Author: Martha Kimber, M.S.
What is a microbial challenge study?
A microbial challenge study is a laboratory study in which a food product is artificially-inoculated with pertinent pathogenic or spoilage microorganisms in order to determine their behavior in the product; and thus whether those organisms present a risk to the microbial safety or quality of the product. Eurofins Microbiology Laboratories, Inc. (Fresno, CA) provides customized challenge studies based on the guidelines put forth by the National Advisory Committee on Microbiological Criteria for Foods (NACMCF): Parameters for Determining Inoculated Pack/Challenge Study Protocols (2).
Microbial challenge studies are an important tool in the development of a new product formulation or process, or in the validation of a control measure as part of establishing a food safety plan. For example, microbial challenge studies may be performed to determine if a product is capable of supporting the growth of pathogenic organisms, to test the efficacy of a natural preservative, or to provide data supporting a scheduled process for a cold-filled acidified food (1).
Types of microbial challenge studies
There are several types of microbial challenge studies, and their design differs depending on the overall objective. In general, challenge studies are aimed to determine the fate of organisms if present in a food matrix. This could be expressed as growth, survival (no growth), or death (log reduction) (2).
- Growth inhibition challenge studies may be performed to evaluate the ability of a given food product matrix to inhibit the growth of pertinent pathogenic or spoilage organisms under given storage conditions (e.g. time and temperature). For these studies, the performance criterion may be absence of measurable growth of the target organism over the storage period. For example, this type of study may be performed in order to demonstrate microbial stability during an extended product shelf life.
- Inactivation challenge studies may be used to determine if a given food product formulation is capable of providing an appropriate level of reduction of the target organism(s). For example, in the case of ambient-filled acidified foods under 21 CFR 114, challenge studies may be designed to determine if adequate reductions of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica can be achieved in the product during a defined storage period.
- Combination challenge studies may be used to determine if a given food product and process is capable of inactivating certain microorganisms, in addition to inhibiting growth of other microorganisms. For example, an initial die-off of pertinent pathogens may occur in a food immediately or shortly after production, yet spoilage organisms (non-pathogenic) may grow later, on rendering the product unacceptable. A properly designed combination challenge study can evaluate both of these aspects concurrently.
Considerations in microbial challenge study testing
When designing a microbial challenge study, a variety of factors must be considered, such as the intrinsic characteristics of the product formulation, storage conditions, the target organism(s), the inoculum preparation method, inoculation method and level, sampling considerations, and interpretation of test results. As each of these considerations is specific to the product, process, and overall study objective, it is important that the study is designed and evaluated by an expert food microbiologist (2). Eurofins Microbiology Laboratories, Inc. (Fresno, CA) has the breadth of expertise required to design and execute microbial challenge studies, as well as to analyze and interpret the results.
Target organisms
We have an extensive culture collection, but below are the more commonly selected organisms for challenge studies:
- Pathogenic organisms: Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica, Staphylococcus aureus, Bacillus cereus, Clostridium botulinum
- Spoilage organisms: yeast, mold, lactic acid bacteria, acetic acid bacteria, Alicyclobacillus
Storage conditions
We have the ability to set our incubators at a variety of temperatures based on client needs. Below are several common storage conditions:
- Typical refrigerated (4°C/39°F)
- Abusive refrigerated (7°C/45°F or 10°C/50°F)
- Ambient (25°C/77°F)
- Abusive ambient (30°C/86°F or 35°C/95°F)
- Thermophilic (55°C/131°F)
Evaluation options
At each sampling time, testing is performed to assess the behavior of the target microorganism(s). Examples of testing may include:
- Microbiological evaluation of samples (enumeration of target organism via the plating method)
- Analytical evaluation of samples (toxin testing, pH, water activity, etc.)
- Visual description of samples (e.g. gas production, turbidity, change in product color or texture, visible microbial growth)
Questions to consider for challenge studies
- What is the overall objective of the study?
- What is the performance criteria for this study (e.g. 5 log reduction, no more than 2 logs of growth, etc.)?
- Will you be using this data to satisfy specific regulatory requirements?
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