Yonago Acta medica 1998;41:83–88

Warming and Sterilizing Towels by Microwave Irradiation
Yoshinori Tanaka, Shoko Fujiwara, Daisuke Kataoka, Tomonobu Takagaki,
Shuichi Takano, Shoko Honda, Michiko Katayose, Yusuke Kinosita and Yuko
Toyoshima
Department of Bacteriology, Faculty of Medicine, Tottori University, Yonago 683-0826, JapanA steamed and sterilized towel was easily obtained using a home microwave oven. A
small piece of gauze (7 × 7 cm) containing approximately 5 × 108 organisms in 1 mL ofsaline was rolled up in the moist towel which was wrapped in heat-resistant vinylidene
polychloride film (wrap film). Staphylococcus aureus, Pseudomonas aeruginosa andCandida albicans in the towel were completely killed by microwave irradiation for 1
min. When three towels were irradiated simultaneously, it took 2 min for complete
killing of the bacteria. In another experiment, the small gauze containing the bacterial
suspension was set in a sterile plastic dish and completely dried using filtered air for aperiod of 3 h. After a 5-min irradiation in a microwave oven, the survival rate of S.
aureus was 17%. These results indicate that a sterile steamed towel can be obtained by
microwave irradiation of a moist towel wrapped in wrap film.
Key words: Candida albicans; microwave irradiation; Pseudomonas aeruginosa; Staphylococcus
aureus; sterilization

activities of caregivers is the wiping of patients’bodies which requires sterile, comfortably
warm towels. In this study, we sterilized towels
by microwave irradiation. The opportunistic
pathogens, S. aureus, P. aeruginosa and C.
albicans, were seeded into the towels, whichwere then subjected to microwave irradiation.
Then, bacteria were cultured in broth or on agar
plates to confirm the survival rate. We compared the survival rate of bacteria contained in asmall moist gauze with that of bacteria attached
to a small dry gauze, following microwave irradiation. The degree of moistness of the towel
is discussed.

Dielectric heating by microwave irradiation iswell known to have an effect on killing microorganisms (Lund, 1975; Metaxas and Meredith,1988). Microwave irradiation is used for thesterilization of food products (Chipley, 1980;
Fung and Cunningham, 1980; Cross and Fung,
1982; Rosenberg and Bögl, 1987), and medical
appliances and materials, such as acrylic resin
dentures (Polyzois et al., 1982; Thomas andWebb, 1995), scalpel blades (Rosaspina et al.,1994a, 1994b), polyethylene catheters (Griffith
et al., 1993) and plastic tissue culture vessels(Sanborn et al., 1982). The polar compounds inthe materials, when exposed to microwave irradiation, generate heat by the collision and friction between molecules. Most microorganisms
are killed by the heat generated. Thus, it is important that water is present in the irradiatedmaterials. However, the possibility of nonthermal microwave effects has been proposed (Frölich, 1975; Cope, 1976; Pickard andRosenbaum, 1978).
In the aging society of Japan, it has becomeof increasing importance to care for aged
persons or bedridden patients. One of theMaterials and Methods

Bacterial strains
The bacterial strains used were Staphylococcus
aureus IFO12732, Pseudomonas aeruginosa
B17 and Candida albicans JCM1542. All areopportunistic pathogens and were stored in ourlaboratory. S. aureus and P. aeruginosa were83

Y. Tanaka et al.

Table 1. Decreased moistness of a towel subjected to microwave irradiation
Towel

Half-sized towel

Amount ofwater added*
(mL)

1

2

0
30

S
W

S
S

Microwave irradiation (min)
3
4
5
D
S

NT
S

NT
S

6
NT
S

Regular-sized face towel

0
S
S
S
S
S
S
50
NT
NT
NT
W
S
S
*A towel was dampened with running water and wrung tightly. Then, a fixed amount of water was added tothe towel.
D, relatively dry; NT, not tested; S, suitable for wiping the body; W, dripping wet.

cultured on nutrient agar slants, and C. albicans
on Sabouraud agar slants.

Enumeration of viable cells
After microwave irradiation, the gauze wastransferred from the towel or from the dish intobroth (or Sabouraud broth), and rinsed. An
aliquot of the broth was serially diluted withsaline, and viable cells were enumerated bycolony counting. The remainder of the broth
was incubated at 37°C without further treatment.

Preparation of bacterial suspensions
Each bacterium in suitable broth medium wascultured overnight at 37°C with shaking. The
bacterial culture was transferred to a sterilized
test tube, and centrifuged at 3,000 rpm for 10
min. The precipitate was resuspended in saline
at a concentration of approximately 5 × 108/mL.
In the case of C. albicans, the concentration wasapproximately 5 × 106/mL.

