Whether you are releasing oral solid dosage forms under Ph. Eur. 2.6.12/2.6.13, filing in the US market under USP <61>/<62> or testing personal care products against ISO 17516, RSSL’s microbial limit testing confirms your product meets its microbial specification before it leaves the manufacturing site.
MLT addresses two dimensions of risk before batch release:
Clean Laboratory physically separated from the Culture Laboratory
Inoculation within a biological safety cabinet for high-value or contamination-sensitive media.
Colonies recovered from your MLT plates can be identified using MALDI-ToF mass spectrometry
Permanent visual records of all culture plates, supporting traceability, OOS investigations, and transparent client reporting
Phase II laboratory investigations and Phase III investigation support (client-side root cause analysis, CAPA proposals) are available on request.
Pharmaceutical and biopharmaceutical manufacturers
MLT is mandatory for batch release under GMP. Acceptance criteria are stratified by route of administration (Ph. Eur. 5.1.4, USP <1111>) with specified organism panels determined by the product's intended use. Without compliant MLT data, your Qualified Person cannot certify the batch for release.
Medical device manufacturers
Non-sterile devices contacting mucous membranes, compromised skin or internal tissues require microbiological characterisation as part of the safety assessment under the Medical Devices Regulation (EU) 2017/745. Product bioburden determination per ISO 11737-1 is one method commonly used to fulfil this requirement.
(Ph. Eur. 2.6.12 / USP <61>)
TAMC and TYMC
(Ph. Eur. 2.6.13 / USP <62>)
E. coli, Salmonella, P. aeruginosa, S. aureus, C. albicans, Clostridia, bile-tolerant gram-negative bacteria.
(USP <60>)
Available for US-filing clients or as a risk-based addition
Full suitability demonstration as a standalone project, with expedited option available.
We work with you to determine the appropriate sample preparation method, pour plate, membrane filtration or most probable number based on your product's formulation and antimicrobial properties.
Before routine testing can begin on your product, the laboratory must demonstrate that the test method recovers microorganisms in the presence of your specific product matrix. This is a pharmacopoeial requirement (Ph. Eur. 2.6.12 & 2.6.13, USP <61>/<62>) and protects you from false negatives caused by product interference with microbial recovery.
RSSL offers method verification as a standalone project.
| Service | Standard | Expedited |
| Microbial Limit Testing | 10 working days | 6 working days |
| MLT Method Verification Protocol | 5 working days | 2 working days |
| MLT Method Verification | 6 Weeks* | 3 Weeks* |
| MLT Method Verification Report | 6 Weeks* | 3 Weeks* |
Our microbial limit testing team combines deep pharmacopoeial expertise with practical understanding of product formulation challenges.
Whether you need guidance on method verification strategy, support interpreting enumeration data or a partner to help resolve an out-of-specification result, our specialists are here to help.
Both terms describe the enumeration of viable micro-organisms in or on a product, and are frequently used interchangeably, but they apply to different industries and regulatory frameworks.
Microbial Limit Testing (MLT): The pharmacopoeial term for testing non-sterile pharmaceuticals and raw materials to Ph. Eur. 2.6.12/2.6.13 and USP <61>/<62>. MLT quantifies total aerobic microbial count (TAMC), total yeast and mould count (TYMC), and detects specified micro-organisms.
Bioburden testing: The medical device sector term, governed by ISO 11737-1, quantifying the microbial population on a device or component prior to sterilisation validation. The term also applies more broadly to microbial load assessment on in-process materials and water systems.
MLT quantifies the bioburden in non-sterile products against defined numerical limits and tests for named pathogens. Sterility testing confirms the complete absence of viable organisms in products labelled as sterile.
They apply to different product types: MLT covers oral medicines, topicals, devices, and personal care; sterility testing covers injectables, ophthalmics, implants, and biologics. The incubation periods also differ: 5–7 days for MLT versus a minimum of 14 days for sterility testing.
Method verification demonstrates that the test method can recover microorganisms in the presence of your specific product. It is a pharmacopoeial requirement and we always recommend it. If you choose not to proceed, we will state on the certificate that the method has not been verified for the sample.
RSSL tests across all three major pharmacopoeias European Pharmacopoeia (Ph. Eur.), United States Pharmacopeia (USP), and Japanese Pharmacopoeia (JP) and serves pharmaceutical, biopharmaceutical, medical device, consumer healthcare and personal care manufacturers from a single UK laboratory, GMP compliant and routinely inspected by the MHRA and FDA.
We typically request at least 50 mL or 50 g. For many products, 20 mL/g is sufficient. For expensive or limited-supply materials, we can work with as little as 3 mL/g. The exact requirement depends on which tests your specification requires and whether method verification has been performed. When looking at limited-supply materials such as biologics, early phase development or products with restricted batch sizes please contact us to discuss minimum viable test volumes.
We do not simply report a failure. If results exceed your specified limits, we perform a full Phase I laboratory investigation as standard. This assesses analyst technique, media integrity, negative controls, and equipment function to determine whether the result is attributable to a laboratory error or genuinely reflects your product's microbial quality.
If a laboratory error is confirmed, the test may be invalidated and repeated. If no error is found, we can proceed to a Phase II investigation (hypothesis testing, retesting), and provide support with your Phase III investigation if required.
Yes. BCC testing per USP <60> is available for clients filing in the US market or requiring BCC screening as part of a risk-based objectionable organism assessment. BCC is the leading cause of non-sterile drug product recalls in the US, particularly affecting aqueous products. There is no Ph. Eur. equivalent chapter, but BCC testing can be added to any programme as a risk-based measure where your product characteristics indicate a risk.
Standard turnaround for routine MLT is 10 working days from receipt of compliant samples. Expedited turnaround is 6 working days. For method verification: protocol delivery within 5 working days of sample receipt; full programme and report within 6 weeks from protocol approval (expedited: 3 weeks).
Method verification is a focused demonstration that a compendial method works for your specific product. As compendial methods are already validated by the pharmacopoeia, you do not repeat full validation.
Full method validation (per Ph. Eur. 5.1.6 or USP <1223>) is required only for alternative or non-compendial methods and encompasses accuracy, precision, specificity, detection limits, linearity, range, and robustness. If your method is based on a pharmacopoeial procedure, you need verification, not validation.
Re-verification is required whenever something changes that could affect microbial recovery: product reformulation, changed preservative system, altered pH, test method changes, site transfer to a new laboratory, or new raw material suppliers with potential antimicrobial properties.