Elimination of bacteria from the circulation of the pond snail Lymnaea stagnalis

Doses of 5 × 10 7 living bacteria ( Staphylococcus saprophyticus and Escherichia coli), injected into the freshwater pulmonate, Lymnaea stagnalis, are rapidly removed from the circulation. Two hrs after injection, 99% of the bacteria have been cleared. In the initial stage of the clearance process, numbers of circulating amoebocytes decreased rapidly. Light microscopy on whole snails and electron microscopy on blood cell pellets and connective tissue showed that in addition to circulating amoebocytes, connective tissue amoebocytes and fixed phagocytes play a role in endocytosis and subsequent digestion of bacteria. The clearance of bacteria seems to be due to phagocytic activity alone, as substances with bactericidal properties were not found in the haemolymph. The role of opsonic factors in in vivo phagocytosis is discussed.