Microbial inactivation by microwave radiation in the home environment.

Introduction

Food is a vehicle for the transmission of numerous infectious diseases caused by bacteria, viruses, protozoa, and helminth parasites. In the United States, it is estimated that foodborne illnesses affect some 6 to 80 million people per year, leading to approximately 9,000 deaths (Altekruse, Cohen, & Swerklow, 1997). In the United States, the cost of foodborne diseases has been estimated at $4-6 billion. According to Roberts (1982), the highest percentages of cases occur in the home environment.

The domestic kitchen represents an important source of contamination in the home environment (Enriquez, Enriquez, & Gerba, 1997; Finch, Prince, & Hawksworth, 1978; Scott & Bloomfield, 1990; Scott, Bloomfield, & Barlow, 1984; Rusin, Orosz-Coughlin, & Gerba, 1998). Cross-contamination of kitchen surfaces was found to be caused by contaminated eggshells (Humphrey, Martin & Whitehead, 1994). Infectious microorganisms originating from raw meat, poultry, fruits, and vegetables contaminate sinks, countertops, sponges, dishcloths, utensils, and draining and cutting boards. An examination of cellulose sponges and cotton dishcloths showed that they harbored pathogenic microorganisms such as Salmonella (15.4 percent of samples for sponges and 13.8 percent for dishcloths) and Staphylococcus aureus (20 percent of samples for sponges and 18.6 percent for dishcloths). Rusin and co-authors (1998) reported that sponges and dishcloths are the most problematic in terms of microbial contamination.

Sponges and dishcloths remain wet in the kitchen environment, a condition that helps in the survival of potential pathogens. Air drying cannot be relied upon to inactivate microorganisms (Scott & Bloomfield, 1990). Some investigators have suggested the use of hypochlorite solutions to disinfect sponges and dishcloths (Scott et al., 1984; Rusin et al., 1998), the use of self-disinfecting sponges (Enriquez et al., 1997).

The home health care industry has approximately 20,000 providers in the United States (up from 1,100 in 1963). It delivers home care services to about 7.6 million patients with chronic illnesses, disabilities, or terminal illnesses. In 2003, the cost of home care was around $38 billion (National Association for Home Care and Hospice, 2004). Home care patients use a wide range of medical devices (e.g., catheters, tracheotomy tubes, nasogastric tubes, syringes), which may become contaminated with pathogenic microorganisms. The pathogens form biofilms on the surface of medical devices and become resistant to antibacterial and antifungal drugs, thus leading to urinary-tract infections and other problems (Anwar & Costerton, 1992; Donlan, 2002). Medical instruments and equipment must be sterilized to protect the spread of pathogens from patient to patient or to healthy people. Also, home care patients generate infectious wastes that are disposed of in household garbage. This circumstance leads to an increase of the pathogen and parasite load in municipal solid waste, with potential for public health problems.

Microwave radiation falls within the radio-frequency band of the electromagnetic spectrum. Most studies on microwave radiation have primarily addressed the effect of radiation on microorganisms associated with food (Farber, Aoust, Diotte, Sewell, & Daley, 1998; Spite, 1984). Microbial suspensions were used in most of the studies dealing with microbial inactivation by microwave irradiation (e.g., Kakita et al., 1995; Wang, Hu, & Lin, 2003). The research reported here also dealt with microorganisms associated with surfaces, especially those adsorbed to plastic-syringe surfaces.

The purpose of the research was to determine the effects of microwave radiation produced by a household microwave oven on the survival of bacterial indicators, phage, and bacterial endospores in wastewater-contaminated kitchen sponges and scrubbing pads, as well as medical devices such as syringes used in the home environment.

Materials and Methods

Microwave Oven

Microwave irradiation was performed in a household microwave oven (Sharp, Model R-630D) with a rotating glass plate, a frequency of 2,450 MHz, and power of 1,100 watts. This microwave oven had 11 power levels ranging from 0 percent to 100 percent.

Thermometers

Two types of thermometers were used. A digital, Type K thermocouple thermometer (Omega, Model HH21) was used to measure temperatures inside liquids, kitchen sponges, and scrubbing pads. A noncontact infrared thermometer (Raytek[R]) was used to measure temperature on the surface of objects.

