Antimicrobial activity of the reef sponge Amphimedon viridis from the Red Sea: Evidence for selective toxicity

Living benthic marine organisms such as sponges and corals are frequently colonized by bacteria that may be pathogenic to them. One of the means by which they are able to combat microbial attack is by chemical defense. We tested the activity of crude organic extracts of 11 dominant Red Sea reef sponges against a panel of bacteria isolated from their natural environment. Amphimedon viridis (Keller) exhibited the highest antimicrobial activity. Bioassay-directed fractionation resulted in the isolation of an active fraction that contained a purified mixture of halitoxin and amphitoxin, which are highly bioactive pyridinium alkaloids. These compounds showed selective activity against specific bacteria rather than being of a broad spectrum. They were highly active against 8 strains of bacteria isolated from the seawater surrounding these sponges, whereas 6 different bacterial strains associated with the sponge A. viridis were resistant to these compounds. This selective toxicity may be important in enabling certain bacteria to live in close association with their sponge host while it maintains a chemical defense against microbial pathogenesis.