The Survival of Marine Vibrios in Mercenaria Mercenaria, the Hardshell Clam

A depuration chamber was used to study the persistence of marine vibrios in the hardshell clam, Mercenaria mercenaria. Specimens of M. mercenaria were incubated for two h in artificial seawater containing 103 cells/ml each of the following bacterial species; Vibrio parahaemolyticus, Vibrio harveyi and Escherichia coli, and then transferred to the depuration chamber (a tank through which U. V.‐sterilized artificial seawater was continually flowing). Numbers of the three bacterial species in tissues of M. mercenaira removed from the chamber at various times were determined by differential plating techniques. The number of each species ranged from 102 to 103 colony‐forming units/gram tissues immediately after transfer to the depuration chamber. After 24 h at 25°C the number of E coli cells detected had decreased over 100‐fold. Generally, V. parahaemolyticus and V. harveyi were found in increased abundance after 24 h. The abundance of V. parahaemolyticus and V. harveyi in clams that had been incubated in the depuration chamber for 72 h at 25°C was approximately 10% of the abundance of these species immediately after transfer to the chamber. Similar results were obtained when the incubation temperature was 8 or 15°C and when initial cell concentrations were altered. Thus, V. harveyi and the potential human pathogen, V. parahaemolyticus which are both of marine origin were not removed from M. mercenaria at a rate comparable to the rate at which M. mercenaria depurated cells of E. coli.