Health-care–associated infections are those that develop in patients during hospitalization. Most HAIs in humans are introduced during surgical procedures or develop secondary to the presence of invasive devices such as a catheter inserted into a blood vessel or the urinary bladder.1 Microbes residing on the surfaces of furnishings, equipment, and instruments have been reported or suspected to serve as sources of HAIs.2–7 In 2002, approximately 1.7 million human HAIs resulted in 99,000 deaths.8
The prevalence of HAIs in the veterinary field and their effects on the health of canine and feline patients have not been extensively studied.9 In a prospective surveillance study10 performed in 4 veterinary referral hospitals, 16.3% of dogs and 12% of cats developed an HAI. Between 2003 and 2008, 31 of 38 (82%) veterinary teaching hospitals identified HAI outbreaks, whereas 19 (50%) reported zoonotic infections among their personnel.11 Similar to humans, HAIs described in dogs and cats are most commonly associated with surgical procedures and urinary and IV catheterizations.10,12
Potential sources of HAIs include microbes residing on a patient's body, transmission by staff members, or exposure to microbes residing on inanimate surfaces (including equipment) with which the patient comes in contact.4,9 Specific bacteria isolated from hospitalized small animal patients are genetically identical or closely related to those found on contaminated surfaces within veterinary hospitals, which suggests cross-contamination of microbes between hospital surfaces and hospitalized patients.13–15 Analysis of the results of these studies suggests that microbes present on inanimate surfaces could serve as sources of HAIs in veterinary patients.
Chemical disinfectants are used to reduce microbial contamination on surfaces, equipment, and instruments that may serve as sources of HAIs. Chemical disinfectants have inherent limitations. Most require lengthy contact times ranging from 5 to 30 minutes, which can be difficult to achieve in a busy practice.9 The effectiveness of chemical disinfectants may be compromised by the presence of organic material, dirt, light, or water frequently found in many veterinary facilities.9 In addition, some chemical disinfectants are potentially toxic or irritating to pets and personnel, emit offensive odors, and can corrode surface materials.16 Bacteria can also develop resistance to chemical disinfectants.16–20
Heat kills microbes by denaturing and damaging cellular proteins.16 Heat, in the form of steam, is used in autoclaves to sterilize surgical instruments. Similarly, the application of heat has the potential to kill microbes on contaminated hospital surfaces. In 1 study,21 the application of steam to frequently used handrails in a human hospital room reduced surface bacterial concentrations by > 90%. Steam treatment of hospital surfaces may provide a quick and effective method for killing unwanted microbes in the presence of dirt and organic material. In addition, it would be expected to be less toxic to patients and veterinary personnel than chemical disinfectants. Microbes may also prove to be more susceptible to the heat generated by steam treatment than they might to chemical disinfection.
Nonindustrial systems that deliver steam for cleaning purposes have only recently become commercially available. These systems have been marketed as allergen-free, nontoxic means for cleaning and disinfecting in home environments. The portability of these devices makes them practical for use in hospital settings.
The objective of the study reported here was to determine whether treatment with steam would reduce the number of bacteria on various inanimate surfaces in a busy veterinary hospital. Total bacteria as determined by HPCs and counts of Staphylococcus aureus, Pseudomonas spp, and total coliforms were chosen as indicators of bacterial contamination. It was hypothesized that treatment with steam would reduce bacterial numbers on a variety of contaminated surfaces in a veterinary facility.
Materials and Methods
Sample—Effects of steam treatments were evaluated at 18 test sites on 5 surface types in a busy 24-hour veterinary specialty and emergency hospital located in Overland Park, Kansas. This facility was chosen because of its high small animal case load and the high amount of staff movement within the hospital, which was expected to contribute to bacterial contamination of the many surfaces within the facility.
Samples for evaluation were obtained from 5 horizontal surface types within the facility. Surfaces included slightly rough poured concrete floors in dog runs (n = 6 test sites), smooth stainless steel kennel floors (6), smooth molded stainless steel tub sinks used for washing dogs (2), smooth flat plastic laminate surfaces (2; one covering a radiology table and the other a laboratory countertop), and ribbed rubber mats (2; one on a weight scale and the other on a stepping stool).
