Influence of water activity and temperature on in vitro growth of surface cultures of a Phoma sp. and production of the pharmaceutical metabolites, squalestatins S1 and S2

A Phoma sp., known to produce the pharmaceutically active metabolites squalestatin 1 (S1) and squalestatin 2 (S2), was cultured on malt-extract/agar (MEA) over a range of water activities (a(w), 0,995-0.90) and temperatures (10-35 degrees C) to investigate the influence on growth and metabolite production. Use of the ionic solute NaCl to adjust a(w) resulted in significantly lower (P < 0.01) squalestatin yields than when the Phoma sp. was grown on MEA amended with the non-ionic solute glycerol. Water activity and temperature and their interactions were highly significant factors (P < 0.001) affecting growth of the Phoma sp., with optimum conditions of 0.998-0.950 a(w) and 35 degrees C. Squalestatin production was similarly influenced by a(w) temperature. time and their interactions (P < 0.001), S1 and S2 production occurred over a narrower a(w) and temperature range than growth, with a slightly lower optimum a(w) range of 0.995-0.980 a(w). The optimum temperature for squalestatin production varied from 20 degrees C (S1) to 25 degrees C (S2) and yields of S2 were up to 1000 times lower than those of S1. The ratio of S1 and S2 produced by the Phoma sp. was influenced by a(w) and temperature, with highest values at 0.99-0.95 a(w) and at 15 degrees C. Incubation times of 25 days gave highest yields of both S1 and S2. Up to 2000-fold increases in squalestatin fields were measured at optimum environmental conditions, compared to the unmodified MEA. This indicates the need to consider such factors in screening systems used to detect biologically active lead compounds produced by fungi.