Antimicrobial factors in solitary ascidians

Solitary ascidians possess a wide range of antimicrobial agents in their bloodand other tissues. A number have been purified and characterised with a view to developing novel pharmacological or commercially useful products. The most well known are the didemnins isolated from whole body homogenates of Caribbean ascidians belonging to the genus Trididemnum. Others include the halocyamines from Halocynthia roretzi, piclavines from Clavelina picta and lissoclinotoxins from Lissoclinum perforatum. For most species, however, little is known about the spectrum of activity, tissue location or mode of action of these compounds, and no details have been forthcoming about their biosynthesis, release or internal regulation. Recent work on Ciona intestinalis has shown that potent antibacterial activity against a range of Gram positive and Gram negative bacteria resides within the circulating blood cells, principally the morula cells. Activity is constitutive, unrelated to lysozyme and does not involve agglutination or direct lysis of the bacterial cell wall. Instead, it appears to be clue to at least two proteins with molecular masses of ca 8–12 kDa and 60–70 kDa respectively. Since the morula cells are known to participate in a variety of cellular host defence responses, there can be little doubt that these proteins aid in the neutralisation of bacterial, and possibly other micro-organisms, which may gain access to the tissues. However, in some ascidian species, peptides with antibiotic properties in vitro have been shown to have other biological effects; for example protection against predation, digestion or prevention of surface epibiosis. There is great scope for finding further novel antimicrobial proteins in the ascidian group, and areas where further research is needed include an analysis of their biochemical and phylogenetic relationship to other biologically active peptides, an understanding of their induction and modulation in vivo and an assessment of the ways they exert their biological effects on susceptible micro-organisms in vitro.