US Environmental Protection Agency
Office of Pesticide Programs
Office of Pesticide Programs
Mic ro bio lo gy La bo ra to ry
Environmental Science Center, Ft. Meade, MD
Standard Operating Procedure for
Quantitative Disk Carrier Test Method (QCT-2) Modified
Modified for Testing Antimicrobial Products Against
Spores of Clostridium difficile (ATCC 43598) on
Inanimate, Hard, Non-porous Surfaces
SOP Number: MB-31-02
Date Revised: 05-06-14

SOP No. MB-31-02
Date Revised 05-06-14
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SOP Number
Quantitative Disk Carrier Test Method (QCT-2) Modified for
Testing Antimicrobial Products Against Spores of Clostridium
difficile (ATCC 43598) on Inanimate, Hard, Non-porous Surfaces.
This quantitative method is used to evaluate the sporicidal efficacy
of liquid disinfectants against spores of Clostridium difficile (ATCC
43598) on inanimate, hard, non-porous surfaces. This SOP is based
on the ASTM Standard E2197-11 (see 15.1) and incorporates
methodologies specific to testing spores of C. difficile.
Data from this method are used to generate the log reduction (LR)
values of viable spores of C. difficile as the quantitative measure of
efficacy for liquid disinfectants on an inanimate, hard non-porous

Approval Date
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Data SOP issued:

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Date Revised 05-06-14
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Contents	Page Number

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1. Definitions
Additional abbreviations/definitions are provided in the text.
1.	Eluate = recovered eluent that contains the test organism.
2.	Eluent = the liquid that is added to a vial containing the carrier to recover
the test organism.
3.	Frozen spore suspension = frozen stocks of spore suspension prepared per
4.	Final test suspension = thawed frozen spore suspension including the
addition of the soil load when specified.
2. Health and
Follow procedures specified in SOP MB-01, Laboratory Biosafety. The Study
Director and/or lead analyst should consult the Material Safety Data Sheet for
specific hazards associated with the test substance.
3. Personnel
and Training
1.	A reference standard (e.g., predetermined concentrations of sodium
hypochlorite) may be used to check method performance and analyst
2.	Refer toSOPADM-04, OPP Microbiology Laboratory Training.
4. Instrument
Refer to SOPEQ-01 (pH meters), EQ-02 (Thermometers and Hygrometers),
EQ-03 (Balances), EQ-05 (Timers) and QC-19 (pipettes) for details on
method and frequency of calibration.
5. Sample
Handling and
Refer to SOPMB-22; Disinfectant Sample Preparation, and SOP COC-01;
Disinfectant Sample Login, Tracking and Disposal.
6. Quality Control
For quality control purposes, the required information is documented on the
appropriate form(s) (see section 14).
7. Interferences
1.	Prior to efficacy testing, verify neutralizer effectiveness using the
procedure outlined in SOPMB-26 (Neutralization of Microbicidal
Activity using the OECD Quantitative Method) with the following
modification: For Titer Control use PBS with Tween-80 instead of PBS.
2.	The test organism (C. difficile ATCC 43 598) must be incubated under strict
anaerobic conditions. The presence of oxygen will severely compromise the
viability and growth of C. difficile.
3.	During testing, do not process a carrier where the test substance runs off
the carrier; replace with an untreated inoculated carrier and vial.
8. Non-
1.	The control carrier counts should be between 106to 107 spores/carrier.
2.	For a valid test, the spore suspension used in efficacy testing must
meet the criteria specified in SOP MB-28.

