Bioactive Compounds Isolated from a Marine Sponge Selectively Inhibit Neisseria gonorrhoeae

Neisseria gonorrhoeae is the third most common sexually transmitted infection (STI), which may become untreatable soon if resistance continues to drastically increase. Due to increases in resistance to recommended antibiotics, alternative sources of novel compounds to combat this threat are being explored. Interestingly, marine sponges have proven to produce a plethora of bioactive compounds that display anticancer, antiviral, antifungal, and antibacterial activity.

In this study, the extracts of the sponge collected from Saint Thomas, US Virgin Islands were examined to determine their antibacterial activity against E. coli, S. aureus, and N. gonorrhoeae.

The ethyl acetate sponge extracts significantly inhibited growth of N. gonorrhoeae, while none inhibited S. aureus and E. coli. The bioassay-guided purification of the ethyl acetate extract resulted in the isolation of 6-desmethyl-6-ethylspongosoritin A (1) and plakortone B (2). To determine if the pure sponge metabolite could improve the efficacy of ceftriaxone against a high-level ceftriaxone (HTX)-resistant gonococcal strain, an antibiotic checkerboard assay was done by combining various concentrations of either precursor fractions or the purified compound 2 with ceftriaxone. Plakortone B (2) and ceftriaxone acted in synergy against gonococcal strains and inhibited growth by increasing membrane permeability when exposed for 4 h and 24 h.

This suggests that marine sponges may serve as a source for novel bioactive compounds against antibiotic-resistant strains of N. gonorrhoeae, as well as improve the efficacy of currently prescribed antibiotics.