A template was used to outline a testing area (30.48 × 60.96 cm) on each surface. The testing area was divided in half to create 2 equal test sites (left and right), each of which measured 30.48 × 30.48 cm. The ribbed rubber mat of the stepping stool was < 30.48 × 60.96 cm; therefore, the entire ribbed surface was measured and divided into equal left and right halves. The boundary of each test site was delineated with a water-resistant marker.
Steam application system—A steam unita was used to treat each test surface. The unit was filled with tap water, powered on, and allowed to reach a boiler pressure of 455,053 Pa prior to use. Vapor pressure at the steam outlet was set at 82,737 to 103,421 Pa, in accordance with the manufacturer's instructions. The unit was outfitted with a hose, which was connected to a triangular cleaning head (14 × 14 × 14 cm). A terry cloth towel (76.20 × 76.20 cm) then was affixed to the cleaning head. To prevent cross-contamination of test sites, a new terry cloth towel was affixed between subsequent treatments.
Sample collection and steam treatment—A pretreatment sample was obtained from the left half of each test site. Five milliliters of sterile neutralizing brothb (1:10 dilution in sterile water purified by reverse osmosis) was poured onto the center of the left half of each test site. Sterile forceps was used to hold a sterile 4 × 4 × 1-cm polyurethane foam sponge; the sponge was used to absorb the broth and then was rubbed over the surface for 10 seconds. The sponge was placed into a large sterile sample collection tube that contained 5 mL of sterile neutralizing broth; a cap was then placed on the tube. Steam was applied to the right half of each test site by slowly advancing the cleaning head over the defined area for a period of 10 seconds. A posttreatment sample was then immediately obtained from the steam-treated test site by use of the same technique described for the pretreatment sample.
Samples were immediately refrigerated at 4.44°C for approximately 4 hours, then shipped on ice by overnight courier to a commercial microbiology laboratory.c Total sample collection volume was considered to be 10 mL for subsequent calculations.
Sample processing and quantitative microbiological techniques—Samples were mixed on a vortexer for approximately 15 to 30 seconds; aliquots were then obtained for bacterial culture and quantitation. Aliquots used for the HPC were dispensed into empty, sterile Petri dishes and counted with the pour plate method by use of tryptic soy agar.d All plates used for HPCs were incubated at a mean ± SD temperature of 25 ± 5°C for 96 ± 6 hours. Aliquots used for culture on selective media were dispensed and spread onto plates containing the appropriate media. All selective media plates were incubated at 36 ± 1°C for 24 ± 6 hours as per the specific instructions for each medium. After incubation was completed, the number of individual colonies was counted, and results were recorded as the number of CFUs per test site. The limit of quantification was 50 CFUs/test site.
Bacterial culture on selective media—For coliform isolation, samples were cultured on m-Endo agar.e Plates were visually examined for the presence of blue-black colonies with a green metallic sheen characteristic of coliforms.
For S aureus isolation, samples were cultured on Staphylococcus agar.f Plates were visually examined for the presence of golden yellow colonies characteristic of S aureus. It was possible that growth of other Staphylococcus spp resulted in yellow colonies, but for the purpose of this study, all such colonies were assumed to be S aureus.
For Pseudomonas spp isolation, samples were cultured on Pseudomonas isolation agar.g Plates were visually examined for the presence of white (sometimes with a greenish tint) mucoid colonies with a characteristic grape-like or tortilla-like odor.
Statistical analysis—All analyses were performed with commercial statistical software.h Descriptive statistics were calculated for the counts obtained.
Results for the variables HPC and counts of coliforms, S aureus, and Pseudomonas spp were statistically evaluated by use of an ANOVA with a generalized linear model program.i Before data were analyzed with the ANOVA, assumptions of normality and homogeneity of variance were tested for all variables with the Shapiro-Wilk and Levene tests, respectively. The data were normally distributed but had heterogeneous variance. A logarithmic transformation (natural logarithm) stabilized the variance and yielded data that were normally distributed.
All hypothesis tests were performed at an α of 0.05. Each value below the limit of quantification was replaced by the limit of quantification divided by the square root of 2.j Summary statistics were obtained for sites with at least 2 observations/treatment for both untreated and steam-treated samples.