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9. Data
Data will be archived consistent with SOP ADM-03, Records and Archives.
10. Cautions
Avoid extended soaking of the carriers in water or detergent and prolonged
rinsing to reduce risk of corrosion or rusting.
11. Special
Apparatus and
1.	Test microbe. Spore suspension of C. difficile (ATCC 43598); prepared
according to MB-28.
2.	Recovery medium. Brain-heart infusion agar with yeast extract, horse
blood and sodium taurocholate (BHIY-HT). Pre-reduced recovery
medium (Anaerobe Systems, Morgan Hill, CA) for enumeration of viable
3.	Reagents
a.	Neutralizer in eluent. The default neutralizer is PBS with Tween-80;
however, an alternative neutralizer may be used.
b.	Phosphate buffer (PB) stock solution. Dissolve 34.0 g of potassium
dihydrogen phosphate (KH2P04)in 500 mL de-ionized water.
Adjust pHto 7.2  0.2 with 0.1 N NaOHor 0.1 NHC1 and bring to
1000 mL with de-ionized water. Alternative phosphate buffers with
the same pH may be used (e.g., commercially prepared 10XPBS
c.	Phosphate buffered saline (PBS). Add 1.25 mL of phosphate buffer
stock solution and 8.75 g of NaCl to a volumetric flask; fill with de-
ionized water to the 1000 mL mark and mix. The pH should be
approximately 7.0 0.5. Sterilize by filtration or autoclaving.
Alternative PBS formulations with the same pH may be used (e.g.,
dilute commercially prepared 1 OX PBS solution to IX using de-
ionized water).
d.	ST-80: Phosphate-buffered saline (PBS) containing 0.1 % Tween 80.
Diluting and washing reagent; add 2.0 mL of polysorbate 80 (Tween
80, or equivalent) to approximately 1.5 L of PBS (IX). Mix
thoroughly and (using a volumetric flask) bring solution to volume (2
L) with PBS. Distribute into bottles and autoclave for 20 min at
12 IT.
e.	Spore stain. 5% aqueous malachite green and 0.5% aqueous
safranine to differentiate spores from vegetative cells. Spores appear
green while vegetative cells appear red.
f Soil load. The recommended soil load (if required) to be
incorporated in the test suspension is a mixture of the following stock
solutions in PBS:

SOP No. MB-31-02
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i.	BSA: Add 0.5 g bovine serum albumin (BSA) to 10 mL of
PBS, mix and pass through a 0.2 pm pore diameter membrane
filter, aliquot and store at -20  5C.
ii.	Yeast Extract: Add 0.5 g yeast extract to 10 mL of PBS, mix,
and pass through a 0.2 pm pore diameter membrane filter,
aliquot and store at -20  5C.
iii.	Mucin: Add 0.04 g mucin (bovine or porcine) to lOmLof
PBS, mix thoroughly until dissolved, and autoclave (15
minutes at 121C), aliquot and store at -20  5C.
The stock solutions of the soil load are single use only and
should not be refrozen once thawed; store up to one year at
-20  5C.
g.	Test substance. Refer to SOP MB-22, Disinfectant Sample
h.	Test substance diluent. The test substance diluent will be used as
specified. For preparation of hard water, refer to SOPMB-30.
i.	Water. Either de-ionized distilled water or water with equivalent
quality for making reagent solutions and culture media.
j. Tween-80 (polysorbate 80). To prepare ST-80.
k. Liquinox. To clean carriers.
4. Apparatus
a.	Calibrated positive displacement pipettes (e.g., 10 uL). For carrier
b.	Micropipettes (e.g., 200 |iL), For deposition of test substance on
c.	Carriers. Disks (1 cm in diameter) made from 0.7 mm thick sheets
of brushed and magnetized stainless steel (AISI #430). The top of
the disk is brushed and has rounded edges; only the top is visually
screened and inoculated. Carriers are single-use only. See
Attachment 1 for complete specifications.
d.	Bottle-top dispensers, squirt bottles, pre-measured volumes in tubes,
or pipettes used in rinsing of vials and filters.
e.	Forceps. Straight or curved, non-magnetic, disposable with smooth
flat tips to handle membrane filters, appropriate to pick up the
carriers for placement in vials.