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Test Microorganisms

Total coliforms and E. coli were enumerated by the ColiPAD[TM] rapid detection method described by Bitton, Koopman, and Jung (1995). Briefly, following serial dilutions of the samples, a 1-mL aliquot of each dilution was added to 1 mL of 2X Lauryl Tryptose Broth-MUG (LTB-MUG). The tubes were incubated for 22 hours at 35[degrees]C. Following incubation, a 10-[micro]L aliquot from each tube was transferred to the ColiPAD assay pads. These pads contain two enzyme substrates, chlorophenol red-[beta]-D-galactopyranoside (CPRG) and 4-methylumbelliferone glucuronide (MUG), for the rapid detection of total coliforms and E. coli, respectively (Bitton et al., 1995). The pads were placed in locking empty petri dishes and incubated for 2 hours at 35[degrees]C. Purple spots on assay pads indicate the presence of total coliforms in samples. To detect E. coli, a 10-[micro]L drop of the alkaline buffer 2-amino-2-methyl-l-propanol (Sigma Diagnostics) was added to each spot. Fluorescence spots on assay pads indicate the presence of E. coli under a long-wave ultraviolet (UV) lamp in the dark. The method gave the most probable numbers of total coliforms and E. coli per 100 mL of sample.

Total bacteria colonies were detected on nutrient agar plates incubated for 48 hours at 30[degrees]C.

Bacillus cereus was used to determine the effect of microwave irradiation on bacterial spores in contaminated materials. The vegetative cells were routinely cultured in tryptic soy agar (or broth) (Difco Laboratories). Spores were produced by cultivation of the bacteria at 37[degrees]C in 1/10-strength Columbia broth (Difco Laboratories). They were separated from vegetative cells by preincubation in a water bath at 37[degrees]C followed by shaking of the spore suspension in a water bath at 60[degrees]C for 15 minutes. The spores were plated on nutrient agar supplemented with 0.1 percent soluble starch and 0.015 g/L of trypan blue, and incubated at 37[degrees]C for 24 hours (Nieminski, 2002; Nieminski, Bellamy, & Moss, 2000; Radziminski et al., 2002).

Bacterial endospores in wastewater from the University of Florida water reclamation plant were enumerated according to a slightly modified methodology proposed by Nieminski (2002).

MS2 phage was obtained from Dr. Sam Far-rah of the Department of Microbiology and Cell Science at the University of Florida. The host cell used was E. coli C3000. The phage was assayed according to the double-layer technique of Adams (1959).

Kitchen Sponges

The kitchen sponges were 3.17 x 3.37 x 0.69 cm when dry The average water-holding capacity of sponges was 64.3 g. To contaminate dry sponges, the authors soaked them in raw wastewater from the University of Florida water reclamation facility. Following exposure of the contaminated sponges to microwave radiation, wastewater was squeezed from the exposed sponges. Serial dilutions of the squeezed wastewater were made and plated on the appropriate media (see above) to enumerate total bacteria, total coliforms, E. coli, and bacterial endospores. A similar protocol was used for MS2 phage and B. cereus spores.

Control sponges were also soaked in wastewater but were not exposed to microwave radiation.

Kitchen Scrubbing Pads (Nonmetallic)

The scrubbing pads were 9.06 x 7.07 x 0.65 cm when dry. The size of the pads did not change significantly following soaking in liquids. The water-holding capacity of the pads was 10.94 g. The inactivation experiments were conducted according to the protocol described for kitchen sponges.

Syringes

The goal of these experiments was to simulate the fate of pathogenic and indicator microorganisms in contaminated syringes following exposure to microwave radiation. The microorganisms under study are those that remain attached to the syringe walls. To simulate microbial contamination, 10-mL sterile plastic syringes were filled with 10 mL of raw wastewater, which remained inside the syringe for 90 minutes. Afterwards, the syringes were emptied and exposed to microwave irradiation. The adsorbed microorganisms were desorbed from the syringe walls with 10 mL of an eluent that contained 3 percent beef extract solution and 0.1 percent Tween 80 (pH = 7). The eluent solution was in contact with the syringe walls for 90 minutes. The eluate was diluted and plated to determine the number of survivors according to the methods described above.

