The major antibiotics in current clinical use were discovered by screening terrestrial microorganisms. Over the past decade, however, the discovery rate of new antibiotics from terrestrial sources has decreased. At this time, the dereplication rate for the terrestrial environment is estimated to be greater than 90%.

In contrast, the marine environment represents a relatively unexplored resource for the discovery of new antimicrobial compounds. The extreme diversity of the marine environment with respect to salinity, temperature, pressure and nutritional availability provides an environment that selects for a high level of genetic and molecular diversity among the microorganisms that exist there.

Marine microorganisms have developed unique metabolic and physiological properties that allow them to survive in these extreme environments. Marine bacteria have the ability to incorporate halogens (Br, Cl, F and I) found in high concentrations in seawater into secondary metabolites. This is important because many antimicrobial compounds are halogenated. The marine environment is therefore likely to be a good source for new antimicrobials.

Aphios has established a library of more than 2,000 unique marine microorganisms from diverse environments, including deep-sea sediments to shallow water mangrove swamps, tropical waters to temperate oceans, hydrothermal vents and hypersaline ponds as well as from sponges, corals, and other marine invertebrates. Aphios has also developed proprietary fermentation techniques that mimic the natural saline marine environment in order to enhance isolation of bioactive compounds and allow large-scale manufacture of novel anti-infectives and anticancer drugs. The microorganisms (bacteria, actinomyces, yeasts, and fungi) are fermented in at least four different media designed to maximize the diversity of secondary metabolites being generated.

In our research to date, 4,800 fractions of 400 marine microorganisms were utilized as a starting point for a systematic search for the discovery of antimicrobial compounds. The ability of the fractions to inhibit the growth of two Gram-positive bacteria, Streptococcus mutans, and Actinomyces viscosus, were evaluated in 96-well format using a plate reader and visually confirmed.

Twelve potential “hits” which exhibited greater than 95 % inhibition of growth of these two organisms were tested for inhibition of growth against MDR Staphylococcus aureusATCC 33592 (gentamicin and methicillin) and Enterococcus faecium ATCC 51559 (ampicillin, ciprofloxacin, gentamicin, rifampin, teicoplanin, and vancomycin). The activity was determined based on the inhibition of cell growth relative to a viability control on the same plate.

Six marine microorganism fractions that showed greater than 25 % inhibition of one or both of these organisms were tested for minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The most active fraction, with an absolute purity < 0.1%, had MICs of 15.6 μg/mL and 3.9 μg/mL and MBCs of 31.25 μg/mL and 15.63 μg/mL respectively against MDR Staphylococcus aureus and Enterococcus faecium.