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f Magnet. Strong enough to hold a treated carrier in place in the vial
while the liquid is being poured out of it for membrane filtration.
g.	Poly ethersulfone membrane filter (PES). For recovery of test
microbe, 47 mm diameter and 0.2 pm pore size. Filtration units
(reusable or disposable) may be used.
h.	Sterile vials (plastic or comparable). To hold test carriers: flat
bottom and wide-mouth (at least 25mm in neck diameter) to
accommodate addition and removal of the carriers, for holding
inoculated carriers to be exposed to the test substance and for
accommodating neutralize r/eluent.
i.	Vortex mixer. To vortex the eluate and rinsing fluid in the carrier
vials to ensure efficient recovery of the test organism(s).
j. Certified timer. That can be read in minutes and seconds for the
contact time specified.
k. Desiccator with desiccant (e.g., CaC ():<). For drying the inoculum on
the carriers.
1. Vacuum source. In-house line or suitable vacuum pump (20-25 in.
mercury) for drying carriers and for filtering.
m. Petri dish (100 x 15mm). To hold carriers.
n. Filter paper. Whatman No. 2, to line Petri dishes.
o. COY Anaerobic chamber. Supported by a compressed gas mixture
consisting of 10% Hydrogen, 5% CO2, and 85% N2. To provide an
anaerobic environment.
p. Anaerobic incubator. Use the incubator at 36  1C inside the COY
anaerobic chamber to support the growth of C. difficile.
q. Microscope. With 1 OX eyepiece and 40X and 100X (oil) objectives
with phase contrast option. To examine spores.
12. Procedure and
Conduct three independent tests (three test days) or as specified by the study
sponsor. The product performance standard for a C. difficile claim is a
minimum 6 log reduction in viable spores for each test. For each test,
evaluate 10 treated and 3 control carriers.
12.1 Preparation
sterilization of
a. Visually check the brushed top surface of the carriers (with the
rounded edge) for abnormalities (e.g., rust, chipping, deep striations)
and discard if observed. Record physical screening of carriers on
form as noted in section 14.

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b.	Soak visually screened carriers in a suitable detergent solution (e.g.,
1% v/v Liquinox) for 2-4 h to degrease and then rinse thoroughly in
distilled water.
c.	Place up to 20 clean dry carriers on a piece of filter paper inside the
bottom surface of a glass Petri dish. Cover the Petri dish with lid and
sterilize. After sterilization, aseptically transfer carriers to sterile
Petri dishes without filter paper for inoculation.
12.2 Preparation of
test organism
a. Preparation of Frozen Stock of Spore Suspensions. Refer to SOP
MB-28 for the generation of purified spores of C. difficile (ATCC
12.3 Preparation of
the final test
with soil load
a.	Defrost a cryovial of C. difficile spores (approximately 10-15 min at
room temperature). Each cryovial is single use only.
b.	Vortex the thawed spore suspension until re-suspended to evenly
distribute the spores.
c.	If soil load is required, to obtain 500 [j,L of the final test suspension
vortex each component and combine the following:
i.	25 |aL BSA stock
ii.	35 |uL yeast extract stock
iii.	100 |iL mucin stock
iv.	340 |aL spore suspension
d.	If different total volumes of the final test suspension are required,
scale each component listed in 12.3c, maintaining the correct ratio of
each component. Following the addition of the soil load, vortex mix
the final test suspension.
12.4 Inoculation
and drying of
a.	Inoculate a minimum of fifteen carriers (e.g., ten for treated carriers,
three for control carriers, and two extras). For inoculation, withdraw
10 |oL of the final spore suspension with a calibrated positive-
displacement pipette and deposit spore suspension in the center of
each carrier.
b.	Avoid contact with carrier and do not spread the spore suspension
with the pipette tip. Use the same pipette tip to inoculate each batch
of carriers. Discard any inoculated carrier where the final spore
suspension has run over the edge.
c.	Dry the carriers inside a Petri dish (with the lid off) in the BSC for
305 min. After the inoculum has dried, place the Petri dish in a
desiccator connected to a vacuum line. Remove the Petri dish lid.
Continue drying under vacuum for 2 h at room temperature inside the

