MSL Solution Providers has called on the cosmetics industry to rework preservative challenge testing, arguing that protocols designed for legacy formulations no longer fit the ingredient systems now in development pipelines.

The appeal, made through trade outlet Cosmetics Business, points at a gap between the microbiological reference methods brand safety dossiers still rely on and the formulation categories R&D is shipping: waterless formats, ferment and postbiotic actives, and "free-from" preservative systems built on multi-hurdle chemistry rather than traditional single-molecule biocides.

What "challenge testing" actually covers

Cosmetic preservative efficacy testing (PET) follows two reference frameworks: ISO 11930 ("Evaluation of the antimicrobial protection of a cosmetic product") and the United States Pharmacopeia chapter <51> ("Antimicrobial Effectiveness Testing"). Both inoculate the finished product with a defined panel of reference organisms — typically ATCC strains of Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, Candida albicans and Aspergillus brasiliensis — and log the log-reduction in viable counts against the inoculum at set intervals (commonly Day 7, 14 and 28).

Pass/fail is anchored to log reduction values: ISO 11930 sets a pass at ≥3 log against bacteria and ≥1 log against yeast/mold at the end of the test, with the 7-day result used to predict the end-of-test outcome. USP <51> applies differentiated criteria depending on product category, with topicals generally under category B (lower thresholds than injectables).

Where the standard tests strain

Three formulation shifts in particular stress the original protocol design:

• Anhydrous and low-a formats. Solid cleansers, shampoo bars, oil sticks and waterless deodorants reduce free water and lower water activity below the ~0.7 threshold most challenge organisms need to proliferate. A standardized water-rich inoculation can overstate the real-world risk profile.
• Ferment, postbiotic and probiotic skincare. Live or inactivated microbial preparations complicate what "preservative efficacy" even measures — viability at end of shelf, total plate count, and consumer-facing microbiome claims all sit outside the ISO/USP frame.
• Free-from preservative stacks. Brands moving off parabens, formaldehyde-releasers and isothiazolinones rely on hurdle systems — chelators like EDTA and sodium phytate, glycols, organic acids and essential-oil phenolics — at lower individual concentrations. These stacks pass PET inconsistently, with formulation-to-formulation variability that brand and lab teams struggle to interpret in reformulation cycles.

What this means for formulators, brands and labs

MSL's positioning as a microbiology and contract-testing specialist places it in the supplier base that brand safety and regulatory teams already hire. A "smarter" testing regime, in practical trade terms, would mean tiered challenge panels matched to formulation archetypes — anhydrous, low-pH, surface-active or fermentation-loaded — and a risk-based read-out rather than a binary pass.

For development chemists, the immediate consequence is a widening accountability gap: a formula can pass PET at Day 28 yet show failure patterns under accelerated stability, or vice versa, depending on the hurdle system in use. For procurement and QA leads, the conversation is shifting from "what lab ran our PET" to "which lab designed a PET scheme appropriate to the format."

What to watch

Whether MSL — or a peer contract lab — publishes a draft protocol or white paper that maps PET categories to formulation archetypes. Until then, dossiers continue to cite ISO 11930 and USP <51> while brief sheets move further outside their assumptions.