Challenge of 20 fish from each experimental group and the
control was by i.p. injection with 0.1 ml suspension of A. hydrophila
in 0.9% (w/v) saline containing 106 cells ml1, 24 h after stopping
feeding trials. Previous work had determined the LD 50% to be
1.7 105 cells ml1. Mortalities were monitored over 14 days, and
any dead or moribund fish examined bacteriologically to confirm
the presence of A. hydrophila [11]. The relative percentage survival
(RPS) was calculated after [14]. Thus as: