To the Editor: A large outbreak of Escherichia coli O157:H7 with more than 6,000 cases occurred in Sakai City, Osaka Prefecture, Japan, in July 1996 (1); after the outbreak, more than 1,000 secondary infections occurred in the families of the patients (2). We studied the resistance of E. coli O157:H7 to dryness because the survival on surfaces of inert materials under dry conditions may be related to the transmissibility of the strains.

E. coli O157:H7 strains grown on 3.0% nutrient broth with 1.5% agar for 20 to 22 hours at 37°C were suspended at a concentration of approximately 5 x 10⁸ cfu/ml in a 10% skim milk, 0.5% NaCl solution. Aliquots (10 µl) of bacterial suspensions were spread to approximately 10 cm² on plastic petri plates for bacterial culture and dried under the air flow of a clean bench until no aliquots were evident. After storage in the dark at room temperature, bacteria were harvested with saline and gauze and the viable number was counted (n = 3).

The log reductions 12 hours after drying were employed to show the resistance levels of E. coli O157:H7 to dry stress. The log reductions of the three strains from the Sakai City outbreak (RIMD0509950, RIMD0509894, and RIMD0509951) were 0.04 ± 0.34, 0.14 ± 0.06, and 0.20 ± 0.60, respectively (mean = 0.13), whereas those of the E. coli O157:H7 strains from the other cases varied from 0.71 ± 0.18 to 4.57 ± 1.02. (RIMD0509861, 0.71 ± 0.18; ATCC35150, 1.15 ± 0.35; RIMD0509826, 1.23 ± 0.60; RIMD0509742, 1.28 ± 0.18; ATCC43890, 1.31 ± 0.17; RIMD0509933, 1.48 ± 0.19; RIMD0509932, 1.48 ± 0.22; RIMD0509765, 3.60 ± 0.24; RIMD0509764, 4.57 ± 1.02; mean = 1.87). Although the strain from the Sakai City outbreak (RIMD0509950) survived for at least 35 days under the dry conditions, the strains from the other cases had no viable cells after 7 days. The log reductions of E. coli JM109 and DH10B strains were more than 10, and no viable cells were detected in 12 hours.

The strains from the Sakai City outbreak also showed marked acid resistance. Acid resistance of E. coli O157:H7 has been reported to depend on rpoS, which is induced in a stationary phase (3). Since the E. coli O157:H7 strains in a stationary phase were more resistant to dry and acid stresses than those in a log phase, rpoS may also be associated with resistance to dry stress. However, no deletions of rpoS were detected by polymerase chain reaction analysis in the E. coli O157:H7 strains used in this experiment. Further study on the mechanism of resistance will be needed to establish new strategies for eradicating the bacteria.

A case-control study by the Ministry of Health and Welfare of Japan showed that uncooked radish sprouts were the vehicle of the largest outbreak in Sakai City (4). In two small outbreaks of E. coli O157:H7 in March 1997, the vehicle of infection might have been radish sprouts; therefore, the possible contamination of white radish seeds with E. coli O157:H7 has been discussed (5). If such contamination was present, dry resistance might be involved in the survival on or in white radish seeds because the bacteria were exposed to dry conditions for a long period before sprouting. We propose that dry resistance be considered an important factor in infection.

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