Survival of Flexibacter sp. strain F-2, the Causative Bacterium of Spotting Disease of Sea Urchin Strongylocentrotus intermedius at Low Temperatures

Abstract

Flexibacter sp. represented by strain F-2, the causative bacterium of spotting disease of sea urchin Strongylocentrotus intermedius, was detected in various samples from mid July to mid August when seawater temperature reached 20°C or more, but the bacterium was not detected in any samples from winter to early summer. From this phenomenon it is suspected that this bacterium enters into a viable but nonculturable (VBNC) state during low-temperature seasons. To obtain fundamental data of survival of this bacterium at low temperatures, we examined some factors for entering the VBNC state. Direct viable count (DVC) was also carried out. The bacterial cells in the mid-log phase quickly became nonculturable at 5°C in artificial seawater (ASW). No decrease in total counts or DVC was observed during storage, indicating that the bacterium entered the VBNC state. During storage periods, a morphological change from long rod cells to short rods or spheroid cells was observed by scanning electron microscopy. Cells in the VBNC state lost the pathogenicity to sea urchin.