SOP No. MB-31-02
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d. At the end of the drying period, turn off the vacuum, and cover the
plate. Observe the dried inoculum on each carrier. Discard any
carrier in which the inoculum has run off the surface. The carriers
can be used at this point or stored overnight in the desiccator without
vacuum for use the next day Use dried carriers within 24 h of
12.5 Exposure of
the dried
inoculum to
the test
substance or
ST-80 (control
a.	Using sterile forceps transfer each dried carrier with the inoculated
side up to a flat-bottom vial and cap the vial. Repeat until all carriers
are transferred.
b.	Use a certified timer to ensure that each carrier receives the required
exposure time (e.g., 5 min  3 sec).
c.	In a timed fashion, deposit 50 [j,L of the test substance, equilibrated
to 20-25C, over the dried inoculum on each test carrier, ensuring
complete coverage, at predetermined staggered intervals. Use a new
tip for each carrier; do not touch the pipette tip to the carrier surface.
Do not cap the vials.
d.	Hold the test carriers at 20-25C for the contact period.
e.	Treat control carriers last - each control carrier receives 50 [j,L of ST-
80, equilibrated to 20-25C, instead of the test substance. Hold the
control carriers at 20-25C for the contact period.
12.6 Neutralization
of test
substance and
elution of test
The neutralizer for the control carriers is the same as that for the treated
a.	Within  3 seconds of the end of the contact period, add 10 mL of
neutralizer at room temperature to each vial in the specified order,
including controls, according to the predetermined schedule (the
neutralized vial is the 10 dilution). Cap the vial and briefly (2-3 sec)
vortex following the addition of the neutralizer.
b.	Following the neutralization of the entire set of carriers, vortex each
vial for 30  5 sec at high speed to recover the inoculum; ensure that
the carrier is vortexing along with the liquid in the vial. Visually
examine each carrier and, in case of incomplete elution, perform
further vortexing to remove inoculum. Do not remove the carrier
from the vial.
12.7 Dilution and
a. Initiate dilutions within 30 min at room temperature after
neutralization. Initiate filtration within 30 min of preparing the
dilutions. Use 0.2 pm PES membrane filters. Direct plating is not

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b.	Process the treated carriers first. Prior to filtering the contents of
tubes and vials, pre-wet each membrane filter with approximately 10
mL of sterile PBS.
c.	For treated carriers, briefly vortex the vial (10) and remove 1 mL to
prepare serial dilutions in 9 mL ST-80 out to a minimum of the 10"1
dilution. For filtration of contents in tubes, briefly vortex and pour
into filter. Rinse each tube once with -10 mL of PBS, briefly vortex,
and pour the contents of the tube into the same filter unit.
d.	For the treated vial, briefly vortex contents and while holding a
magnet to the bottom of the vial (to keep the carrier in place) pour
the contents into a filter unit. Rinse the vial with -20 mL of PBS,
briefly vortex, and while keeping the magnet in place, pour the wash
liquid into the same filter unit. Repeat this step three more times.
Swirl the contents of the filter unit and apply the vacuum. With the
vacuum on, rinse the inside surface of each filter unit with an
additional -40 mL PBS.
e.	For control carriers, prepare serial dilutions using 1 mL from the vial
(10) in 9 mL ST-80 out to the 10"7 dilution to provide countable
filters (up to 200 CFU/filter). Filter the contents of the 10"6and 10~7
tubes. For filtration of contents of tubes, briefly vortex and pour into
filter. Rinse each tube once with -10 mL of PBS, briefly vortex, and
pour the contents of the tube into the same filter unit. With the
vacuum on, rinse the inside surface of each control filter unit with an
additional -20 mL PBS.
f Aseptically remove the membrane filter (treated samples first,
followed by the controls) and place on the recovery medium (pre-
reduced BHIY-HT). Open each sealed package inside the BSC just
prior to placement of the membrane filter. Avoid trapping any air
bubbles between the membrane filter and the agar surface.
g. Place BHIY-HT plates with membrane filters under anaerobic
conditions within 5010min of opening the package of plates.
Incubate BHIY-HT plates with membrane filters under anaerobic
conditions at 3 6 1 C.
12.8 Recording
a. For control carriers, record results as colony forming units (CFUs)
per filter at484 h of incubation. For treated carriers, record results
at 724 h of incubation. If no or few colonies are observed after
724 h of incubation, continue to incubate for an additional 484 h.
Colony counts in excess of 200 CFUs per membrane filter should be