Statistical Analyses

Statistical Package for Social Science (SPSS) for Windows was used to determine the effects of microwave irradiation on various microorganisms. An [alpha]-level of.05 was used for each analysis to determine the statistical significance evaluated by paired t-test.

Results

Temperature Profile with Varying Microwave Power Level

100 mL of deionized water was placed in a 250-mL Pyrex beaker at the center of the rotating plate in the microwave oven and exposed to microwave radiation at different power levels for one minute. Figure 1 shows the temperature profile with increasing power level. The temperature increased in 1 minute from 23.2[degrees]C (room temperature) to 96.8[degrees]C at 100 percent power level. Kakita and co-authors (1995) reported a similar increase in water temperature (26[degrees]C to 90[degrees]C) following 1-minute irradiation.

The experiments discussed below were carried out at 100 percent power level.

Microbial Inactivation in Kitchen Sponges

The mean surface temperature of the sponges ranged from 79.4[degrees]C after 1 minute to 80.8[degrees]C at 4 minutes, and the mean inside temperature ranged from 91.4[degrees]C after 1 minute to 90.4[degrees]C after 4 minutes (Figure 2).

The extent to which different microorganisms in contaminated sponges were inactivated after microwave radiation is shown in Table 1. For total bacteria, inactivation was 99.1 [+ or -] 0.2 percent and 99. 5 [+ or -] 0.1 percent after 1- and 2-minute irradiation, respectively. The levels of total coliforms and E. coli in wastewater were 2.2 x [10.sup.6] per 100 mL and 9 x [10.sup.4] per 100 mL, respectively. Complete inactivation of these indicators was obtained following only 30 seconds of microwave treatment at 100 percent power level. Similarly, complete inactivation of phage MS2 was achieved following 1-minute microwave irradiation. The initial concentration of MS2 in the sponges was 2.8 x [10.sup.9] per mL.

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Kitchen sponges were soaked in a Bacillus cereus spore suspension. As expected, bacterial spores were hardier then the other microorganisms under study. A 2-minute microwave irradiation period was enough to inactivate 99.99 percent of bacterial spores, and no spore survived after 4 minutes of irradiation. Surface temperature of the sponges rose to 87.2[degrees]C after 4 minutes (Table 1).

Wastewater from the UF water reclamation facility was found to harbor 5.7 x [10.sup.4] bacterial spores per 100 mL. When exposed to microwave radiation, spore inactivation was 99.4 percent after 1 minute and 100 percent after 1.5 minutes (Table 1).

Microbial Inactivation in Kitchen Scrubbing Pads

Microbial inactivation in scrubbing pads showed the same pattern as microbial inactivation in sponges (Table 1). A 2-minute irradiation time resulted in a 4-log reduction in total bacteria, and no bacteria were detected after 4-minute irradiation. Complete inactivation of total coliforms and E. coli was achieved following 1-minute irradiation. The temperature inside the scrubbing pads increased to 84.2[degrees]C.

Inactivation of Microorganisms in Contaminated Syringes

Because of the rising costs of hospital care, home care of patients is becoming increasingly attractive. It was therefore important to study the effect of microwave irradiation on microorganisms associated with medical devices (e.g., syringes) that are often disposed of in the patient's home garbage. In syringes, the microbial pathogens are likely to adhere to the plastic and rubber surfaces.

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Experiments were undertaken to study the fate of syringe-associated microorganisms following microwave irradiation. Sterile, 10-cc syringes were filled with wastewater and then emptied following 90 minutes of contact time at room temperature. The empty syringes with attached microorganisms were then exposed to microwave irradiation.

It was observed that microorganisms attached to syringe surfaces were generally more resistant than those associated with kitchen sponges or scrubbing pads. After a 10-minute exposure time, the surface temperature of syringes exposed to microwave radiation climbed to around 120[degrees]C for the main polyethylene body and 130[degrees]C for the rubber end of the syringe piston (Figure 3). For total coliforms and E. coli, a 2-minute exposure to microwave radiation resulted in only 30.7 percent and 40.9 percent inactivation, respectively. No survivor organisms were detected after a 4-minute exposure, however (Figure 4, Figure 5). The surface temperature of syringes rose to 86.4[degrees]C in 4 minutes when syringes were exposed to microwave irradiation. It took 7 minutes, however, to achieve a 4-log reduction of total bacteria, and total inactivation was obtained only after 10-minute irradiation (Table 2). Exposure periods of 1 and 2 minutes resulted in 3- and 4-log inactivation of MS2 bacteriophage, respectively.