Results
HPCs—All 18 test sites had pretreatment arithmetic mean HPCs ranging from 8.00 × 102 CFUs/test site to 7.95 × 107 CFUs/test site (Table 1). Stainless steel tub sink 1 and the plastic laminate laboratory countertop had the highest pretreatment HPC (7.95 × 107 CFUs/test site and 1.60 × 107 CFUs/test site, respectively). After steam treatment, the bacterial numbers at these sites were reduced by > 99% (to 3.02 × 104 CFUs/test site and 1.00 × 102 CFUs/test site, respectively). The ribbed rubber mat of the weight scale was the only test site at which steam treatment failed to cause a reduction in the HPC (3.29 × 104 CFUs/test site before steam treatment vs 6.62 × 104 CFUs/test site after steam treatment). Excluding the results for the ribbed rubber mat of the weight scale, the HPC was decreased by 88% to > 99% for the various test sites.
Mean bacterial counts on surfaces before and after steam treatment for disinfection at various sites in a veterinary hospital.
| Test site | Time of sample collection* | HPC | Coliforms | Staphylococcus aureus | Pseudomonas spp |
|---|---|---|---|---|---|
| Concrete floor of dog run (n = 6) | Before | 5.16 × 104 (2.87 × 104) | < 50 (5.02 × 101) | 2.16 × 102 (8.35 × 101) | 1.66 × 102 (8.75 × 101) |
| After | 2.36 × 103 (1.37 × 103)† | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) | |
| Stainless steel kennel floor (n = 6) | Before | 4.22 × 104 (2.78 × 104) | 8.33 × 101 (9.69 × 101) | 1.41 × 102 (7.59 × 101) | < 50 (< 50) |
| After | 1.58 × 102 (1.25 × 102)‡ | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) | |
| Stainless steel tub sink (n = 2) | Before | 3.99 × 107 (6.01 × 106) | 1.00 × 106 (2.00 × 104) | 8.95 × 103 (7.82 × 103) | 2.20 × 106 (6.56 × 105) |
| After | 1.76 × 104 (1.25 × 104) | 2.52 × 103 (5.04 × 102) | 3.75 × 102 (1.88 × 102) | < 50 (< 50)‡ | |
| Ribbed rubber mat Weight scale (n = 1) | Before | 3.29 × 104 (3.29 × 104) | 3.00 × 102 (3.00 × 102) | 7.50 × 102 (7.50 × 102) | < 50 (< 50) |
| After | 6.62 × 104 (6.62 × 104) | 2.00 × 102 (2.00 × 102) | 4.00 × 102 (4.00 × 102) | < 50 (< 50) | |
| Stepping stool (n = 1) | Before | 9.50 × 104 (9.50 × 104) | < 50 (< 50) | 5.00 × 101 (5.00 × 101) | < 50 (< 50) |
| After | 4.50 × 103 (4.50 × 103) | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) | |
| Flat plastic laminate Laboratory counter | Before | 1.60 × 107 (1.60 × 107) | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) |
| (n = 1) | After | 1.00 × 102 (1.00 × 102) | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) |
| Radiology table | Before | 8.00 × 102 (8.00 × 102) | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) |
| (n = 1) | After | 1.00 × 102 (1.00 × 102) | < 50 (< 50) | < 50 (< 50) | < 50 (< 50) |
Values reported are arithmetic mean (geometric mean) CFUs per test site.
Samples for bacterial culture were collected before steam treatment and immediately after steam treatment for 10 seconds.
Value differs significantly (P < 0.05) from the value for the corresponding test site before treatment.
Value differs significantly (P < 0.001) from the value for the corresponding test site before treatment.
Steam treatment caused significant reductions in the HPC for the concrete dog runs (P = 0.002) and stainless steel kennels (P < 0.001). Steam treatment did not cause a significant (P = 0.152) reduction in bacteria for the stainless steel tub sinks. None of the other test sites had sufficient data to enable statistical analysis.
