Preservative Testing – Choice of Challenge Isolates

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Publish Date: September 27, 2018

“What you see is that the most outstanding feature of life’s history is a constant domination by bacteria.”

– Stephen J. Gould.

Preservation capacity is a central element of consumer product quality.  Though necessary to any microbiological risk assessment, adequate preservative capacity is not by itself sufficient to ensure appropriate quality.  Preservatives are relatively weak biocides, typically incapable of achieving in 28 days microbial kill disinfectants achieve in less than 10 minutes.  As microbiological contamination of such disinfectants and even 70% ethanol has been reported (1,2), control of microbiological compromise in manufacturing and in typical use and reasonably-expected misuse are clearly necessary to satisfy a positive risk assessment for establishment and maintenance of microbiological integrity.

Preservative Efficacy Testing

Confirmation of preservative efficacy is a routine product development effort.  Methods of preservative efficacy testing vary by product category and by product form within categories.  The USP <51> protocol originally developed in the 1960s (4), has served as the protocol for preservation risk assessment for drugs and the model for other categories.  Although USP <51> is used by some for cosmetic risk assessment, the Personal Care Products Council (PCPC) Guideline methods (5) serve as the nominal industry standard for cosmetics.  There are also many proprietary, in-house methods used for cosmetic risk assessment.  Most unique and diverse are methods for household and industrial products (6).  Each of these methods exposes product to challenge microorganisms based on unique dynamics of their production and use.

Drug Preservative Testing
Microbiological risk in production of these products is largely a function of personnel training, sanitary and hygienic practices, and protocols well documented in regulatory and industrial compendia.  For many drug products, these are advanced to establish sterile manufacturing conditions.  In use, drug products are typically manipulated and applied by clinical professionals whose training and experience also mitigates microbiological risk in use.  Despite these controls, preservatives are typically necessary and are mandated for some multidose liquid and semi solid products (7).

Preservative testing and performance standards are defined in USP <51>.  As with preservative effectiveness testing for other product categories, USP <51> challenges products with high levels of specified microorganisms which are introduced to the product during use.  These isolates (Table 1) are primarily of clinical origin whose original isolations date from 60 to almost 90 years.  In practice, these offer a consistent response to antimicrobial capacity.  With GMPs, manufacturing controls and hygiene as well as appropriate use practice, USP <51> has served the industry well in establishing and maintaining a high standard of microbiological quality (8).

Table 1

Isolate ATCC® Number Approximate Origin: Date and Source
Staphylococcus aureus 6538™ 1930/1940; Clinical/Leison
Pseudomonas aeruginosa  9027™  1943; Clinical/Outer Ear
 Escherichia coli  8739™  1949; Clincal/Feces
 Candida albicans  10231™  1950; Clinical/Bronchomycosis
 Aspergillus brasiliensis  16404™  1965; Botanical/Bluberry

Cosmetic Preservative Testing
Spurred by concerning mid-twentieth century reports of in-market contamination, cosmetic manufacturers developed in-house methods and standards largely focused on contamination risks encountered in manufacturing.  These methodologies brought to the USP <51> battery isolates relevant to cosmetic manufacturing contamination including Burkholderia cepacia, Serratia marcescens,  Enterobacter gergoviae and Enterobacter cloacae.  In-use contamination concerns drove the addition of relevant microorganisms including Staphylococcus epidermidis, Klebsiella spp. and additional fungi such as Penicillium spp.  Some of these in-house methods were validated to effective preservation in consumer use (9).  Challenge concepts embodied in these methods were evolved into a single protocol and industry standard established in PCPC Guidelines (5), validated (10) and tacitly endorsed by the FDA (11).

Whereas PCPC methods largely specify American Type Culture Collection (ATCC) strains more relevant to species reported to contaminate cosmetics, they also observe the usefulness of product-relevant isolates.  To this point, major manufacturers continue to use their own unique isolates recovered from production systems and contaminated materials from manufacturing and consumer use contamination events.  In the hands of major cosmetic companies, cosmetic microbiological quality established a consistent record of quality.  Unfortunately recent regulatory enforcement records indicate substantially increased reports of cosmetic product recalls based on microbiological contamination.  Many appear to be associated with the use of “natural” and alternative preservatives systems as produced by smaller manufacturers possibly less familiar with the stringencies of manufacturing hygiene and without access to product-relevant isolates that would drive greater preservative efficacy beyond that of USP <51>.

Household and Industrial Products
Concern for microbiological quality came late to many household and industrial product manufacturers.  An unpublished study conducted by this author (Dr. Philip Geis) in 1981 found many major household products including fabric softeners and household cleaners contaminated.  Unlike microbial contaminants typical of drugs and cosmetics, many of the contaminants recovered were incapable of growth on conventional media, requiring media modified in pH consistent with the relevant product.