SOP No. MB-31-02
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recorded as Too Numerous to Count (TNTC). If no colonies are
present, record as zero.
b. Inspect the growth from one of the membrane filters for purity and
typical characteristics of the test microbe (see Table 1). Observe
growth from a typical colony on a membrane filter using spore
staining or under phase contrast microscopy. Record results on the
Test Microbe Confirmation Sheet.
Table 1. Characteristics of C. difficile (ATCC 43598)
Typical Diagnostic Characteristics
BfflY-HT plate
Growth circular, entire edge, convex, smooth and grey colonies.*
Spores appear bright and ovular while vegetative cells appear dark and
Spore staining
Spores appear green while vegetative cells appear red.
* At 484 h
13. Data Analysis/
1.	Colony counts (CFUs) at each dilution are recorded and used to calculate
the log reduction in viable spores.
2.	To calculate the CFU/carrier when two (2) serial dilutions are filtered, use
the following example formula:
f CFU for 10 ' + CFU for 10 : ^
w c
(axWy) + (bxW2)
where 10"y and 10"zare the dilutions filtered, "a" and "b" are the volumes
filtered at each dilution (typically 9 or 10 mL), and "c" is the volume (10
mL) of neutralizer originally in the vial with the carrier.
a.	When TNTC values are observed for each dilution filtered, substitute
200 for the TNTC at the highest (most dilute) dilution and scale up
accordingly for the calculations.
b.	When zeroes are observed for each dilution filtered, substitute 0.5 for
the zero at the lowest (least dilute) dilution and scale up accordingly
for the calculations.
3.	Calculate the logio density (LD) of each carrier by taking the logio of the
4.	Calculate the mean logio density across treated carriers.
5.	Calculate the mean logio density across control carriers.

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6. Calculate the logio reduction (LR) for treated carriers:
logio reduction = mean logio control - mean logio treated

7. If no spores are recovered for any treated carrier, report the LR as greater
than or equal to the mean logio density for the control carriers.
14. Forms and Data
1. Attachment 1: Carrier Specifications

2. Test Sheets. Test sheets are stored separately from the SOP under the
following file names:

Physical Screening of Carriers Record Form

QCT-2 Method for Sporicidal Activity: Organism
Culture Tracking Form

QCT-2 Method for Sporicidal Activity: Test
Microbe Confirmation Sheet (Quality Control)

QCT-2 Method for Sporicidal Activity: Test
Information Sheet
MB-31-02 F4.docx

QCT-2 Method for Sporicidal Activity: Time
Recording Sheet

QCT-2 Method for Sporicidal Activity: Serial
Dilution Plating/Tracking Form
MB-31-02 F6.docx

QCT-2 Method for Sporicidal Activity: Results

QCT-2 Method for Sporicidal Activity: Test
Microbe Confirmation Sheet
MB-31-02 F8.docx
15. Reference
1. ASTM Standard E2197-11. Standard Quantitative Disk Carrier Test
Method for Determining Bactericidal, Virucidal, Fungicidal,
Mycobactericidal and Sporicidal Activities of Chemicals. ASTM
International, West Conshohocken, PA, 201 1.

SOP No. MB-31-02
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Attachment 1
Carrier Specifications
	Ferritic stainless steel: Consist of chromium (17%) and iron and essentially nickel-free.
	AISI Type 430 (European equivalent name X6Crl7 and number 1.4016) belongs to Group 2,
which is the most widely used family of ferritic alloys.
	Dimensions: 1 cm (0.39370 inch) in diameter; 0.7 mm (0.27559 inch) thick.
	AISI 430 - ASTM A240; Japanese Industrial Standard (US) G4305; EN 10088-2
	No. 4 Finish (EN 10088-2 1J/2J): Aground unidirectional finish obtained with 150 grit
abrasive (AISI).
	Passivation: A soak in a mild acid bath for a few minutes to remove any impurities and
accumulated debris from the disk surface.
	Tumbling: To remove the punching burrs from the edges of the discs they are tumbled in a
barrel together with ceramic chips and a cleanser.