Total coliform counts—Four of the 18 test sites had pretreatment total coliform growth (arithmetic mean) > 100 CFUs/test site (range, 200 CFUs/test site to 2.00 × 106 CFUs/test site). These included both stainless steel tub sinks, stainless steel kennel floor 5, and the ribbed rubber mat of the weight scale. Stainless steel tub sink 1 (2.00 × 106 CFUs/test site) and stainless steel kennel floor 5 (2.50 × 103 CFUs/test site) had the highest pretreatment coliform counts. After steam treatment, coliform counts for those 2 test sites decreased by > 99% and 98% (to 5.00 × 103 CFUs/test site and < 50 CFUs/test site, respectively). Steam treatment of the ribbed rubber mat of the weight scale resulted in the smallest reduction (33%) in coliform numbers (3.00 × 102 CFUs/test site before steam treatment vs 2.00 × 102 CFUs/test site after steam treatment). Excluding results for the rubber mat of the weight scale, total coliform counts after steam treatment were decreased by 75% to > 99% for the other 3 test sites. However, there were no significant differences between pretreatment and posttreatment total coliform counts for the concrete dog runs (P = 0.341), stainless steel kennels (P = 0.297), or stainless steel tub sinks (P = 0.537). None of the other test sites had sufficient data to enable statistical analysis.
S aureus—Five of 18 test sites had pretreatment S aureus growth (arithmetic mean) > 100 CFUs/test site (range, 6.00 × 102 CFUs/test site to 1.33 × 104 CFUs/test site). These included both stainless steel tub sinks, stainless steel kennel floor 4, concrete dog run 1, and the ribbed rubber mat of the weight scale. Stainless steel tub sinks 1 (4.60 × 103 CFUs/test site) and 2 (1.33 × 104 CFUs/test site) had the highest pretreatment S aureus counts. After steam treatment, the S aureus counts from these sites were reduced by 98% and 94% (to < 50 CFUs/test site and 7.00 × 102 CFUs/test site, respectively). The ribbed rubber mat of the weight scale had the smallest reduction in S aureus counts (47%) after steam treatment. Excluding the ribbed rubber mat of the weight scale, S aureus counts after steam treatment were decreased by 94% to 98% for the other 4 test sites. However, there were no significant differences between pretreatment and posttreatment S aureus counts for the concrete dog runs (P = 0.341), stainless steel kennels (P = 0.341), or stainless steel tub sinks (P = 0.133). None of the other test sites had sufficient data to enable statistical analysis.
Pseudomonas spp—Four of 18 test sites had pretreatment Pseudomonas spp growth (arithmetic mean) ≥ 100 CFUs/test site (range, 1.00 × 102 CFUs/test site to 4.30 × 106 CFUs/test site). These included both stainless steel tub sinks and concrete dog runs 1 and 4. Stainless steel tub sinks 1 and 2 had the highest pretreatment Pseudomonas spp counts (4.30 × 106 CFUs/test site and 1.00 × 105 CFUs/test site, respectively). After steam treatment, Pseudomonas spp growth at these sites was reduced by > 99% (to < 50 CFUs/test site for both stainless steel tub sinks).
Pretreatment and posttreatment Pseudomonas spp counts did not differ significantly (P = 0.204) for the concrete dog runs. There was a significant (P = 0.035) reduction in Pseudomonas spp counts after steam treatment of the stainless steel tub sinks. None of the other test sites had sufficient data to enable statistical analysis.
Discussion
In the present study, S aureus, Pseudomonas spp, coliform bacteria, and total heterotrophic bacteria were used as indicators of environmental contamination. Heterotrophic plate counts were performed to reflect total bacterial numbers. Staphylococcus aureus and Pseudomonas spp were chosen as representatives of gram-positive and gram-negative bacteria because of reports of their persistence in the environment where they may serve as causes of HAIs.14,22–25 In addition, the US EPA, which is responsible for the approval and registration of disinfectants in the United States, lists both species as indicator organisms against which disinfectants must prove efficacy.26 Total coliform bacteria were chosen as an indicator of fecal contamination, although their detection does not necessarily confirm the presence of feces.27
Treatment with steam was effective against the specific indicator organisms (S aureus, Pseudomonas spp, and coliforms) in the present study. The application of steam reduced bacterial numbers on most contaminated surfaces by 75% to 99%. Surfaces for which there were sufficient sample numbers had numeric reductions of at least 1 order of magnitude. Significant reductions were detected in the HPCs for concrete dog runs and stainless steel kennels and in Pseudomonas spp for stainless steel tub sinks. Nonsignificant results for other test sites may have been a result of high variability between the quantity of bacteria before and after treatment rather than a lack of numeric reduction after steam treatment. Low pretreatment bacterial colony counts that precluded evaluation of the effect of steam treatment on many of the test surfaces could have been attributable to less contamination of those test surfaces or satisfactory bacterial suppression from the existing disinfection procedures.