Products in this category were not and still are largely not produced on hygienic manufacturing systems.  Even with such exposure to potential contamination, these products characterized by factors such as extremes of pH, presence or solvent, and relatively high concentrations of surfactant and salt are typically hostile to contamination.  Products so designed will likely satisfy preservative challenge tests such as USP <51> with or without added preservation, a capacity that with typical use practices effectively eliminates concerns for in-use contamination.

When contamination does develop, the microorganism (that in some cases may be considered an extremophile) is often so unique in its growth requirements that it may go undetected when cultured on media appropriate for recovery and growth of mesophilic microorganisms.  Effective preservative challenge will address the contaminants of the relevant product form (12) and may require culture conditions reproducing the unique formation characteristics to which the organism has apparently adapted.  More than other categories, household and industrial microbiologists must rely on testing versus authentic manufacturing isolates.  Relevant microorganisms include species of the genera Bacillus, Gluconoacetobacter, Klebsiella, Nesterenkonia, Dermacoccus, Halomonas and Roseomonas (13,14,15,16,17,18).

Key Takeaways
Risk assessment of any product must consider relevance of manufacturing and in-use contamination risk.  Preservation alone will not be sufficient to overcome major compromise in either realm.  However, control of both, failing sterile manufacturing and complete barrier packaging (e.g., aerosol), will not alleviate the need for preservation as confirmed effective by preservative testing versus the appropriate microbiological culprits. It is necessary that challenge in preservative testing be category relevant and that isolates used are appropriate for the product risk and maintained in a manner that provides consistent results.  For USP <51>, such isolates are obtained as stabilized (e.g., lyophilized) product and their further transfer is controlled by protocol.  For other categories, similar stabilization of manufacturing and consumer isolates is advised to maintain consistency in their response to preservatives.


Dr. Philip Geis earned a PhD in microbiology and mycology from the University of Texas. His career in microbiology began in the clinical laboratory the U.S. Army’s 45th Field Hospital, moving to commercial media production, and in 1981 to The Proctor & Gamble Company (P&G). Through almost three decades with P&G microbiology, Dr. Geis managed preservative and disinfectant development and studies of household and skin microbial ecologies and hygienic manufacturing. He was the first recipient of P&G’s namesake award – Dr. Philip Geis Microbiology Quality Award.

Dr. Geis is now an independent consultant – Geis Microbiological Quality affiliated with the Advanced Testing Laboratory of Cincinnati Ohio. Dr. Geis brings unique global expertise and experience in diverse regulatory, manufacturing, product quality and consumer realities for a broad range of products from OTC drugs to fabric softeners and dog food.

Microbiologics EZ-PEC™ quantitative microorganisms help laboratories conduct Antimicrobial Effectiveness and Preservative Efficacy testing with convenience and confidence. Watch the video below to learn how to use EZ-PEC.


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  4. Sutton S, Porter D 2002. Development of the Antimicrobial Effectiveness Test as USP Chapter <51>. PDA J Pharma Sci Technol. 56:301.
  5. Personal Care product Council Microbiology Committee. 2016.  M-3 Method for Preservation Efficacy Testing of Water‑Miscible Personal Care Products In: Personal Care Products Council Technical Guidelines.
  6. Geis PA, Rook A. 2011. Microbiological quality of consumer and industrial household products. Happi May/June:82.
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  10. Machtiger NA, et al. 2001. Determination of the efficacy of preservation of non-eye area water-miscible cosmetic and toiletry formulations: collaborative study. J AOAC Int 84:101.
  11. Huang J. et al. .2017. Chapter 23. Microbiological Methods for Cosmetics. In: Bacteriological Analytical Manual.
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  13. Témoin‐Fardini S, et al., 2017. New test method for the evaluation of the preservation efficacy of soaps at very alkaline pH made by saponification. Int J Cosm Sci 39:476.
  15. Obidi OF, et al. 2009. Microbial evaluation and deterioration of paints and paint-products. J Environ Biol 30:835.
  16. Tang X, et al. 2017. Halomonas alkalicola sp. nov., isolated from a household product plant. Int J Syst Evol Microbiol 67:1546.
  18. Beadle IR, et al. 1995. Studies on the Growth of Klebsiella sp. in a Cleaning Fluid. Int Biodeter Biodegrad 36:468.

Written by microbiologics

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1 Comment

  1. Danne Labs

    There are plenty of things that needs to be considered while performing a risk assessment

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