As shown for kitchen sponges and scrubbing pads, Bacillus cereus spores were hardier than vegetative bacteria. The average extent of inactivation was 86.5 [+ or -] 4.6 percent after 7-minute exposure and 97.4 [+ or -] 3.4 percent after 12-minute exposure. The surface temperature ranges after 7 minutes and 12 minutes were 86.4-105.2[degrees]C and 107.2-123.6[degrees]C, respectively.

Effect of Container Type on Inactivation of B. cereus Spores Adhering to Syringe Surfaces

Syringes contaminated with B. cereus spores were placed either in a glass container or in a ceramic bowl (i.e., a household clay bowl) and were microwaved for 5 minutes at the 100 percent setting. In the glass container, the surface temperature increased up to 76.8-89.2[degrees]C, whereas in the ceramic container, the surface temperature was always above 100[degrees]C (115.8-176.4[degrees]C). This greater increase in temperature was probably responsible for the increase in average inactivation from 84.1 percent to 95.8 percent (Figure 6).

Discussion and Conclusions

The study showed that total bacteria and indicator organisms are inactivated by microwave radiation within 1 to 2 minutes. A 5-log reduction of the viable count was also reported for E. coli suspension exposed to full power (600 watts) in 80 seconds (Woo, Rhee, & Park, 2000). Park and Cliver (1997) reported up to a 7-log reduction of pure cultures of E. coli in sponges subjected to microwave radiation (at 800 watts) within 30 to 60 seconds.

Sponges, pads, and dishcloths in kitchens retain moisture and are potential sources of pathogenic and opportunistic bacteria (Enriquez, Enriquez-Gordillo, Kennedy, & Gerba, 1997; Rusin et al., 1998). Moreover, Bottone, Perez, and Oeser (1994) have reported loofah sponges to be reservoirs and vehicles of Pseudomonas aeruginosa, which causes folliculitis. Sponges are generally left to dry on kitchen surfaces, but drying is not sufficient to significantly inactivate microbial pathogens, particularly microbial spores and protozoan cysts and oocysts (Scott & Bloomfield, 1990). Placing the contaminated sponges in a microwave oven for a few minutes will significantly inactivate potential pathogens.

The study reported here also showed that microorganisms attached to syringe surfaces were generally more resistant than those associated with kitchen sponges or scrubbing pads. Total inactivation of the syringe-associated bacteria was achieved only after an exposure period of 4 to 10 minutes. Similarly, a 2-minute exposure of phage MS2 did not result in total inactivation. Phage MS2 appears to be more resistant than Phage PL-1, for which a 5-log reduction was achieved in about 1 minute (Kakita et al., 1995). Thus, wastewater microorganisms in liquid-saturated kitchen sponges and scrubbing pads are more sensitive to microwave treatment than are those attached to syringe surfaces, which hold relatively little water. Attachment to the syringe surface appears to provide a protective effect.

[FIGURE 6 OMITTED]

As expected, the authors found that bacterial spores displayed a higher resistance to microwave radiation than did vegetative bacteria or bacterial phages. Because of their generally higher resistance to environmental stresses, it has been suggested that bacterial spores could be used as potential indicators for protozoan parasites such as Cryptosporidium (Radziminski et al., 2002). Protozoan cysts and oocysts are generally more resistant than vegetative bacteria or even viruses to disinfection and adverse environmental factors (Bitton, 2005). Moreover, there are differences in the resistance of Bacillus spores (B. licheniformis, B. cereus, B. coagulans, and B. subtilis) to microwave radiation, with B. licheniformis spores having the highest resistance (Wang et al., 2003).

