10CFU/ml remains relevant, a sensitivity limit of <100GC/ml has been introduced for NAT-based methods, creating new opportunities for product-specific validation using reference materials that are aligned with the analytical principle of molecular testing.

Why reference materials matter in mycoplasma method validation

Reliable and well-characterized reference materials are essential for demonstrating that a mycoplasma detection method performs as intended. In regulatory contexts, this is a central requirement for method validation.

EP chapter 2.6.7 refers to this as the “test for inhibitory substances”, while USP <77> describes it as the “method suitability test”. In both cases, the aim is to confirm that the sample matrix does not interfere with mycoplasma detection and that the assay can reliably detect low contamination levels.

For this purpose, relevant sample matrices are spiked with defined quantities of well-characterized mycoplasma reference materials. The assay must then show that it can detect these organisms despite potential matrix-related effects.

The validity of the entire method validation therefore depends on the quality and accurate characterization of the reference material. Inaccurate preparation or quantification can compromise the assessment of assay performance, making compliant mycoplasma material one of the most critical and technically demanding parts of the validation process.

Moving from CFU to GC

Historically, mycoplasma method validation has been based on colony forming units (CFU). CFU is derived from culture- based methods and reflects the ability of viable organisms to form colonies under defined culture conditions.

However, for PCR-based methods, genome copies (GC), is the more suitable unit. PCR, qPCR, and dPCR detect nucleic acids rather than colony formation. Therefore, genome copy-based reference materials are more directly aligned with the analytical principle of NAT-based mycoplasma detection.

This distinction is particularly important when working with inactivated reference materials. In such materials, CFU values can only be assigned indirectly, while genome copy values remain analytically meaningful and independent of culturability.

Choosing the right mycoplasma standard for method validation

For EP 2.6.7 and USP <77> compliant method validation, both 10CFU® Mycoplasma Standards and 100GC® Mycoplasma Standards can be used, as both pharmacopoeias allow validation based on either 10 CFU/ml or 100 GC/ml.

For NAT-based methods such as PCR, qPCR, or dPCR, 100GC® Mycoplasma Standards are the more suitable choice because these methods detect nucleic acids rather than viable colony-forming organisms.

In addition, 100 GC represents the highest allowed target level based on the maximum GC:CFU ratio of <10 within the regulatory requirement. This provides a favorable setup for demonstrating method sensitivity in PCR-based mycoplasma validation.

	100GC® Mycoplasma Standards	10CFU® Mycoplasma Standards	100CFU® Mycoplasma Standards
Contents	100GC Purified nucleic acid standard	10CFU Irreversibly inactivated mycoplasma	100CFU Irreversibly inactivated mycoplasma
Use for	Spike material for matrix-specific validation acc. to EP 2.6.7 and USP <77> Spike material for EPC Also suitable for direct testing	Spike material for matrix-specific validation acc. to EP 2.6.7 and USP <77> GC:CFU ratio < 10 Spike material for EPC	GC:CFU ratio < 10 Spike material for EPC
Features	✓ Non-infectious ✓ Lyophilized ✓ GC content quantified by dPCR ✓ Derived from native mycoplasma material used during validation ✓ Availabe for 10 relevant mycoplasma species

Use as EPC and EIC in routine testing

Beyond product-specific validation, mycoplasma standards can also be used as controls in routine testing. All Minerva Biolabs standards — 10CFU®, 100CFU®, and 100GC® Mycoplasma Standards — can be used as external positive control (EPC) and extraction inhibition control (EIC).

According to EP 2.6.7 (12.2), the EPC must contain a defined number of target-sequence copies or CFUs and should be set close to the assay cut-off.

CFU or GC: Aligning the standard with the method

The final choice of reference material for product-specific validation and for use as EPC/EIC should always be aligned with the product, assay type, validation strategy and be discussed with the responsible local authority. Whether transitioning to GC-based validation or continuing with CFU-based approaches, 10CFU®, 100CFU®, and 100GC® Mycoplasma Standards provide easy-to-use, non-infectious, and well-characterized tools for reliable mycoplasma method validation and routine assay control.

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