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 c olonized by bacteria that may be pathogenic to them. One of the means by wh ich they are able to combat microbial attack is by chemical defense. We tes ted the activity of crude organic extracts of 11 dominant Red Sea reef spon ges against a panel of bacteria isolated from their natural environment. Am phimedon viridis (Keller) exhibited the highest antimicrobial activity. Bio assay-directed fractionation resulted in the isolation of an active fractio n that contained a purified mixture of halitoxin and amphitoxin, which are highly bioactive pyridinium alkaloids. These compounds showed selective act ivity against specific bacteria rather than being of a broad spectrum. They were highly active against 8 strains of bacteria isolated from the seawate r surrounding these sponges, whereas 6 different bacterial strains associat ed with the sponge A. viridis were resistant to these compounds. This selec tive toxicity may be important in enabling certain bacteria to live in clos e association with their sponge host while it maintains a chemical defense against microbial pathogenesis.