Finally, the study reported here showed that the type of container used may have some effect on microbial inactivation in syringes. A higher degree of inactivation was observed when the contaminated syringes were placed in a clay container. An effect of container type on microbial inactivation by microwave radiation was also reported by Wang and co-authors (2003), who showed that the resistance of microbial spores to microwave radiation was lower in glass containers than in polypropylene containers or aluminum foil-enclosed pouches. These preliminary data suggest the possibility that open or closed ceramic containers could be used to increase the inactivation of microorganisms subjected to microwave radiation.

Microwave ovens have become common household appliances in developed countries and, to some extent, in developing countries. This relatively inexpensive technology is commonly used to cook or warm foods in homes, offices, and some restaurants. With respect to consumer safety, the research reported here shows that microwave radiation can also be used to control (to reduce or sometimes to completely eliminate) microbial potential pathogens in the kitchen environment by irradiation of sponges and scrubbing pads for a few minutes. Reports are conflicting as to whether microbial inactivation is due to heating or to the microwave energy itself (Salvatorelli, Marchetti, Betti, Rosaspina, & Finzi, 1996). Celandroni and co-authors (2004), however, attributed the killing of Bacillus subtilis spores to both causes. Microwave radiation was found to lead to the release of DNA and proteins but not to cause cell lysing (Woo et al., 2000).

Homebound patients can also use this technology to reduce microbial contamination by exposing nonmetallic medical wastes to microwave radiation. Examples of the successful application of this technology are its use in treatment of infectious hospital wastes, which are commonly subjected to incineration or irradiation with ultraviolet or [gamma]-rays (Pellerin, 1994; Tata & Beone, 1995); industrial food processing (Aziz, Mahrous, & Youssef, 2002); decontamination of mildew-infected cotton (Chen, 2001), towels (Tanaka et al., 1998), corks in wine bottling (Murphy & Curran, 2000), and contact lenses (Crabbe & Thompson, 2001; Hiti, Walochnik, Faschinger, Haller-Schober, & Aspock, 2001); dentures cleaning (Goodson, Glass, Bullard, & Conrad, 2003); air decontamination (Hamid et al., 2001); and pyrolysis of wastewater biosolids (Menendez, Inguanzo, & Pis, 2002).

In conclusion, consumers and homebound patients can use microwave ovens to significantly reduce microbial pathogens in the home environment. Microwave irradiation is a cost-effective, practical, fast, easy, and safe method of disinfecting household and medical devices.

Acknowledgements: The authors thank the University of Florida Research Foundation for funding this research.

Corresponding Author: Gabriel Bitton, Ph.D., Professor, University of Florida, Department of Environmental Engineering Sciences, Black Hall, Gainesville, FL 32611. E-mail:[e-mail].

REFERENCES

Adams, M.H. (1959). Bacteriophages. New York: Wiley-Interscience.

Altekruse, S.F., Cohen, M.L., & Swerklow, M.D. (1997). Emerging foodborne diseases. Emerging Infectious Diseases, 3(3), 285-293.

Anwar, H., & Costerton, J.W. (1992). Effective use of antibiotics in the treatment of biofilm-associated infections. American Society of Microbiology News, 58, 665-668.

Aziz, N.H., Mahrous, S.R., & Youssef, B.M. (2002). Effect of gammaray and microwave treatment on the shelf-life of beef products stored at 5[degrees]C. Food Control, 13, 437 444.

Bitton, G. (2005). Wastewater microbiology (3rd. ed.). Hoboken, NJ: Wiley-Liss.

Bitton, G., Koopman, B., & Jung, K. (1995). An assay for the enumeration of total coliforms and Escherichia coli in water and wastewater. Water Environment Research, 67, 906-909.

Bottone, E. J., Perez II, A.A., & Oeser, J.L. (1994). Loofah sponges as reservoirs and vehicles in the transmission of potentially pathogenic bacterial species to human skin. Journal of Clinical Microbiology, 32, 469-472.

Celandroni, F., Longo, I., Tosoratti, N., Giannessi, F., Ghelardi, E., Salvetti, S., Baggiani, A., & Senesi, S. (2004). Effect of microwave radiation on Bacillus subtilis spores. Journal of Applied Microbiology, 97, 1220-1227.

Chen, H. L. (2001). Microwave radiation decontamination of mildew infected cotton. Textile Research Journal, 71(3), 247-254.