Results

Microwave irradiation of towels
Degree of moistness of towel before andafter microwave irradiation

One milliliter of the bacterial suspension was
pipetted onto a small sterilized gauze (7 × 7cm), which was rolled up in a moist towel or ahalf-sized towel. The towel was wrapped inheat-resistant vinylidene polychloride film
(wrap film; Saran Wrap, Asahi Kasei Kogyo
Co., Ltd., Tokyo, Japan). When three towels
were used simultaneously, each towel was
wrapped in wrap film, and the three wrapped
towels were put side by side on the microwave
oven dish. Microwave irradiation (500 W of output power; 2,450 MHz) was performed using a
microwave oven (Corona Co., Ltd., Tokyo).
In another experiment, a small gauze containing 1 mL of the bacterial suspension was set
in a sterile plastic dish and dried in filtered air
for 3 h in a safety cabinet. The dried gauze inthe plastic dish was irradiated in the microwave
oven.

A regular-sized face towel and a half-sized
towel were dampened with running water andwrung tightly. The towels were wrapped in
wrap film, irradiated at regular intervals, andtested to determine whether they were suitablefor wiping the body of an aged person.
In the case of the half-sized towel, the addition of water to the wrung towel was not necessary when a 2-min irradiation was used, but 30
mL of water had to be added for irradiation of 2
min or more (Table 1). The regular-sized towelrequired no addition of water when 5-min irradiation was used. Thus, in subsequent experiments, no water was added to the half-sized
towel for a 2-min microwave irradiation, or to
the regular-sized towel for a 3-min irradiation.

84

Microwave-irradiated towels

Table 2. Bacterial growth in broth or Sabouraud broth after microwave irradiation
Towel

Duration of
microwave irradiation(min)

S. aureus

Bacterial growth*
P. aeruginosa C. albicans

Half-sized towel

0
1
2

+
–
–

+
–
–

+
–
–

Regular-sized face towel

0
1
2

+
–
–

+
–
–

+
–
–

Three face towels usedsimultaneously

0
+
+
+
1
+
+
+
2
–
–
–
3
–
–
–
*S. aureus and P. aeruginosa were grown in broth and C. albicans was grown in Sabouraud broth.
+, growth was observed; –, no growth was observed.

after rolling up in the moist towel was about
35% in two separate experiments (data notshown). When the half-sized or the regularsized single towel was irradiated for 1 min, nobacteria were recovered from the gauze (Table
2 and Fig. 1). All three types of towels were
hot, and of a suitable degree of moistness for
wiping the body.

Survival rate of bacteria in the moistened
towel following microwave irradiation
A bacterial suspension was pipetted onto asmall piece of gauze which was then rolled up
in a towel and subjected to microwave irradiation. Recovery rate of bacteria in the gauze
(log)
0

(log)
0

(log)
0

Survival rate

A

B

–2

C

–2
–2

–4

–4

–6

–6

–4

0

1

2

Time

3

(min)

0

1

2

Time

3

(min) 0

1

2

3

(min)

Time

Fig. 1. Lethal effects of microwave irradiation on bacteria in a moist towel. Approximately 5 × 10 8organisms (or 5 × 106 organisms in the case of C. albicans) in saline were pipetted onto a small gauze, rolled
up in a half-sized or regular-sized towel, and then wrapped in wrap film. A half-sized towel ( ), a regularsized towel ( ) and three towels simultaneously ( ) were irradiated in a domestic microwave oven. A: S.
aureus; B: P. aeruginosa; C: C. albicans. Colony counting was performed as described in Materials andMethods. Recovery of bacteria in the gauze after rolling up in the moist towel was about 35% (in two
separate experiments). The symbols on the transverse axis show that no bacterial growth was observed.

85

Y. Tanaka et al.

creased as the irradiation time increased. However, S. aureus survived even after a 5-min irradiation (survival rate, 17%). In contrast, S.
aureus on the surface of the moist gauze waskilled by 1-min irradiation. In another experiment, an aliquot of an S. aureus suspension wasplaced in a sterile plastic dish, dried up in filtered air and irradiated for 5 min. Then, 5 mL ofbroth was added to the plate, mixed well andtransferred to a test tube for incubation overnight at 37°C. The results indicated that viableorganisms could be observed even after a 5-min
irradiation (data not shown).
The above data and Fig.2 show that thebacteria in the dried condition were not completely killed by microwave irradiation. In bothof the cases mentioned above, the plastic dishes, if not moistened, were not hot after a 5-min
irradiation.

When the S. aureus-containing towel was irradiated together with two other towels for 1
min, the towels were warm but not hot, and thesurvival rate of S. aureus was 1.6%. However,no bacteria survived after a 2-min irradiation.
The survival rates of P. aeruginosa and C.
albicans in three towels after 1-min irradiation
were 33% and 11%, respectively (Fig. 1). The
bacteria were completely killed by a 2-min
microwave irradiation. Thus, the survival rates
of bacteria were dependent on the towel volume. For the effective killing of bacteria alonger microwave irradiation is required as thevolume of towels increases.