Treatment with steam as performed in the present study may have resulted in increased efficacy against bacteria if the testing had been performed in a laboratory setting. Results of studies performed in human hospital settings suggest that chemical disinfectants underperform in routine conditions relative to results for laboratory testing. Investigators in one study28 found that conventional disinfection failed to reduce the presence of methicillin-resistant S aureus on 82 of 124 (66%) tested surfaces, whereas investigators in another study29 found that conventional disinfection failed to reduce vancomycin-resistant Enterococcus faecium contamination in 60 of 376 (16.0%) tested sites. To be registered with the EPA as a disinfectant for medical use, a product must have specific efficacy to kill > 98% of bacteria on laboratory test samples containing both gram-negative and gram-positive bacteria, including specific Salmonella spp, Staphylococcus spp, and Pseudomonas spp on repeated sampling.26 It was not the purpose of the study reported here to determine whether steam treatment could be used as a disinfectant as defined by the EPA, but instead to determine whether steam treatment could reduce bacterial numbers in a veterinary hospital setting.
Many chemical disinfectants such as chlorine, formaldehyde, and chlorhexidine can be toxic or irritating to the mucous membranes, skin, and respiratory tract. A connection between diluted bleach and asthma has been reported.30 Investigators in 1 study31 reported a potential for patients to develop contact allergies with exposure to formaldehyde. In addition, serious, although rare, anaphylactic reactions to chlorhexidine have been described.32,33 In contrast, except for accidental exposure to excessive heat, deleterious effects from the use of steam disinfection would be unlikely.
The application of steam for a relatively short period was effective for killing bacterial contaminants in the present study. Greater reductions of bacterial numbers may have been achieved with longer steam exposure times, especially on surfaces that are not flat. In contrast, most chemical disinfectants require a contact time of 5 to 30 minutes to be effective, which would be an inconvenient amount of time in a busy veterinary hospital. The time required to disinfect a surface with steam may be less than that associated with chemical disinfectants, although this would need to be further evaluated.
The development of microbial resistance is another concern associated with the use of chemical disinfectants.19,25 The mechanisms responsible for microbial resistance to disinfectants would not be expected to protect these same bacteria from the excessive heat associated with steam treatment. The efficacy of steam treatment on heat-resistant bacterial endospores warrants evaluation.
It can be difficult to uniformly apply chemical disinfectants to many types of surfaces found in a veterinary hospital setting. The ability to deliver steam to hard to reach or irregular surfaces, such as cage doors and the underside of tables and equipment, could prove beneficial. However, the application of steam in the study reported here had diminished efficacy for decreasing HPCs and numbers of Pseudomonas spp, S aureus, and coliforms on the ribbed rubber mat of the weight scale. The irregularly contoured surface of the ribbed rubber mats made it difficult to maintain contact between the cleaning head of the steam device and the surfaces being treated, which allowed steam to escape. Similarly, in a human hospital, lower logarithmic reductions in bacterial numbers were found when rounded surfaces (eg, bedrails and sinks) were treated with steam, compared with bacterial numbers for treated flat surfaces.21 Treatment with steam may be more effective on smooth, flat surfaces where constant contact with the steam cleaning head can be achieved.
Dirt or organic material that had accumulated on the uneven and worn surface of the ribbed rubber mat of the weight scale may also have interfered with the ability of the steam to reach bacteria on its surface. Dirt and debris can collect in porous, uneven, cracked, and pitted surfaces, thus hiding microbes and making it difficult for disinfectants to reach the bacteria.9,34 In contrast, the steam system reduced bacterial numbers on the irregular surfaces of the slightly rough poured concrete floors in the dog runs. Composition of the tested surfaces may have contributed to differences in the responses. Rubber mats may have failed to adequately conduct heat to kill the bacteria, whereas poured concrete and stainless steel may have conducted heat to the surrounding surfaces more effectively. Additional studies are warranted to investigate the effect of surface condition, contour, and material composition on the ability of steam to eradicate surface microbes.
