EVIDENCE FOR ORAL INGESTION AS THE PRINCIPAL ROUTE OF ANTIGEN ENTRY IN BATH-IMMUNIZED FISH

This study re-examines the route of antigen entry in bath-immunized fi sh by making use of a 150 kDa toxin molecule (from Clostridium botulin um type E) which acted as both prototype antigen and marker for its ow n entry, through its neurotoxic effects. Initially, toxin uptake was e stablished as a function of dose by seeding serial toxin dilutions int o small aquaria containing 4-10 cm goldfish (Carassius auratus). The p ossibility that gills or lateral lines were principal toxin entry poin ts was ruled out by plugging fish oesophagi with a rubber-like, dental -impression compound. Toxin uptake was reduced nearly sixfold by this procedure. Water ingestion rates were measured by exposing groups of 8 -10 cm fish for varying time intervals to a constant flow of water con taining 0.2% toxic supernatant from a botulinum culture. The potency o f this supernatant was determined simultaneously in additional fish by intragastric inoculation. The measured ingestion rate in 8-10 cm fish was at least 0.37 ml h(-1) (with an additional 0.18 ml h(-1) entering fish either by other routes or leaking past the oesophageal plugs). T herefore, if goldfish were immersed for 2 min in a bath containing 2 x 10(10) bacteria ml(-1), their gastrointestinal tract would accumulate at least 2.5 x 10(8) bacteria.