Survival rate of bacteria in the dried gauze
A small gauze containing an S. aureus suspension was dried up in a plastic dish in filtersterilized air for 3 h. After microwave irradiation at regular intervals, the gauze was transferred into 5 mL of broth followed by serial
dilution and colony counting. As shown in Fig.
2, the survival rate of S. aureus gradually de-Discussion
In addition to the thermal effects of microwave
irradiation (Coote et al., 1991; Diaz-Cinco andMartinelli, 1991; Fujikawa et al., 1991; Welt etal., 1994), nonthermal effects on inactivation of
microorganisms have also been reported (Frölich,
1975; Cope, 1976; Wayland et al., 1977). Kakita
and colleagues (1995) analyzed the DNA frommicrowave-irradiated Lactobacillus phage PL1 by agarose gel electrophoresis, and showedthat the phage DNA chain was broken by microwave irradiation. However, they claimed thatinactivation of phage PL-1 was due to the heatproduced by microwave irradiation.
In this study, a moist towel was wrapped in
wrap film, and the heat generated by microwave
irradiation effectively killed the microorganisms present in the wrapped towel (Table 2 andFig. 1). The temperature within the wrapped
towel may reach close to 100°C. Thus, it is conceivable that spores of Bacillus and Clostridium
would not be effectively killed by microwave
irradiation in our system. Heating by autoclaving (121°C) may be required for the complete killing of spores. When the dry gauze
containing bacteria was irradiated for 5 min, thebacteria were not completely killed (Fig. 2). It

1.0
0.8

Survival rate

0.6

0.4

0.2

0.1

0

1

2

3

4

5 (min)Time
Fig. 2. Survival rate of S. aureus in the dried gauze.
Approximately 5 × 108 S. aureus cells in saline were
pipetted onto a small gauze which was set in a sterile
plastic dish. The gauze was dried ( ) with filtered
air for 3 h in a safety cabinet, or not dried ( ). Microwave irradiation and colony counting were carried out as described in Materials and Methods.
The symbols on the transverse axis show that nobacterial growth was observed. The data are typicalin the three separate experiments.

86Microwave-irradiated towels

is considered that sufficient heat was not generated in the dehydrated gauze by microwave
irradiation or that the heat generated may havediffused in the air. However, when a dry towel
was irradiated with or without wrapping for 2 to
3 min, it smoked. These findings indicate that
towels should be moistened and wrapped forthe effective inactivation of microorganisms by
microwave irradiation.
Recently, it was reported that many medical
instruments, such as catheters (Griffith et al.,1993), acrylic resin dentures (Polyzois, 1995;
Thomas and Webb, 1995), stainless-steel
scalpel blades (Rosaspina et al., 1994a, 1994b)
and plastic tissue culture vessels (Sanborn et al.,1982), are sterilized by microwave irradiation.
Our data on the killing of bacteria in towels
showed that the dielectric heating of towels by
microwave irradiation generates towels suitable
for the care of aged persons and bedridden patients. The heating and sterilization of a towel
using a domestic microwave oven was achieved
using a 1-min irradiation, and a 2-min irradiation when three towels were used simultaneously. Thus, at home and in small hospitals,
it is easy to prepare sanitary towels using domestic microwave ovens.
Due to the aging society in Japan, the importance of the care of bedridden patients is gradually increasing. Caregivers should protect
aged persons against infection by opportunistic
pathogens. There are many opportunistic pathogens present in the environment, such as enterobacteria (Escherichia coli, Klebsiella pneumoniae, Serratia, Enterobacter, Proteus), gramnegative bacilli (Pseudomonas, Acinetobacter),
Staphylococcus, fungi (Candida albicans, Aspergillus), protozoa (Pneumocystis carinii) andviruses (cytomegalovirus, human herpes virus1 and -2). We tried to inactivate opportunistic
pathogens and prepare sterile, comfortably hot
towels using a domestic microwave oven.
Methicillin-resistant S. aureus (MRSA) has
become a major factor in nosocomial infection,and is, together with P. aeruginosa, resistant tomany antibiotics (Locksley et al., 1982). It isthus reasonable to use sterilized, comfortably
hot towels for aged persons and bedridden patients to protect them from decubitus and chron-ic infection. Our finding that S. aureus, P. aeruginosa and C. albicans when present in a moist
towel covered with wrap film, were killed by a1- or 2-min microwave irradiation, indicatesthat towels prepared in this manner are sufficiently sanitary to be used for care of aged
persons.
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(Received April 20, Accepted May 21, 1998)

88