The application of steam may not be feasible for treating all surfaces in a veterinary hospital. Excessive heat can dissolve glues, fixatives, and sealants. Other materials, especially those containing plastic, are prone to melting, cracking, or deformation.35,36 Heat damage to surfaces could make subsequent disinfection more difficult. Therefore, steam would not appear to be a method for disinfecting all surfaces, and chemical disinfectants would need to be used on heat-sensitive materials. Manufacturers of items to be steam disinfected would need to be consulted to determine whether a product was heat sensitive.
The goal of disinfection is not necessarily to eradicate all potential pathogens but to reduce their numbers below those that might be necessary to induce an HAI. Potential pathogenic microbes differ in their ability to subsist in the environment and in their ability to be transmitted to potential hosts. Disinfectant guidelines are developed in many human hospitals with the goal of decreasing potential pathogen numbers in the environment. For example, hygiene standards in many human hospitals call for quantitative Staphylococcus spp counts obtained from culture of surface samples to be < 2.5 CFUs/cm2.37 The quantity of specific microbes that must be present to induce an HAI has not been evaluated in the veterinary field and would be expected to differ on the basis of the characteristics of the specific microbe and status of a particular patient.
Steam treatment was effective for reducing bacterial numbers on most surface types evaluated in the present study. This study was limited by the number and variety of test sites evaluated. In addition, the failure to retrieve > 100 CFUs/test site for S aureus, Pseudomonas spp, and coliforms at many of the test sites reduced the number of sites that could be evaluated. The low bacterial counts obtained for many of the pretreatment culture samples would be expected to limit the overall reduction in HPCs resulting from steam treatment. Replicate testing in a more contaminated environment and on a larger number and variety of surfaces would be required to determine efficacy limits of steam disinfection. We also did not examine the effectiveness of steam against viruses, fungi, or bacterial spores. An ideal disinfectant would prove effective against a wide variety of these microbes. The efficacy of steam treatment against viruses, fungi, and bacterial spores warrants further investigation.
Steam in the present study was applied by slowly advancing the steam cleaning head with an affixed terry cloth towel in accordance with the manufacturer's instructions. Other products use alternative methods of steam application. Whether other methods would prove less or more effective for reducing bacterial numbers warrants investigation. The role of the terry cloth towel, irrespective of the steam, for reducing bacterial numbers also warrants evaluation.
The present study revealed that the application of steam by use of the described methods significantly reduced bacterial numbers on a variety of surfaces within a veterinary facility. The steam system we used was portable, environmentally friendly, and rapidly efficacious, which are qualities that would be beneficial in a veterinary practice setting. However, further studies are needed to quantify the ability of steam to consistently disinfect irregular surfaces of a variety of materials and its ability to kill a broader spectrum of potential microbial pathogens. Detrimental effects of high temperatures on materials commonly used for veterinary surfaces also need to be explored. Steam disinfection may prove to be an effective alternative or adjunct to chemical disinfection within veterinary practices.
ABBREVIATIONS
| HAI | Health-care–associated infection |
| HPC | Heterotrophic plate count |
TANCS-equipped VaporJet-PC 2400 dry steam vapor unit, Advanced Vapor Technologies, Everett, Wash.
D/E Neutralizing broth, Becton, Dickinson and Co, Franklin Lakes, NJ.
Antimicrobial Test Laboratories, Round Rock, Tex.
Tryptic soy agar (7100), Acumedia Manufacturers Inc, Lansing, Mich.
m-ENDO agar (7724), Acumedia Manufacturers Inc, Lansing, Mich.
Staphylococcus agar No. 110 (R01845), Thermo Scientific, Waltham, Wash.
Pseudomonas isolation agar (7329), Acumedia Manufacturers Inc, Lansing, Mich.
SAS/STAT software, version 9.3, SAS Institute Inc, Cary, NC.
PROC GLM, SAS/STAT software, version 9.3, SAS Institute Inc, Cary, NC.
Croghan CW, Egeghy PP. Methods of dealing with values below the limit of detection using SAS (oral presentation). Southeastern SAS User Group, St Petersburg, Fla, September 2003.
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