Characterization and virulence retention of viable but nonculturable Vibrio harveyi

Vibrio harveyi has been reported to enter into a viable but nonculturable (VBNC) state. Two clinical V. harveyi strains, SF1 and CW2, and the type strain, VIB295(T), were incubated in sterilized seawater at 4 degrees C. Plate counts in these strains declined to undetectable levels (<0.1 CFU mL(-1)) within 69, 67 and 65 days, respectively. The direct viable count (DVC) declined from 10(6) to 10(4) active cells mL(-1) and remained constant at this level by a DVC. VBNC cell numbers could be restored via a temperature upshift when grown in yeast extract with the addition of Tween 20 or compound vitamin B. Reverse transcriptase-PCR was used to monitor virulence gene expression within VBNC cells. No expression of the hemolysin gene was detected in VBNC cells. VBNC and resuscitative cells were intraperitoneally injected into zebra fish separately. No death was observed in the groups inoculated with VBNC cells. The fish inoculated with the resuscitative cells died within 7 days, the lethal dose 50% (LD(50)) being 2.85 x 10(4) CFU mL(-1), a value similar to that for groups inoculated with normal cells (2.28 x 10(4) CFU mL(-1)). This suggested that VBNC V. harveyi might retain pathogenic potential.