Varin Tanasomwang

Effect of sodium nifurstyrenate on the reduction of bacterial contamination of rotifers brachionus plicatilis

Abstract 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 ZoBells 2216e agar 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 μg/ml NFS-Na. On the other hand, the counts on TCBS agar at the three densities of rotifers showed a marked downward trend from 2.6 × 10 6 CFU/g to 2.9 × 10 4 , 2.0 × 10 4 and 4.0 × 10 3 CFU/g after exposure to 5 μg/ml, and to 4.0 × 10 3 , 4.0 × 10 3 and 7.0 × 10 3 CFU/g after exposure to 10 μg/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 × 10 8 and 1.1 × 10 8 CFU/g on ZoBell agar but 1.6 × 10 7 and 1.3 × 10 7 CFU/g on TCBS agar, respectively. The number of bacteria in the control on both ZoBell and TCBS agar 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 × 10 7 CFU/g to 9.1 × 10 3 and 2.9 × 10 3 CFU/g after exposure to 5 μg/ml, and to 3.2 × 10 3 and 8.2 × 10 2 CFU/g after exposure to 10 μg/ml of the drug for 5 h, respectively. No pathogenic Vibrio sp. INFL was detected from any treated rotifers after 3 h exposure.