Forty two strains of Vibrio anguillarum were isolated from the spleen of young Ayu in Hamana Lake in 1978. Of the 42 strains 17 were isolated from apparently normal 81 fish captured by small set nets (Kakudate-ami), isolation rate being 21%. The remaining 25 strains were isolated from 25 diseased fish which had been kept in a stocking pond for a few days, isolation rate being 100%. Sixteen strains out of the former 17 strains were classified as serotype B and C, but 24 strains out of the latter 25 strains were serotype A. Twenty five strains were isolated from 34 adult fish which had been cultured in ponds for a few months, and almost all of them were serotype A. These results suggest that the succession of serotype of the bacterium is affected by medication and some other unknown factors during the period of cultivation in ponds.
The drug sensitivity of 56 strains of Vibrio anguillarm, which were isolated from young (normal and diseased) Ayu from Hamana Lake, to nalidixic acid (NA), oxolinic acid (OA), tetracycline (TC), chloramphenicol (CP), nifulpirinol (NP), sulfamonomethoxine (SMM), and sulfadoxine-trimethoprim (SDX-TMP) were examined and expressed as minimal inhibitory concentration (MIC). It is of interest that distinct correlation was found between serotypes and drug sensitivity; Serotype A strains were resistant to NA, OA, and NP, but serotype B and C strains were sensitive to these drugs. As shown in the previous paper, serotype A strains were isolated from diseased adult Ayu from culture ponds, though serotype B and C strains were predominantly isolated from young Ayu caught in Hamana Lake. This change in serotypes was attributed to a selection by certain chemotherapeutics used in culture ponds.