Chemotactic attraction of Schistosoma mansoni miracidia to acidic carboxyl-containing solutions in vitro.

The life cycle of Schistosoma mansoni includes a short-lived miracidial stage, during which miracidia must locate and penetrate suitable Biomphalaria snail hosts. Although several snail-derived attractants have been reported, the physicochemical properties underlying miracidial chemotaxis remain poorly defined. To examine the contribution of acidity and carboxyl groups to miracidial attraction, we first evaluated chemotactic responses to a previously reported snail-derived peptide (P12) and its amino acid-substituted derivatives under acidic and neutral conditions. Under acidic conditions, miracidia accumulated toward all peptide solutions. Neutralization did not significantly affect miracidial attraction to P12, whereas it significantly reduced attraction to P12-D/N at all concentrations tested and to P12-P/A at intermediate concentrations. Circular dichroism analysis revealed that all peptides adopted random conformations irrespective of pH, indicating that peptide secondary structure did not account for the observed differences in attraction. Based on these findings, we examined miracidial chemotaxis toward simple acidic solutions. S. mansoni miracidia were attracted to organic acids containing carboxyl groups, including acetic, succinic, and citric acids, at specific concentrations, whereas hydrochloric acid induced attraction only within a limited concentration range and not at higher acidity. These results indicate that miracidial attraction is influenced by both acidity and the presence of carboxyl groups, rather than by acidity alone. As several organic acids tested here can be produced by snail hosts, the present findings provide insight into the chemical basis of host-finding behavior in S. mansoni miracidia in vitro.