EFFECT OF SODIUM NIFURSTYRENATE ON THE REDUCTION OF BACTERIAL-CONTAMINATION OF ROTIFERS (BRACHIONUS-PLICATILIS)

Although sodium nifurstyrenate (NFS-Na) has been shown effective in suppressing bacterial contamination of live-food rotifers (Brachionus plicatilis), the earlier experiment dealt with a density of live food lower than used in a commercial marine hatchery. This study attempted to assess the effectiveness of NFS-Na with rotifers at greater densities. The drug was also tested with rotifers artificially contaminated with a bacterial pathogen ( Vibrio sp. INFL) of Japanese flounder larvae (Paralichthys olivaceus). The viable bacterial counts on ZoBell's 2216e apr inoculated with samples drawn from the treated rotifers at densities of 1610, 8060 and 16 100 individuals/ml were slightly lower than the counts of untreated rotifers. There was no difference between samples treated with 5 or 10-mu-g/ml NFS-Na. On the other hand, the counts on TCBS apr at the three densities of rotifers showed a marked downward trend from 2.6 X 10(6) CFU/g to 2.9 X 10(4), 2.0 X 10(4) and 4.0 X 10(3) CFU/g after exposure to 5-mu-g/ml, and to 4.0 X 10(3), 4.0 X 10(3) and 7.0 X 10(3) CFU/g after exposure to 10-mu-g/ml of the drug for 5 h, respectively. A lower number of Vibrio isolates was observed to correspond with greater numbers of Moraxella and/or Pseudomonas in the NFS-Na-treated rotifers at all densities tested. For the Vibrio sp. INFL-loaded rotifers, the total and Vibrio sp. INFL counts at the beginning were 1.2 X 10(8) and 1.1 X 10(8) CFU/g on ZoBell apr but 1.6 X 10(7) and 1.3 X 10(7) CFU/g on TCBS apr, respectively. The number of bacteria in the control on both ZoBell and TCBS apr media was slightly lower during a 5-h period. In the NFS-Na-treated groups, the number of bacteria on the two media, particularly on the TCBS agar, was much lowered. The TCBS counts of rotifers at densities of 970 and 9700 individuals/ml decreased from 1.6 X 10(7) CFU/g to 9.1 X 10(3) and 2.9 X 10(3) CFU/g after exposure to 5-mu-g/ml, and to 3.2 X 10(3) and 8.2 X 10(2) CFU/g after exposure to 10-mu-g/ml of the drug for 5 h, respectively. No pathogenic Vibrio sp. INFL was detected from any treated rotifers after 3 h exposure.