Prevalence of bacterial contamination in uncooked sticky rice samples from markets in Japan

Abstract

We investigated the prevalence of bacterial contamination in uncooked sticky rice samples obtained from markets
in Japan. Between June and August, 2019, a total of 25 sticky rice samples were purchased from shops in the Tokyo
area. Twenty-two (88.0%) and 10 (40.0%) of 25 sticky rice samples harbored 3.06±0.54 log spc/g of standard plates
counts and 2.17±0.23 log cfu/g of enterobacteriaceae bacteria counts, respectively. Three species of Enterobacteriaceae,
Pantoea dispersa, P. ananati, and Kosakonia cowanii, were identified by MALDI-TOFMS-based test. Three (12.0%) of 25
sticky rice samples harbored 2.00 log cfu/g of Bacillus species. Bacillus spp. was not isolated after heat treatment (98 °C
for 20 min.) of sample homogenates. Two isolates were identified B. cereus by both API 50 CH and MALDI-TOFMS
based test. However, one isolate was B. mycoides by API 50 CH, but B. cereus by MALDI-TOF MS based test. These 3
Bacillus strains harbored no gene encoding the enzyme responsible for cereulide synthesis and had not produced
enterotoxin. Our study suggested that some uncooked sticky rice has Enterobacteriaceae bacteria and Bacillus spp.
contamination, however there is no isolate from heat treatment samples in this study. Isolated P. dispersa, P. ananati,
and K. cowanii, are usually in the environment and these organisms have been isolated from patients with opportunistic
infections. Preventing cross-contamination, from sticky rice to cooked food may be important during preparation and
storage in kitchen.