Crabbe, A., & Thompson, P. (2001). Effects of microwave irradiation on the parameters of hydrogel contact lenses. Optometry and Vision Science, 78, 610-615.

Donlan, R.M. (2002). Biofilms: Microbial life on surfaces. Emerging Infectious Diseases, 8, 880-890.

Enriquez, C.E., Enriquez-Gordillo, R., Kennedy, D.I., & Gerba, C.P. (1997) Bacteriological survey of used cellulose sponges and cotton dishcloths from domestic kitchens. Dairy, Food, and Environmental Sanitation, 17(1), 20-24.

Enriquez, C.E., Enriquez, V.E., & Gerba, C.P. (1997). Reduction of bacterial contamination in the household environment through the use of self-disinfecting sponges. Dairy, Food, and Environmental Sanitation, 17(9), 550-554.

Farber, J.M., Aoust, J.Y.D., Diotte, M., Sewell, A., & Daley, E. (1998). Survival of Listeria spp. on raw whole chickens cooked in microwave ovens. Journal of Food Protection, 61, 1465-1469. Finch, J. E., Prince, J., & Hawksworth, M. (1978). A bacteriological survey of the domestic environment. Journal of Applied Bacteriology, 45, 357-364.

Goodson, L.B., Glass, R.T., Bullard, J.W., & Conrad, R.S. (2003). A statistical comparison of denture sanitation using a commercially available denture cleaner with and without microwaving. General Dentistry, 51,148-151.

Hamid, M., Thomas,T., El-Saba, A., Stapleton, W., Sakla, A., Rahman, A., Byrne, P. VanLandingham, D., & McCombs, C. (2001). The effects of microwaves on airborne microorganisms. Journal of Microwave Power & Electromagnetic Energy, 36(1), 7-45.

Hiti, K., Walochnik, J., Faschinger, C., Haller-Schober, E.M., & Aspock, H. (2001). Microwave treatment of contact lens cases contaminated with Acanthamoeba. Cornea, 20, 467-470.

Humphrey, T.J., Martin, K.W., & Whitehead, A. (1994). Contamination of hands and work surfaces with Salmonella enteritidis PT4 during preparation of egg dishes. Epidemioliogy & Infection, 113, 403-409.

Kakita, Y., Kashige, N., Murata, K., Kuroiwa, A., Funatsu, M., & Watanabe, K. (1995). Inactivation of Lactobacillus bacteriophage PL-1 by microwave irradiation. Microbiolology and Immunology, 39 (8), 571-576.

Menendez, J. A., Inguanzo, M., & Pis, J.J. (2002). Microwave-induced pyrolysis of sewage sludge. Water Research, 36, 3261-3264.

Murphy, M., & Curran, C. (2000, July 10). Microwaves stop corked wine. Chemistry & Industry, 13, 424.

National Association for Home Care and Hospices. (2004). Basic statistics about home care. Washington, DC: Author. Retrieved August 9, 2006, from http://www.nahc.org/04HC_Stats.pdf.

Nieminski, E.C. (2002). Aerobic endospores. In G. Bitton (Ed.), Encyclopedia of environmental microbiology (pp. 140-148). Hoboken, NJ: Wiley-Intescience.

Nieminski, E.C., Bellamy, W.D., & Moss, L.R. (2000). Using surrogates to improve plant performance. Journal of American Water Works Association, 92(3), 67-78.

Park, P.K., & Cliver, D.O. (1997). Disinfection of kitchen sponges and dishcloths by microwave oven. Dairy, Food, & Environmental Sanitation, 17(3), 146-149.

Pellerin, C. (1994). Alternatives to incineration: There's more than one way to remediate. Environmental Health Perspectives, 102, 840 845.

Radziminski, C., Ballantyne, L., Hodson, J., Creason, R., Andrews, R.C., & Chauret, C. (2002). Disinfection of Bacillus subtilis spores with chlorine dioxide: A bench-scale and pilot-scale study. Water Research, 36, 1629-1639.

Roberts, D. (1982). Factors contributing to outbreaks of food poisoning in England and Wales: 1970-1979. Journal of Hygiene, 89, 491-498.

