2.1. Bacterial strain and preparation
Pathogenic V. parahaemolyticus strain BCC 24339 with the
thermostable direct hemolysin (tdhþ/trhe) genewas used in this
study. This bacterium was isolated from prawn (Litopenaeus
vannamei) (Vandamei) by the Faculty of Public Health, Mahidol
University, Thailand. The prawn was cultivated in Thailand.
Virulence gene tdh and pandemic clonal marker orf8 were
confirmed positive by a multiplex PCR method. The isolated
V. parahaemolyticus cells were maintained at 80 C at
a concentration of 109e1010 cfu/ml in Tryptic soy broth (TSB;
Merck, Darmstadt, Germany) with 3% NaCl (w/v) and 20% glycerol
(Sigma Chemical Company, St. Louis, Mo, USA). For each
experiment, 10 ml of thawed stock culture was inoculated into
a 25 ml Erlenmeyer flask containing 9 ml of sterile TSB with 3%
NaCl (w/v) and incubated at 37 C for 20e24 h at 140 rpm on
a rotary shaker (SK-300, Jeio Tech, Korea). The start culture was
re-cultured with the same procedure for 18 h prior to use. One
ml of the start culture was then transferred into 9 ml of phosphate
buffered saline (PBS, pH 7.4). The phosphate buffered
saline was made of 285 mmol/L NaH2PO4 with 3% NaCl (w/v) and
adjusted to pH 7.4 with NaOH. This cellular suspension was
diluted with the phosphate buffered saline prior to inoculate
into prawn.