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Pesticide Registration

Methods and Guidance for Testing the Efficacy of Antimicrobial Products Against Spores of Clostridium difficile on Hard Non-Porous Surfaces (February 2018)

This webpage provides test methods for evaluating the efficacy of antimicrobial pesticides against spores of Clostridium difficile (C. difficile), as well as regulatory guidance for pesticidal claims for those products. These test methods and guidance provide a framework for registrants who seek to make a claim for antimicrobial pesticide products to control C. difficile spores on hard, non-porous surfaces. 

EPA solicited comments on the clarity of the test methods and the regulatory guidance in December 2016. The Agency’s response to comments and other documents associated with this action are available in docket EPA-HQ-OPP-2016-0753 at

This guidance supersedes the Agency’s previous guidance for the efficacy evaluation of antimicrobial pesticides registered or proposed for use against C. difficile on hard, non-porous surfaces. This guidance is not binding on EPA or any outside parties, and EPA may depart from the guidance where circumstances warrant and without prior notice. Registrants and applicants may propose and submit alternative practices (e.g. modifications to the recommended test methodology) to the Agency for assessment. The Agency will evaluate any proposed method modifications for appropriateness on a case-by-case basis. This guidance may be updated in the future.

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Public Health Significance of C. difficile Spores

C. difficile is an anaerobic spore-forming bacterium and a frequent cause of hospital-acquired infections. The Agency recognizes the potential public health implications of these spores in healthcare settings and the importance of having antimicrobial products with demonstrated efficacy against C. difficile. EPA considers claims to control C. difficile spores to be public health claims for which the Agency must receive, review, and approve appropriate efficacy data in order to evaluate such claims for possible approval.

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Products That May Be Eligible for C. difficile Claims

C. difficile claims may be added to liquid, spray, and towelette formulations of a sterilant, sporicide, or hospital disinfectant product provided that the product meets the performance criteria for testing outlined in the most current version of Antimicrobial Efficacy Test Guidelines – 810.2100: Sterilants – Efficacy Data Recommendations or 810.2200: Disinfectants for Use on Hard Surfaces – Efficacy Data Recommendations.

The test methodologies described below may be used for testing the efficacy of water soluble powders or liquid formulations against C. difficile; spray formulations may be evaluated as a liquid by removing/dispensing an appropriate amount of liquid from the spray device.

Towelettes may be evaluated using liquid directly expressed from the towelette. In addition, data should be generated to assess the amount of liquid remaining on a hard, non-porous surface treated (wiped) with the towelette per the label instructions. The Agency recommends conducting visual and gravimetric wetness tests in an environmental chamber per EPA Microbiology Laboratory Branch (MLB) SOP MB-31. 

Applicants should consult with the Agency for other formulation types (aerosols, foams, etc.) prior to developing data in support of registration. The Agency may require submission of protocols for review and approval prior to commencement of testing. 

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Test Procedures for Developing Efficacy Data Supporting C. difficile Claims

  • EPA MLB SOP MB-28:  Procedure for the Production and Storage of Spores of Clostridium difficile for Use in the Efficacy Evaluation of Antimicrobial Agents, and
  • EPA MLB SOP MB-31: Quantitative Method for Testing Antimicrobial Products Against Spores of Clostridium difficile (ATCC 43598) on Inanimate, Hard, Non-porous Surfaces

EPA MLB SOP MB-28 is used to produce spores of C. difficile. The spores are used in the procedure described in EPA MLB SOP MB-31 to determine the effectiveness of the antimicrobial product. 

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Test Criteria

Applicants should apply the following product specific test criteria when evaluating products against C. difficile spores:

Acceptable Test Strain and Spore Production: Use C. difficile strain (ATCC 43598). Produce spore suspensions following EPA MLB SOP MB-28, meeting all the qualifications outlined in the procedure before use in efficacy testing.

Number of Batches and Test Coupons per Batch: Test three batches of the product at the lower certified limit(s) (LCL) listed on the confidential statement of formula of the product. Conduct each of the three tests on independent test days. Evaluate ten carriers against the product, three test system control carriers (against sodium hypochlorite, 1500 ppm) and three unexposed carriers as controls in each test.

Verification Testing: Previously, registrants were encouraged to conduct verification testing of the same three batches in (1) a second laboratory or (2) in the same laboratory with a different qualified spore preparation, separate study director, technical staff, and Quality Assurance auditor. Registrants still may conduct this second level verification to ensure confidence in their data. However, the need and test criteria for verification of a C. difficile claim is currently under review by the Agency. The guidance will be updated following the Agency’s assessment.

Carriers: Use 430 stainless steel (SS) carriers for qualifying spores, for the test system controls and product evaluations. For testing products with oxidative chemistries such as peracetic acid and peroxides, 304 SS carriers may be used instead of 430 SS carriers.

Neutralizer Confirmation: Conduct neutralization testing to confirm and document the neutralizer’s effectiveness for the product. Refer to EPA MLB SOP MB-31.

Contact Time: Do not use a contact time for efficacy testing that exceeds 10 minutes.

Diluent: For testing products that require dilution, use a diluent that is consistent with that used for the sterilant, sporicide, or hospital disinfectant claim. If product dilution for a ready to use product is performed, use a diluent that is not expected to increase product efficacy.

Soil Load: Use the three-part soil load specified in EPA MLB SOP MB-31 for all testing including product, control, and test system control carriers.

Control Carrier Counts: Attain a minimum of 1.0×106 to a maximum of 1.0×107 spores per carrier.

Test System Control: Run a test system control using three 430 SS carriers against a 1,500±150 ppm (total chlorine) NaOCl solution concurrently with each product test (per test day) using a three-minute contact time. A log reduction (LR) of <3 for the test system control ensures the quality of the test system. LR above 3 invalidates the test and test repetition will be needed.

Evaluation of Success: Demonstrate a minimum mean 6.0 log reduction in viable spores for each of the six tests.

Modifications to the Test Procedures: When recommended methods are modified by the applicant to support specific claims for a product, submit the complete testing protocol, identifying and describing each modification to the Agency for review and evaluation prior to the initiation of the tests. Describe fully all materials and procedures employed in the testing.

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Data Submission Procedures for Efficacy Data

To assist in the proper review and evaluation of product performance, submit complete descriptions of the test employed and the results obtained to the Agency following the data submission guidelines. All product performance data must be developed in compliance with the Good Laboratory Practice Standards (40 CFR Part 160). 

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Label Guidance

Example of Label Claim: Kills and/or inactivates 99.9999% of spores of Clostridium difficile on a hard, non-porous surface.

Special Label Instructions for Cleaning Prior to Disinfection against Clostridium difficile

Include the following specific cleaning directions on all products bearing Clostridium difficile claims:

  • Personal Protection: Refer to the product label for appropriate personal protective equipment.
  • Cleaning Procedure: Special attention is needed for high-touch surfaces; cleaning in an appropriate manner and adherence to manufacturer’s label instructions for use and contact/dwell times is necessary. Pre-clean surfaces to remove soil and filth. Wipe dry. Thoroughly wet pre-cleaned surface with product. Allow surface to remain wet for [contact time]. Surfaces in patient rooms are to be cleaned in an appropriate manner, such as from right to left or left to right, on horizontal surfaces, and top to bottom, on vertical surfaces, to minimize spreading of spores. Restrooms are to be cleaned last. Do not reuse soiled cloths.
  • Infectious Waste Disposal: Materials used in the cleaning process that may contain feces/wastes are to be disposed of immediately in accordance with local regulations for disposal of infectious materials.

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