Rusin, P., Orosz-Coughlin, P., & Gerba, C.P. (1998). Reduction of faecal coliform, coliform and heterotrophic plate count bacteria in the household kitchen and bathroom by disinfection with hypochlorite cleaners. Journal of Applied Microbiology, 85, 819-828.

Salvatorelli, G., Marchetti, M.G., Betti, V., Rosaspina, S., & Finzi, G. (1996). Comparison of the effects of microwave radiation and conventional heating on Bacillus subtilis spores. Microbios, 87(2), 169-174.

Scott, E., & Bloomfield, S.F. (1990). The survival and transfer of microbial contamination via cloths, hands and utensils. Journal of Applied Bacteriology, 68(3), 271-278.

Scott, E., Bloomfield, S.F., & Barlow, C.G. (1984). Evaluation of disinfectants in the domestic environment under "in use" conditions. Journal of Hygiene, 92(2), 193-203.

Spite, G.T. (1984). Microwave-inactivation of bacterial pathogens in various controlled frozen food compositions and in a commercially available frozen food product. Journal of Food Protection, 47, 458-462.

Tata, A., & Beone, F. (1995). Hospital waste sterilization: A technical and economic comparison between radiation and microwave treatments. Radiation Physics & Chemistry, 46, 1153-1157.

Tanaka, Y., Fujiwara, S., Kataoka, D., Takagaki, T., Takano, S., Honda, S., Katayose, M., Kinosita, Y., & Toyoshima, Y. (1998). Warming and sterilizing towels by microwave irradiation. Yonago Acta Medica, 41(2), 83-88.

Wang, J.C., Hu, S.H., & Lin, C.Y. (2003). Lethal effect of microwaves on spores of Bacillus spp. Journal of Food Protection, 66, 604-609.

Woo, I.S., Rhee, I.K., & Park, H.D. (2000). Differential damage in bacterial cells by microwave radiation on the basis of cell wall structure. Applied & Environmental Microbiology, 66, 2243-2247.

Dong-Kyoo Park

Gabriel Bitton, Ph.D.

Richard Melker, M.D., Ph.D.

TABLE 1 Effect of Microwave Radiation* on Microbial Inactivation in
Kitchen Sponges and Scrubbing Pads Contaminated with Wastewater

Microbial Category (Initial
Concentration/100mL)              Exposure Time  % Inactivation

Sponges
Total bacteria (3.5x[10.sup.7])   1 min          99.1 +/- 0.23
                                  2 min          99.5 +/- 0.15
Total coliform (2.2x[10.sup.6])   30s            Total inactivation
E.coli (9.0x[10.sup.4])           30s            Total inactivation
Phage MS2 (2.8x[10.sup.11])       1 min          Total inactivation
                                  2 min          99.99 +/- 0.05
Bacillus cereus spores            4 min          Total inactivation
  (5.8x[10.sup.7])
Wastewater bacterial spores       1 min          99.99 +/- 0.005
  (5.7x[10.sup.4])                1.5 min        Total inactivation

Scrubbing pads
Total bacteria (1.6 x[10.sup.8])  2 min          99.99 +/- 0.01
Total coliforms (9.0x[10.sup.6])  1 min          Total inactivation
E. coli (2.2x[10.sup.6])          1 min          Total inactivation

*Microwave oven operated at 100% power level (1000 w).

TABLE 2 Effect of Microwave Radiation* on Inactivation of Microorganisms
Attached to Plastic Syringes Contaminated with Wastewater

Microbial Category (Initial        Exposure Time
Concentration/100mL)               (min)          % Inactivation

Total bacteria (1.1x[10.sup.8])     7             99.95 +/- 0.05
                                   10             Total inactivation
Total coliforms (1.3x[10.sup.6])    2             40.9
                                    4             Total inactivation
E. coli (2.2x[10.sup.5])            2             30.7
                                    4             Total inactivation
MS2 phage (5.1x[10.sup.10])         1             99.90
                                    2             99.99
B. cereus spores (1.2x[10.sup.7])   7             86.5 +/- 4.6
                                   12             97.4 +/- 3.4

*Microwave oven operated at 100% power level (1000 w).